a0005 4.13 Volcano Seismologygachon.eri.u-tokyo.ac.jp/.../Papers/data/TOGP73_proof2.pdfELSEVIER...
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4.13a0005 Volcano SeismologyH. Kawakatsu, University of Tokyo, Tokyo, Japan
M. Yamamoto, Tohoku University, Sendai, Japan
ª 2007 Elsevier Ltd. All rights reserved.
4.13.1 Introduction 1
4.13.2 Volcanic Seismic Signals 2
4.13.2.1 Signals Observed by Short-Period Seismometers 2
4.13.2.2 Broadband Signals 3
4.13.3 Description of Volcanic Seismic Sources 5
4.13.3.1 Equivalent Forces of General Seismic Sources 6
4.13.3.2 The Seismic Moment Tensor 7
4.13.3.3 The Single Force 8
4.13.3.4 Waveform Analysis of Volcanic Seismic Sources 9
4.13.4 Physical Mechanisms for Volcanic Seismic Signals 10
4.13.4.1 Slow Waves in Solid/Fluid Composite 10
4.13.4.2 Resonating Sources: A Crack 12
4.13.4.3 Other Resonating Sources 13
4.13.4.4 Flow-Induced Oscillation 14
4.13.4.5 Bubble Dynamics 16
4.13.5 Observation and Analysis Aspects 16
4.13.5.1 Broadband Seismometry 16
4.13.5.2 Array Analysis 17
4.13.6 Models for Volcanic Seismic Signals 18
4.13.6.1 Aso 19
4.13.6.2 Kilauea 20
4.13.6.3 Miyake 21
4.13.6.4 Stromboli 23
4.13.7 Other Volcano-Specific Issues 24
4.13.7.1 Explosion Quakes 24
4.13.7.2 Triggered Seismicity 26
4.13.8 Concluding Remarks 26
References 27
s0005 4.13.1 Introduction
p0005 Volcano seismology is a field of volcanology in which
seismological techniques are employed to help under-
standing the physical conditions and dynamic states of
volcanic edifices and volcanic fluid systems to such a
level that it eventually contributes to predictions of
initiation and cessation of hazardous volcanic activ-
ities. Seismology has been a powerful tool for this
purpose, especially in the past 10–15 years as new
observational and analysis techniques in seismology
have become available (e.g.,b0140
Chouet, 1996a,b0145
1996b;b0610
McNutt, 2005); they include digital and broadband
seismometry, and array and moment tensor waveform
analyses. While the subjects that should be coveredunder the title of volcano seismology are quite broad
and diverse, this chapter, however, neither intends tonor is capable to cover all of its areas; instead we try to
focus on some of seismological phenomena whichappear to be specific to volcanoes. In a sense, volcanoseismology here is a field of seismology which deals
with problems specific to volcanic areas. Some of thesubjects, such as seismic structure of volcanoes (tomo-
graphy) and eruption monitoring (seismicity), aretherefore intentionally omitted. The readers who are
interested in these subjects are advised to refer tothe excellent reviews by
b0140
Chouet (1996a,b0145
1996b,b0150
2003)and
b0600
McNutt (1996,b0605
2002,b0610
2005).
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s0010 4.13.2 Volcanic Seismic Signals
p0010 For seismologists who have only observed seismo-grams of ordinary faulting earthquakes, the mostintriguing aspect of volcano seismology may be theample variety of waveforms recorded in volcanic
environments. Such volcanic seismic signals rangefrom totally chaotic continuous vibrations lastingfor minutes, hours, or sometimes days to purelymonotonic vibrations decaying with constant
damping rates; they often show several spectralpeaks whose frequency may vary during the courseof volcanic activities, possibly reflecting the changeof physical conditions of volcanic fluid systems; when
eruptions occur, variety of long-period signals up toas long as a few hundred seconds are generated andsometimes observed at seismic stations around theworld. In the following, some of such intriguing
volcanic seismic signals will be presented.p0015 Terminology. Based on their appearance on short-
period seismometers, volcanic seismic signals have
been traditionally classified into four types: high-fre-quency or A type, low-frequency or B type, explosionquakes, and volcanic tremors (e.g.,
b0630
Minakami, 1974; for
the detail of different terminology, see table 1 ofb0600
McNutt (1996)). In this chapter, following this tradi-tion, we use high frequency for frequencies higher than5 Hz, and low frequency for frequencies between 5 and
0.5 Hz. On the other hand, the usage of period remainsa complicated issue in volcano seismology; the most
widely accepted notation to describe volcanic seismicsignals is that of
b0145
Chouet (1996b), in which events ofabout 1 s are called ‘long-period’ (LP) events and thosewith longer periods are called ‘very-long-period’(VLP) and ‘ultra-long-period’ (ULP) events. This defi-nition, however, contradicts with the conventionalusage of period in traditional earthquake seismology,where periods longer than the dominant period of theambient microseismic noise (around 5–10 s) are calledlong-period (
b0025
Aki and Lee, 2003;b0600
McNutt, 1996,b0610
2005).In this chapter, as we try to cover the subject in a widercontext of seismology, we follow the convention ofearthquake seismology, and use the term long-periodfor periods longer than 5 s and short period forperiods shorter than or around 1 s; for events with aperiod longer than 50 s, we may also use very long-period. To avoid confusion, whenever appropriate, wewill try to associate actual number in seconds todescribe a period, for example long-period (10 s).
s00154.13.2.1 Signals Observed by Short-PeriodSeismometers
p0020The conventional seismometry at active volcanoes,except for a few rare cases, has been conducted usingshort-period seismometers. Even with such band-restricted sensors, wide varieties of waveformsrecorded in volcanoes have led scientists to wonderwhat may be the origins of those signals. Figure 1shows a classical yet the most fascinating one of those
V L T
0 1 2 3 4 5 6Hz
0 1 2 3 4 5 6Hz
0 1 2 3 4 5 6Hz
V
L
T
GON
5 10 15 20 25 30 35 40 s
f0005 Figure 1 Volcanic tremor observed at Sakurajima. Top three traces show 40 s long seismograms for vertical, longitudinal, and
transverse components. Corresponding spectra are shown below. Adapted from Kamo K, Furuzawa T, and Akamatsu J (1977)
Some natures of Volcanic micro-tremors at the Sakurajima Volcano. Bulletin of the Volcanological Society of Japan 22: 41–58.
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recordings of volcanic tremors observed at
Sakurajima Volcano, Japan (b0480
Kamo et al., 1977).
Volcanic tremors are those sustained ground vibra-
tions commonly observed at volcanoes, typically
lasting for longer than 1 min, up to hours or days;
they often show periodic spectral features as seen
here with the lowest peak frequency around 1 Hz.
During the course of activities, relative spectral
content and peak frequencies may vary from time
to time.p0025 Figure 2 shows examples from Redoubt Volcano,
Alaska, where different types of events are displayed:
from the top, (A) high-frequency or volcano-tectonic
(VT) earthquake, which most likely represents an
ordinary brittle failure on a fault, (B) hybrid or
mixed-frequency event, (C) low-frequency (LF)
event with frequency around 1 Hz, and (D) volcanic
tremor. Note that the volcanic tremor here appears
more random compared to those in Figure 1, and
also its frequency content seems similar to that of the
low-frequency event (C). It is sometimes considered
that tremors and low-frequency events share a com-
mon excitation mechanism.p0030 Low-frequency events in some volcanoes show
characteristic similar spindle-shape waveforms, sug-
gesting a possible common origin for these events in
different volcanoes.p0035 Explosion quakes are observed when explosive
eruptions occur, and often associated with a high-
frequency arrival on seismograms due to a shock
wave originated at the vents. Figure 3 shows series
of such events recorded in Langila Volcano, PapuaNew Guinea (
b0645
Mori et al., 1989), sorted by the ratio ofground-transmitted explosion-wave and air-waveamplitudes. As those without an air wave areclassified as low-frequency events, this figure alsoindicates that explosion quakes and low-frequencyevents may share a common origin.
p0040As noted earlier, the spectral content of these low-frequency volcanic seismic signals may vary through-out the course of activities. Figure 4 shows such anexample from Montserrat Volcano, West Indies; thetemporal change of the system prior to an explosionis clearly manifested.
p0045It appears clear that the low-frequency nature ofthese seismograms is the most characteristic of vol-canic seismic signals. Although they arecharacterized here just in terms of the frequencycontent, there exist similarities and diversitiesamong them; their origins may be different from avolcano to another, and may change from time totime even in a single volcano. Understanding of theirorigins and monitoring of their appearances to inferphysical conditions and dynamic states of volcanicedifices and volcanic fluid systems are interesting andchallenging tasks.
p0050Deep (volcanic) low-frequency signals. Most ofvolcanic seismic signals originate within a shallowpart of the crust (<5 km). There are, however, acertain type of volcanic seismic signals whichoriginate either from the lower crust or from theuppermost mantle (15–50 km) right beneath activevolcanoes. As they show similar low-frequencycontent as volcanic tremors, they are often calleddeep low-frequency volcanic tremors (or events).They are now reported in many volcanoesaround the world (e.g.,
b0020
Aki and Koyanagi, 1981;b0745
Pittand Hill, 1994;
b0345
Hasegawa and Yamamoto, 1994;b0935
White, 1996;b0660
Nakamichi et al., 2003). As deep low-frequency volcanic tremors may be relatedwith deep magma-supplying system of volcanoes,understanding of their origin seems extremelyimportant for the long-term prediction of theactivities of volcanoes.
s00254.13.2.2 Broadband Signals
p0055Availability of broadband seismic records hasbrought wider perspectives in our view of volcanicactivities. Figure 5 shows a very good exampleobserved at Satsuma-Iwojima Volcano, Japan(
b0735
Ohminato and Ereditato, 1997). The short-periodsensor recording at ST3 shows amplitude modulation
(a)
(b)
(c)
(d)
10 sec
f0010 Figure 2 Typical waveforms observed at Redoubt. After
McNutt SR (1996) Seismic monitoring and eruption
forecasting of volcanoes: A review of the state-of-the-art
and case histories. In: Monitoring and Mitigation of VolcanoHazards, pp. 99–146. New York: Springer.
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Fof high-frequency signal with a periodicity of
46–50 min. When this amplitude becomes small,
the lowpass-filtered broadband record at ST1
shows long-period (10 s) spike-like signals. This
synchronicity of two different band signals continues,
while the exact periodicity may fluctuate. Such an
interplay of different band signals forces us to view
the origin of these signals as a system (i.e., volcanic
fluid system).p0060Figure 6 is another such example of a broadband
record of a small phreatic eruption observed at Aso
Volcano, Japan (b0505
Kaneshima et al., 1996;b0520
Kawakatsu
et al., 2000). While the integrated displacement
record exhibits a very long-period (>100 s)
signal corresponding to the inflation and deflation
stages of a crack-like conduit (b0955
Yamamoto et al.,
1999) before and during the eruption, the raw velo-
city record exhibits short-period signals apparently
due to the quick flow of volcanic fluids only seen in
the deflation stage (i.e., eruption). Again signals seen
in two different bands allow us to vividly image
the process of the eruption of a volcanic fluid
system. Different physics apply at different
frequencies, and this makes volcanic seismic signals
essentially broadband.
Explosion
Continuous tremor
Time (min)0 12 24 36 48 60
Freq
uenc
y (H
z)
9
8
7
6
5
4
3
2
1
0
f0020 Figure 4 Example of tremor at Montserrat before anexplosion. After Jousset P, Neuberg J, and Sturton S (2003)
Modelling the time-dependent frequency content of low-
frequency volcanic earthquakes. Journal of Volcanology
Geothermal Research 128: 201–223.
Gradation in typesof seismic events
0 10 20 30 s
Air phase fromsummit explosion
Low-frequencyeventwith air phase
Low-frequencyeventwith air phase
Low-frequencyevent
f0015 Figure 3 Range of volcanic seismic signals from the high-frequency air-wave phase to LF event observed at Langila.
Arrows indicate timings of air-wave phases. Adapted from Mori J, Patia H, and McKee C, et al. (1989) Seismicity associatedwith eruptive activity at Langila Volcano, Papua New Guinea. Journal of Volcanology Geothermal Research 38: 243–255.
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Fp0065Global observation. The last example (Figure 7)
comes from the recordings of very long-period(200–400 s) seismic signals generated by the erup-tion of Pinatubo Volcano, Philippines, observed atbroadband stations around the world (
b0500
Kanamoriand Mori, 1992;
b0940
Widmer and Zurn, 1992). Thelong-period disturbances of the atmosphere gener-ated by the eruption clouds are transmitted back tothe solid Earth through the coupling between thetwo systems. Two spectral peaks observed in theseismic records are well explained by a theorywhich treats the solid Earth and the atmosphereas a single system (
b0925
Watada, 1995); such a couplingbetween the solid and fluid parts of the Earth isalso observed in so-called background free oscilla-tions of the Earth (
b0690
Nishida et al., 2000). Thebroadband recordings of the Pinatubo eruptionappear to have accelerated the emergence of anew class of seismology which treats the solidEarth, atmosphere, and ocean as a single system(cf. Chapter 4.14).
s00354.13.3 Description of VolcanicSeismic Sources
p0070The diversity of observed volcanic seismic signalspartly reflects the wide variety of seismic sourcegeometries and dynamics present in active
Inflation Deflation
Velocity
10–30 s
Displacement
Broadband seismograms
200 μm s–1
2 μm s–1
20 μm
50 s
f0030 Figure 6 Broadband waveforms of a phreatic eruptionobserved at Aso. Adapted from Kawakatsu H,
Kaneshima S, and Matsubayashi H, et al. (2000) Aso94:
Aso seismic observation with broadband instruments.
Journal of Volcanology Geothermal Research 101:129–154.
50(a)
(b)
Vel
ocity
(×
10E
–6 M
/S)
–50
2.5
–2.5
10
4–10
Vel
ocity
(×
10E
–6 M
/S)
2.5–4
–2.54/21
3:45:40
ST1(UF)
ST1(LO)
ST3(UF)
4/216:00
ST1(UF)
ST1(LO)
ST3(UF)
4/213:46:40
4/211:00
4
–4
112
f0025 Figure 5 (a) Five-hour records of vertical components of
velocity at Satsuma-Iwojima. A trace lowpass-filtered (LO)at 0.2 Hz is shown in between the two unfiltered records.
(b) Expansion of the 1 min portion of the top panel.
Adapted from Ohminato T and Ereditato D (1997)
Broadband seismic observations at Satsuma-IwojimaVolcano, Japan. Geophysical Research Letters 24:
2845–2848.
0 5 10 15 20hour
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0mHz
f0035Figure 7 Broadband record of the Pinatubo eruption in1991 observed in Japan. Adapted from Watada S (1995)
Part I: Near-source Acoustic Coupling Between the
Atmosphere and the Soild Earth During Volcanic Eruptions.
PhD Thesis, California Institute of Technology.
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volcanoes. A general kinematical description of
seismic sources in terms of ‘moment tensor’ and‘single force’ has become common in volcano
seismology, and the waveform inversion solving for
them has been becoming a general tool to understand
the generation mechanisms of long-period signalsobtained with broadband instruments.
s0040 4.13.3.1 Equivalent Forces of GeneralSeismic Sources
p0075 The concept of the seismic moment tensor was firstintroduced by
b0315
Gilbert (1970) as a description of
seismic sources more general than the conventional
double-couple description. Laterb0080
Backus and
Mulcahy (1976) gave a physical basis for such ageneral description within the framework of linear
elastodynamics by introducing concepts of
indigenous source and stress glut. An indigenous
source is a seismic source which originates withinthe Earth, and its equivalent force system exerts
neither total force nor total torque at any instance
to the Earth. Seismologists, however, soon realized
that there exist such indigenous seismic events whichrequire single force equivalent force systems (e.g.,b0490
Kanamori and Given, 1982;b0495
Kanamori et al., 1984;b0350
Hasegawa and Kanamori, 1987;b0510
Kawakatsu, 1989);
most of them happened to occur around active vol-canoes. To circumvent this apparent conflict,
b0855
Takei
and Kumazawa (1994) extended Backus and
Mulcahy’s treatment to include fluid-dynamical
description with a mass advection term, and
introduced a concept of inertial glut; they furthergave a clear definition of single force (and torque)
source as a momentum (and angular momentum)
exchange between the seismic source volume and
the rest of the Earth. In the following, we brieflysummarize their treatment.
p0080 To make the argument simple, we omit theEarth’s self-gravity in the following; although gravity
often takes the central role generating single forces in
the actual Earth, this simplification does not alter theessence of the argument presented below. The true
Eulerian equation of motion of the Earth without
external force may be written as
�tD2t uj ¼ qiSij ½1�
where �t(x, t) denotes the true density, uj(x, t) is thedisplacement, Sij(x, t) is the true stress field, and Dt isthe particle derivative Dt¼ qtþ vkqk with vk(x, t) as the
velocity. We compare this true equation of motionwith the linear elastodynamic equation of motion,
�mq2t uj ¼ qi�ij ½2�
where �m(x, t) and �ij (x, t) respectively denote themodel density and model stress field due to the linearstress–strain constitution equation (Hooke’s law),which we use to describe the wave propagation inthe Earth. The equivalent body force �V
j ðx; tÞ in thelinear elastodynamic system arises as the differenceof the two equations [1] and [2],
�mq2t uj ¼ qi�ij þ �V
j ½3�
where
�Vj ¼ �S
j þ �Dj ½4�
with
�Sj ¼ – qi�ij ¼ – qið�ij – Sij Þ ½5�
�Dj ¼ �mq2
t uj – �tD2
t uj ½6�
p0085�ij(x, t)¼�ij (x, t) � Sij(x, t) in [5] is the stressglut introduced by
b0080
Backus and Mulcahy (1976) whichrepresents failure of Hooke’s law due to some non-linear effect in the source region, and is zero outside ofthe source region. The second term in [4], �D
j ðx; tÞ, isnamed as inertial glut by
b0855
Takei and Kumazawa (1994),and is the difference of the inertial forces in the actualvalue and in the model, which is again zero outside ofthe source region. It can be shown that total force andtotal torque due to �S
j ðx; tÞ integrated over thevolume is zero at any instance, while those due to theinertial glut �D
j ðx; tÞ are necessarily not.p0090The spatial integral of [6] over the whole space
�ð0ÞDj ¼
Zð�mq2
t uj – �tD2
t uj ÞdV ½7�
can be shown to be
�ð0ÞDj ¼ – q2
t
ZVs
ð�t – �mÞuj dV ½8�
which shows that �ð0ÞDj ðtÞ originates from a difference
between �t and �m in the source region Vs and thus maynot vanish. On the other hand, it is shown that the timeintegral of �
ð0ÞDj ðtÞ during the event vanishes, that is,
Z 10
�ð0ÞDj dt ¼ 0 ½9�
the requirement that any indigenous source has tosatisfy. The authors further defined single force andtorque sources as linear and angular momenta
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exchange between the source region and the rest ofthe Earth, and showed that, in the long-period limit,the single force source is equal to �
ð0ÞDj ðtÞ.
s0045 4.13.3.2 The Seismic Moment Tensor
p0095 The spatial integral of stress glut is the seismicmoment tensor Mij which describes the overallfeature of the seismic source. When the wavelengthof observed seismic waves is much longer than thespatial extent of the source, we may approximate thesource by a point source moment tensor. Eachcomponent of Mij corresponds to one set of opposingforces (dipole or force couple; Figure 8). The seismicmoment tensor is a symmetric second-order tensor,and thus has six independent components, while adouble-couple equivalent body force for a sheardislocation has only four degrees of freedom. Thesetwo extra degrees of freedom in the moment tensorare called non-double-couple components. For thegeneral discussion on non-double-couple compo-nents, readers may refer to the following reviewarticles (
b0475
Julian et al., 1998;b0625
Miller et al., 1998).Seismic sources observed in the volcanic environ-ments often show non-double-couple components.The end members of such non-double-couplesources are discussed below.
p0100 Spherical source. Let us consider an initial condi-tion that a reservoir contains a certain amount ofvolcanic fluid (V ) under a static pressure P. When asudden volume increase of �V (measured under thesame pressure condition) is introduced in the
reservoir, either due to an injection of a new volcanicfluid or a thermal expansion, the reservoir shouldexpand and act as a seismic source. This volumetricincrease�V corresponds to the stress-free volumetricstrain introduced by
b0255
Eshelby (1957) (b0030
Aki andRichards, 2002, p. 53) and characterizes the amount
of the increase of the volcanic fluid. On the otherhand, due to the confining pressure of the surround-ing elastic medium (matrix), the actual volumetricincrease of the reservoir �Vm may be smaller than
�V, the ratio �Vm/�V depending on the geometry ofthe reservoir, with a resultant pressure increase �Pm.
p0105The corresponding moment tensor for a sphericalreservoir is given as
M ¼ �V
�þ 2
3� 0 0
0 �þ 2
3� 0
0 0 �þ 2
3�
0BBBBBB@
1CCCCCCA
½10�
where the ratio
�Vm
�V¼ �þ ð2=3Þ�
�þ 2�½11�
and the actual volumetric strain and pressureincrease may be related as
�Pm ¼4
3�
�Vm
V½12�
(b0030
Aki and Richards, 2002, p. 61). As the actual volu-metric change �Vm is also the volume change oftenused in geodetic analysis (e.g.,
b0635
Mogi, 1958), we pro-pose to call this ‘Mogi volume’ to distinguish it fromthe stress-free volume increase �V.
p0110Cylinder. A moment tensor corresponding to theradial expansion of a cylinder whose symmetry axis isx1 may be given as
M ¼ �V
� 0 0
0 �þ � 0
0 0 �þ �
0BB@
1CCA ½13�
where
�Vm
�V¼ �þ ��þ 2�
½14�
and
�Pm ¼ ��Vm
V½15�
M11 M12 M13
M21 M22 M23
M31 M32 M33
1
2
3
f0040 Figure 8 Nine force couples corresponding to moment
tensor components.
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p0115 Tensile crack. A moment tensor corresponding to an
opening of a thin crack in the direction of x1-axis may
be given as
M ¼ �V
�þ 2� 0 0
0 � 0
0 0 �
0BB@
1CCA ½16�
In case of a thin crack, two volumetric changes turnout to be the same. The Mogi volume here is given as�V ¼ �Vm ¼ S?��u, where S and ��u are the crackarea and opening width, respectively, and it is oftenreferred to as ‘potency’ in seismological literatures(e.g.,
b0090
Ben-Menahem and Singh, 1981).p0120 Compensated linear vector dipole. The isotropic
components of the moment tensors of these three
volumetric sources, when expressed in terms of
stress-free volumetric change �V, are the same:
I ¼ 1
3traceðMÞ ¼ �þ 2
3�
� ��V ¼ ��V ½17�
where � is the bulk modulus of the elastic matrix. Whena movement of magma from a reservoir to anotherexcites seismic waves, such a seismic source should beobserved as a summation of two volumetric sourceswith opposite signs. Equation [17] indicates that thecorresponding moment tensor has zero isotropic com-ponent called compensated linear vector dipole(CLVD) by
b0550
Knopoff and Randall (1970) (Figure 9).p0125 Determination and interpretation of a moment
tensor. Due to structural complexities of volcanic
edifices, the conventional first motion polarity ana-
lysis method to determine source mechanisms of
volcanic events is unlikely to give reliable estimates
of moment tensors. Although the amplitude of
radiated seismic waves contains more information,
often the absolute amplitude also suffers from struc-
tural complexities. Methods that utilize relative
amplitudes of waves which share common raypaths, such as the spectral ratio method ofb0705
Nishimura et al. (1995), the amplitude ratio methodof
b0465
Julian and Foulger (1996), and the relativemoment tensor inversion method of
b0190
Dahm (1996),appear to be better suited; among them, the linear-programming approach of
b0465
Julian and Foulger (1996)has been extensively used to show the existence ofmany non-double-couple earthquakes in differentvolcanic/geothermal environments (e.g.,
b0775
Ross et al.,1996;
b0280
Foulger et al., 2004).p0130An arbitrary moment tensor observed at volcanoes
may be decomposed as a linear combination of thosefour end-member moment tensors (plus a doublecouple). As such a decomposition is nonunique, therecan be different decompositions for a given momenttensor, and thus there exists always nonuniqueness ininterpreting moment tensor solutions in terms of actualphenomenon that is occurring at volcanoes. It is often acustom to choose the simplest possible one or to chooseone which is most consistent with known source geo-metry of corresponding volcano (e.g., the presence ofknown crack-like conduit). Without such a priori infor-mation, on the other hand, one may wish to display afull moment tensor. The source-type plot introducedby
b0390
Hudson et al. (1989) may be a useful tool to helpinterpreting different moment tensors observed in dif-ferent volcanic environments (e.g.,
b0280
Foulger et al., 2004;Figure 10). It displays moment tensors without regardto their orientations, and different types of momenttensors (different in relative magnitudes of principalmoments) are projected on a rhombic plane in such away that the area is proportional to the probability ofthe occurrence of different source types.
s00754.13.3.3 The Single Force
p0135As described above, the single force equivalent bodyforce is due to a momentum exchange between thesource volume and the rest of the Earth. Whenever apart of the Earth is detached from the rest and gains amomentum, the counter force of this acceleration isfelt by the rest of the Earth. As the detached masseventually has to stop somehow, there must be adeceleration stage in this process, giving anothercounter force with the opposite direction. The totalof these two counter forces must cancel out asrequired from [9].
p0140The single force equivalence of long-period seis-mic waves excited by a landslide and explosionsassociated with the 1980 eruption of Mt. St. Helenswas established by
b0490
Kanamori and Given (1982) and
(a) (b)
f0045 Figure 9 Geometry of selected transport models.
Adapted from Chouet B (1996b) New methods and futuretrends in seismological volcano monitoring. In: Monitoring
and Mitigation of Volcano Hazards, pp. 23–97 New York:
Springer.
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b0495
Kanamori et al. (1984). At a long-period limit, a land-slide may be viewed as a sliding of a box along a slope(
b0350
Hasegawa and Kanamori, 1987;b0510
Kawakatsu, 1989),and its single force equivalence may be obvious.b0510
Kawakatsu (1989) developed a centroid single force(CSF) inversion method, an extension of the centroidmoment tensor (CMT) inversion method ofb0235
Dziewonski et al. (1981) to include single force com-ponents, and applied it successfully to severallandslide/slump events recorded by global seismicnetworks.
b0245
Ekstrom et al. (2003) applied the CSF inver-sion to ‘glacial earthquakes’ observed in southernAlaska, and showed that they are represented bystick-slip, downhill sliding of a glacial ice mass whichtakes about 30–60 s in duration.
p0145b0495
Kanamori et al. (1984) also suggested that anequivalent force system for a volcanic eruption canbe decomposed into a vertical downward single forceand an isotropic moment tensor (Figure 11). A singleforce source may be also realized when high-viscousmagma flows along a narrow conduit, as suggested byb0905
Ukawa and Ohtake (1987) to explain the deep mono-chromatic low-frequency (1 Hz) event observedbeneath Izu-Oshima Volcano, Japan, about 1 yearprior to the 1986 eruption.
b0160
Chouet et al. (2003) alsosuggested that a piston-like rise of a slug of gas in theconduit produced observed single force componentsin the long-period signals associated with explosionsin Stromboli Volcano, Italy.
s00804.13.3.4 Waveform Analysis of VolcanicSeismic Sources
p0150The long-period part of volcanic seismic signals hasbeen recorded by seismic networks of regional and/or global scales. One of the advantages of using suchlong-period signals is that waveform analysis to inferexcitation mechanisms can be easily performed com-pared to short-period seismograms. As mentioned,b0490
Kanamori and Given (1982) modeled the long-periodseismic signals accompanied with the eruption of theMt. St. Helens in 1980 recorded at stations of the
1
2
3
4 56
78
91011
12
13
14
15
16
17
18
19
20 21 22 23
24
2526
km0 5 10
–CLVD
–Dipole
–Crack
+CLVD
+Dipole
+Crack
+V
–V
5
11
18
19
20+
+
+ +
+
+
118° 45′11° 50′118° 55′119° 00′119° 05′
37° 35′
37° 40′
37° 45′
DC
(a)
(b)
f0050 Figure 10 (a) Focal mechanism plot (upper focal
hemisphere) of earthquakes in Long Valley. (b) Source-typeplot of
b0390
Hudson et al. (1989) for the same earthquakes. The
vertical axis measures the size of volumetric component,
and deviatoric moment tensors are located on the horizontal
axis (a pure double couple at the origin). Different colorsindicate mechanism solutions determined by different
researchers. Adapted from Foulger GR, Julian BR, Hill DP,
Pitt AM, Malin PE, and Shalev E (2004) Non-double-couplemicroearthquakes at Long Valley caldera, California,
provide evidence for hydraulic fracturing. Journal of
Volcanology and Geothermal Research 132: 45–71.
A B
P
FT
FB
FSFS l2a
FTUp
0t = 0 Time
FS
00
Out-ward
FB
00
Down
τ
FT10
0
(downward)
FT2
00
FS
00
FB
00
(a)
(b)
(c)
Implosivesource
Verticalsingleforce
f0055Figure 11 A force system equivalent to a volcanic
eruption. (a) A pressure release model for a volcaniceruption. (b) Forces acting on the top, side, and bottom
walls. (c) Decomposition of the force to a vertical single
force and an implosive force. Adapted from Kanamori H,
Given JW, and Lay T (1984) Analysis of seismic body wavesexcited by the Mount St. Helens eruption of May 18, 1980.
Journal of Geophysical Research 89: 1856–1866.
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global seismic networks. They showed that the single
force mechanism can explain the radiation pattern of
long-period surface waves, and attributed it to the
gigantic mass movement of the volcano edifice due to
a landslide.b0495
Kanamori et al. (1984) also analyzed the
long-period body waves generated by the same erup-
tion, and showed that vertical single forces due to the
depressurization of the magma chamber explain the
data. With these two single force mechanisms, they
could vividly picture what had occurred during the
whole sequence of the eruption. These two studies
initiated the applications of ‘long-period seismology’
to study volcanic activities.p0155 For long-period seismic signals, it is often appro-
priate to treat the source originating from a point in
space. Then seismic wave field un (x, t) may be
written as
unðx; tÞ ¼ Gnp; qðx; x0; tÞ �MpqðtÞ þ Gnpðx; x0; tÞ � FpðtÞ
where Mpq(t) and Fp(t) denote a moment tensor andsingle-force source time function at x0, respectively,and Gnp (x, x0, t) and Gnp,q (x, x0, t) respectivelyrepresent Green’s function and its spatial derivative(e.g.,
b0030
Aki and Richards, 2002); the asterisk denotesconvolution, and the summation convention forrepeated indices is assumed. Knowing the Earth struc-ture (i.e., knowing the Green’s function) and havingenough waveform data, it is then possible to solve forthe time history of general seismic sources, Mpq(t)and Fp(t).
p0160b0870
Takeo et al. (1990) extended the waveformmoment tensor inversion technique commonly used
in the analysis of far-field records in earthquake
seismology (e.g.,b0540
Kikuchi and Kanamori, 1982) to
volcano seismology by including the single force
contribution. They applied it to the records observed
in a lava drain-back stage of the 1987 eruption of Izu-
Oshima Volcano, Japan, and showed that the resolved
vertical single forces with opposing directions are
due to a rapid collapse of overloading lava into the
deeper vent.b0730
Ohminato et al. (1998) applied a similar
technique to near-field broadband records observed
at Kilauea Volcano, USA, and showed that long-
period signals can be explained by a subhorizontal
crack which periodically injects magma into a larger
reservoir.b0160
Chouet et al. (2003,b0155
2005) andb0720
Ohminato
(2006) further incorporated the effect of complex
topography of volcanic edifice that may
significantly distort seismic signals observed in the
near-field by calculating Green’s functions using the
finite difference scheme developed byb0725
Ohminato andChouet (1997).
p0165The general source analysis for long-period sig-nals using moment tensor and single force hasbecome common in seismological studies of volcanicactivities. Examples of such studies includeMt. St. Helens (
b0490
Kanamori and Given, 1982;b0495
Kanamori et al., 1984;b0510
Kawakatsu, 1989); Long Valley(
b0455
Julian, 1983;b0010
Aki, 1984;b0920
Wallace, 1985;b0225
Dreger et al.,2000); Kilauea, Hawaii (
b0240
Eissler and Kanamori, 1987;b0730
Ohminato et al., 1998), USA; Asama (b0865
Takeo et al., 1984;b0740
Ohminato et al., 2006); Izu-Oshima (b0870
Takeo et al., 1990);Sakurajima (
b0895
Uhira and Takeo, 1994); Unzen(
b0975
Yamasato et al., 1993;b0900
Uhira et al., 1994b); Ito-oki(
b0860
Takeo, 1992); Tori-shima (b0485
Kanamori et al., 1993),Japan; Iceland (
b0470
Julian et al., 1997;b0675
Nettles andEkstrom, 1998); Stromboli, Italy (
b0160
Chouet et al., 2003);and Popocatepetl, Mexico (
b0155
Chouet et al., 2005).Applications of such analysis techniques for shorter-period (�1 s) near-field records obtained at volcanoesare also becoming available (e.g.,
b0890
Uhira et al., 1994a;b0705
Nishimura et al., 1995;b0055
Aoyama and Takeo, 2001;b0660
Nakamichi et al., 2003;b0665
Nakano et al., 2003;b0720
Ohminato, 2006;b0575
Kumagai et al., 2005).
s00854.13.4 Physical Mechanisms forVolcanic Seismic Signals
p0170The presence of ample fluid (gas, vapor, magma, ortheir mixtures) in volcanic edifice introduces anadditional interesting class of vibration/wave phe-nomena to the well-studied elastic formulations. Forexample, the so-called ‘crack wave’, which isintroduced by
b0130
Chouet (1986) to explain the low-frequency nature of volcanic events, may be under-stood as a part of a class of waves due to solid/fluidcoupling.
b0460
Julian (1994) showed that the nonlineareffect of such solid/fluid coupling due to fluid flowtransient can generate a wide variety of waveformswhich have similar characteristics as volcano seismicsignals. This section summarizes some such topics,relevant to generation of volcano-specific seismicsignals.
s00904.13.4.1 Slow Waves in Solid/FluidComposite
p0175There exists a class of waves in the two-phase systemof a solid–liquid composite that travel slower thanany of the sound velocities of the pure material con-stituting the two-phase system (i.e., the wave speed
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10 Volcano Seismology
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can be slower than that of the liquid phase). These
waves include the tube wave (b0100
Biot, 1952), so-called
Biot’s slow wave in a porous medium (b0105
Biot, 1956),
so-called crack wave (b0130
Chouet, 1986;b0265
Ferrazzini and
Aki, 1987), and waves in solid–liquid alternating
layers (b0830
Schoenberg and Sen, 1983) (Rayleigh wave
and Stoneley wave may also be included in the same
class, considering that in the high-frequency limit, all
the above slow waves have the same characteristics as
an interface wave).p0180 The simplest (and the most relevant to volcano
seismology) example of slow waves may be the plane
wave propagation in a medium which consists of a
fluid layer sandwiched by two elastic half-spaces
studied byb0265
Ferrazzini and Aki (1987), who investi-
gated the physical basis of the crack wave found byb0130
Chouet (1986). Here we followb0965
Yamamura (1997) to
formulate the problem.p0185 Following
b0030
Aki and Richards (2002, p. 263), we firstexpress a plane-wave solution propagating parallel to
the fluid layer as
u
w
!¼
r1ðzÞ
ir2ðzÞ
!e iðkx –!tÞ ½18�
where u and w are respectively the displacements forx- and z-directions. The frequency domain equationof motion in x-direction for the fluid layer may thenbe written as
–!2�f r1 ¼ – k2�f r1 – k�fqr2
qz½19�
where �f and �f denote density and bulk modulus ofthe fluid, respectively. Integrating [19] over thethickness of the fluid layer (i.e., z-direction) withappropriate boundary conditions (e.g., free-slip con-dition at the solid/fluid interface), we get thefollowing expression:
–!2�f
Z H
0
r1dz ¼ – k2�f
Z H
0
r1dz – k�f r2ðHÞ ½20�
where we assume a symmetric solution for thez-dependency (
b0265
Ferrazzini and Aki, 1987) and thethickness of the fluid layer 2H.
p0190 Equation [20] clearly demonstrates the role of theinterface: without the last term r2(H ), [20] indicates
the averaged displacement in the fluid layer traveling
with its own sound velocity; that is, the term repre-
sents the way the fluid layer is coupled with the
surrounding elastic half-space. If we further rewrite
[20] as
–!2�f
Z H
0
r1dz¼ –k2�f 1þ 1
k
r2ðHÞZ H
0
r1dz
2664
3775Z H
0
r1 dz ½21�
it is seen that
�ef X�f 1þ 1
k
r2ðHÞZ H
0
r1dz
2664
3775
represents the effective bulk modulus for the fluidlayer; when �e
f < �f , that is,
r2ðHÞZ H
0
r1 dz
< 0 ½22�
the corresponding traveling wave has a smaller wavespeed relative to the sound velocity; thus we have aslow wave. The condition [22] is satisfied if thedenominator and numerator have an opposite signto each other. This makes quite good sense intui-tively; when the fluid ‘tries’ to contract (or expand) asa passage of the wave, due to the ‘elastic coupling’along the solid/fluid boundary, there exists a solutionin which the solid behaves out of phase to the fluid toexpand (contract), deforming the boundary in such away that fluid ‘feels’ easy to contract (expand), com-pared to the single-phase case; that is for a givenamount of displacement in the propagation direction,the actual pressure increment of the fluid phasebecomes lower than that of the pure fluid case(Figure 12). The net effect of this elastic coupling
Rigid
Elastic
Fluid
Fluid
(a)
(b)
f0060Figure 12 Conceptual image of the slow wave in solid/
fluid composites. Out-of-phase motion of the solid (b)lowers the ‘effective bulk modulus’ of the fluid, and as a
result the wave speed becomes slower than that in the pure
fluid (a).
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Volcano Seismology 11
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is to reduce the ‘effective bulk modulus’ (i.e., therestoring force) of the fluid, and as a result the wavespeed becomes slower than that of the pure fluid;stronger the elastic coupling, slower the wave speed(Figure 13).
p0195 This kind of the out-of-phase condition is satisfiedby other slow waves such as Biot’s slow wave inporous medium (
b0105
Biot, 1956), and waves in solid/fluidalternating layers (
b0830
Schoenberg and Sen, 1983). Indeed,using the argument presented above,
b0965
Yamamura(1997) was able to derive a similar set of equations todescribe the elastic coupling for the Schoenberg’sproblem to those Biot’s equations of motion, andsuggested that all these slow waves have a similarcharacteristic as an interface wave, and that in theshort-wavelength limit they reduce to the Stoneleywave.
s0095 4.13.4.2 Resonating Sources: A Crack
p0200 The presence of regularly spaced spectral peaks oftenobserved in volcanic seismic signals immediatelyreminds us of the presence of resonators within thevolcanic edifice filled with volcanic fluid. To quanti-tatively model the volcanic tremor observed atKilauea, Hawaii,
b0015
Aki et al. (1977) considered fluid-filled tensile cracks as the most plausible resonatingsources. Using two-dimensional finite differencemethod, they computed the dynamics of a series offluid-filled cracks connected by narrow channelswhich are excited by jerky extension/opening of
channels due to excess fluid pressure. Althoughtheir model offers both the driving force and thegeometry adequate for the magma transport beneathactive volcanoes, the fluid inside the crack actspassively like a cushion so that the stress at thecrack wall depends only on local displacements ofthe crack wall; the fluid also dose not support thepropagation of pressure perturbation caused by thedeformation of the crack wall. To assess active rolesof the fluid,
b0165
Chouet and Julian (1985) extendedAki et al.’s model, and investigated dynamic interac-tion between fluid and elastic solid inside and outsidea two-dimensional crack by solving equations for theelastic solid and the fluid simultaneously. Themodel was further extended to three-dimension byb0130
Chouet (1986).p0205Chouet’s three-dimensional model consists of a
single isolated fluid-filled crack in an infinite elasticsolid body, and no external mass transfer into and/orout of the crack is taken into consideration. In themodel, the thickness of the crack d is assumed to bemuch smaller than the wavelength of interest, and thusthe motion of the fluid inside the crack is treated astwo-dimensional. Assuming Poisson’s ratio to be 0.25(i.e., �¼�) and taking L and L/� as the characteristiclength and time, where L and � are the length of thecrack and the P wave velocity in the solid, theresponse of the crack is calculated by solving followingdimensionless equations simultaneously with bound-ary conditions at the crack surface and perimeters:
Solid:q~ui
q~t¼ 1
3
q~ik
q~xk
q~ij
q~t¼ q~uk
q~xk
ij þq~ui
q~xj
þ q~uj
q~xi
Fluid:q ~Ul
q~t¼ –
1
3
�
�f
q ~P
q~xl
q ~P
q~t¼ –
�f
�
q ~Um
q~xm
– 2�f
�
L
d~uz
~x ¼ x=L; ~t ¼ t�=Lð Þ
½23�
In these equations, the quantities with tilde representdimensionless ones, and ~u and ~ are the velocity andthe stress of the solid; ~U and ~P are the velocity andthe pressure of the fluid, � and �f are the density ofthe solid and the fluid, � and �f are the rigidity of thesolid and the bulk modulus of the fluid, respectively.As shown in [23], the behavior of fluid-filled crackcritically depends on a nondimensional parameterc¼�f/� ? L/d, which was first introduced by
b0015
Akiet al. (1977) and named as ‘crack stiffness’ by
b0130
Chouet(1986).
0 2 4 6 8 10 12
0 2 4 6 8 10 12
0.6
0.8
1.0
1.2
1.4
1.6
0.6
0.8
1.0
1.2
1.4
1.6
λ /h
v/c ρs /ρf = 2.5
β/c = 1.5
σ = 0.25
f0065 Figure 13 Phase velocity divided by the acoustic velocityof the fluid is plotted as a function of the wavelength divided
by the fluid layer thickness. The fundamental branch having
a phase velocity below 1 corresponds to the slow wave.
Adapted from Ferrazzini V and Aki K (1987) Slow wavestrapped in a fluid-filled infinite crack: Implication for volcanic
tremor. Journal of Geophysical Research 92: 9215–9223.
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p0210 Synthetic seismograms calculated by the fluid-filled model show strong similarities to observedvolcanic signals in terms of both peaked spectra andlong-lasting oscillations, and demonstrated theimportance of active participation of fluids in sourcedynamics generating volcanic seismic signals. Thesestudies include the interpretations of volcanic signalsobserved at Redoubt, Alaska (
b0175
Chouet et al., 1994);Galeras, Colombia (
b0310
Gil Cruz and Chouet, 1997,Figure 14); Kusatsu-Shirane, Japan (
b0670
Nakano et al.,1998;
b0580
Kumagai et al., 2002); Kilauea, Hawaii(
b0575
Kumagai et al., 2005); and other volcanoes.p0215
b0565
Kumagai and Chouet (1999,b0570
2000) andb0655
Morrisseyand Chouet (2001) further studied the dependencies offrequencies and attenuations of crack-resonant oscilla-tions on the properties of fluid in more detail usingvarious models of fluid–gas mixtures and fluid–particlemixtures, and demonstrated that the fluid propertiesand compositions can be estimated from frequencies
and attenuations of observed signals. These results areused to assess the change of magmatic and hydrother-mal system beneath the volcano at Kusatsu-Shirane,Japan (
b0580
Kumagai et al., 2002; Figure 15) andTungurahua, Ecuador (
b0640
Molina et al., 2004).
s01004.13.4.3 Other Resonating Sources
p0220Although the presence of regularly spaced spectralpeaks may not straightforwardly warrant the pre-sence of a resonator (
b0460
Julian, 1994), it seems indeednatural to think of such presence like a crack as adike, a cylinder as a conduit, and a sphere as a magmachamber, as they are all fundamental constituents of avolcano.
p0225Resonators as models of the origins of volcanicseismic signals besides a crack have been suggestedfor a cylinder (
b0275
Ferrick et al., 1982;b0125
Chouet, 1985), anda sphere (
b0810
Sassa, 1935;b0555
Kubotera, 1974).b0185
Crosson andBame (1985) obtained an impulsive response of aspherical magma chamber with a spherical cavityinside.
b0290
Fujita et al. (1995) solved eigenoscillation of afluid sphere embedded in an infinite elastic medium.b0285
Fujita and Ida (2003) conducted a systematic surveyof the geometrical effects on the eigenoscillations,and showed that the ratios of higher modes to thefundamental mode frequencies can be indicators ofthe geometry of the resonator. They also found aclass of modes with a low attenuation named ‘low-attenuation mode’ (LAM), which share the similarout-of-phase characteristics of the slow wavesdiscussed above, and suggested that LAM may bethe origin of long-lasting low-frequency oscillationof volcanic seismic signals.
p0230The complicated nature of vibrations of cylindri-cal conduits has been numerically studied (e.g.,b0680
Neuberg et al., 2000;b0450
Jousset et al., 2003;b0700
Nishimuraand Chouet, 2003).
b0680
Neuberg et al. (2000) introduced adepth-dependent seismic velocity model to accountfor the varying gas content in the magma, andshowed that a pressure change in the conduitwas the most likely candidate for the physicalprocess causing the peak-spectral shift often observedin many of volcanoes (Figure 4).
b0450
Jousset et al.(2003) also showed that the end points of the conduitcan act as secondary sources of volcanic seismicsignals.
p0235The excitation mechanism of these resonators is,on the other hand, not well understood. The pro-posed models include a jerky extension of the cracktip (
b0015
Aki et al., 1977), acoustic emissions from collap-sing bubbles (
b0135
Chouet, 1992), rapid discharge of fluids
0 5 100
1
Frequency (Hz)
Nor
mal
ized
vel
ocity
spe
ctru
m
10 s
(a)
(b)
(c)
f0070 Figure 14 Example of the application of the fluid-filledcrack model: (a) Vertical velocity seismogram observed at
Galeras Volcano, Colombia. (b) Synthetic waveform obtained
from the fluid-filled crack model. (c) Spectrum of observeddata (thin line) and synthetic data (dotted line). Adapted from
Chouet B (1996a) Long-period volcano seismicity: Its source
and use in eruption forecasting. Nature 380: 309–316.
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(b0275
Ferrick et al., 1982), and a sudden pressure drop
caused by unsteady choked flow (b0650
Morrissey and
Chouet, 1997).b0670
Nakano et al. (1998) developed a
method to extract the effective excitation function
from seismic records based on an inhomogeneous
autoregressive (AR) model of a linear dynamic sys-
tem.b0665
Nakano et al. (2003) further extended the
method to deal with the nonorthogonal nature of
eigenfunctions of a resonator embedded in a rock
matrix by using the biorthogonal eigenfunction
expansion method ofb0970
Yamamura and Kawakatsu
(1998); the resolved effective excitation functions of
the low-frequency events in Kusatsu-ShiraneVolcano, Japan, are then modeled by the waveforminversion method of
b0730
Ohminato et al. (1998) to obtainthe moment tensor and single-force source timefunction, which are interpreted as a result of repeatedactivation of a subhorizontal crack located beneaththe summit crater lake.
s01054.13.4.4 Flow-Induced Oscillation
p0240b0460
Julian (1994) proposed nonlinear flow-induced oscil-lations in channels transporting volcanic fluid as a
1992/09/13 09 : 46
1992/09/14 06 : 56
1992/10/22 18 : 31
1992/11/18 05 : 05
0 10 20 30 40 50 60 70Time/sec
8/1 9/1 10/1 11/1 12/1 1/1 2/1
8/1 9/1 10/1 11/1 12/1 1/1 2/1
Date (1992–1993)
QFr
eque
ncy/
Hz
0
50
100
150
200
0
1
2
3
4
5
1st period 2nd period 3rd period
Crackgrowth Drying process Crack collapse
Misty gas (gas wt %)
Misty gas (gas wt %)
10%
30%50%
100%
10%30%
50%
100%
3%
f0075 Figure 15 Temporal change of waveforms observed at Kusatsu-Shirane Volcano and the fluid properties estimated from
the frequency and the attenuation of signals. From Kumagai H, Chouet B, and Nakano M (2002) Temporal evolution of a
hydrothermal system in Kusatsu-Shirane Volcano, Japan, inferred from the complex frequencies of long-period events.Journal of Geophysical Research 107: ESE 9–1.
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possible generating mechanism for volcanic seismicsignals, particularly for sustained volcanic tremors.The mechanism may have analogs such as the vibra-tion of vocal chords, musical wind instruments, etc.,and the periodic behavior, often characteristic ofvolcanic seismic signals, occurs without the presenceof a resonator.
p0245 Figure 16 shows the simplified lumped-para-meter model of the flow of a viscous fluidthrough a constricted channel with elastic walls.The channel walls are modeled as masses con-nected by a spring and dashpot to represent theeffects of the elastic stiffness and radiation loss.The mechanism may be summarized as follows:an increase in a flow speed leads to a decrease influid pressure by the Bernoulli effect; as a result,the walls narrow the channel to constrict the flow,causing a pressure increase and forcing the channelopen again; this again decreases the pressure andincreases the flow speed. This positive feedbackmechanism sustains the oscillation.
p0250b0460
Julian (1994) derived a third-order nonlinearsystem of ordinary differential equations that describethe complete behavior of the system and may be solvednumerically. The system without radiation dampingexhibits two different types of behavior: oscillationsthat grow and approach a stable tremor-like limitcycle, and transient oscillations that decay, leading toa steady flow. Inclusion of the damping further com-plicates the behavior, as shown in Figure 17. Byincreasing the upstream pressure, the system experi-ences so-called period doubling, that is, an appearance
of a subharmonic with a frequency half that of the
fundamental frequency; further increase with succes-
sive period doubling eventually leads to chaotic
behaviors. Julian suggests that these features resemble
behavior of some of the volcanic tremors and the deep
low-frequency event observed in Hawaii (b0020
Aki and
Koyanagi, 1981).p0255
b0085
Balmforth et al. (2005) extended Julian’s treatmentto include the dynamic behavior of the fluid and the
elasticity of the surrounding walls, and considered
the excitation of propagating waves. Their results
confirm that Julian’s mechanism is plausible, but
the physical condition required for the instability
is different. They conclude that magma itself is
unlikely to generate flow-induced oscillations, but
that the rapid flow through fractured rock of low-
viscosity fluids exsolved from magma may. A similar
flow-induced oscillation is also studied byb0410
Ida (1996)
to explain the sawtooth nature of geodetic signals.p0260In fluid/gas two-phase system. The flow-induced
oscillation described above assumes flows of a single-
phase fluid. Volcanic fluids such as magma often con-
tain ample gases, especially at a shallow depth, and it
may be more reasonable to consider a flow of two-
phase fluids consisting of liquid and gas phases rather
than single-phase fluids as a source of volcanic seismic
signals. Such a flow of liquid/gas two-phase fluid is
also seen in, for example, the evaporation pipes in
boiler systems, nuclear reactor cooling systems,
x
y
z
L
h(t )
p2
v (x, t )p1
2M 2M2A 2A2k
2k
f0080 Figure 16 Lumped-parameter model of the generation of
volcanic tremor. Viscous, incompressible fluid flows inx-direction from upstream (bottom) to downstream (top)
reservoir through a channel with imperfectly elastic walls.
Adapted from Julian BR (1994) Volcanic tremor: Nonlinear
excitation by fluid flow. Journal of Geophysical Research 99:11859–11878.
p1 = 13.0 MPa
16.0
17.5
18.0
19.0
0 5 10 15
0
10
10
10
10
1
h (m
)
t (s)
f0085Figure 17 Synthetic tremor time series (channel thicknessh versus time) for different values of the upstream pressure
p1. Behavior changes from a simple limit cycle (top) to chaos
(bottom), with some period doubling in between. Adapted
from Julian BR (1994) Volcanic tremor: Nonlinear excitationby fluid flow. Journal of Geophysical Research 99:
11859–11878.
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chemical industry equipments, and many studies havebeen conducted extensively in the field of engineering.These studies revealed the existence of flow-inducedoscillatory phenomena, which is not observed in a flowof single-phase fluid, caused by complex interactionsbetween two phases.
p0265b0425
Iwamura and Kaneshima (2005) considered one ofthese flow-instability phenomena called ‘densitywave oscillation’, which was originally studied inthe nuclear reactor engineering, as a model of volca-nic seismic signals, and evaluated the properties ofgenerated seismic waves through a series of numer-ical experiments. Another kind of flow-inducedoscillation in fluid/gas two-phase flows called ‘pres-sure drop oscillation’ was also invoked to interpretthe cyclic tilt change observed at Miyake-jimaVolcano by
b0295
Fujita et al. (2004).
s0115 4.13.4.5 Bubble Dynamics
p0270 Gasses dissolved in magma are one of the main driv-ing forces to cause various volcanic phenomena. Thepresence of gas bubbles in magma decreases thedensity of magma, and drives the magma ascentfrom the deep part of volcanoes (e.g.,
b0945
Wilson andHead, 1981). At shallower depths, the relativelyhigh compressibility of bubbles contributes to thegeneration of oscillatory phenomena specific to vol-canoes (e.g.,
b0535
Kieffer, 1977; Chouet,b0140
1996a), and thetransformation and the fragmentation of bubblesaffect the behavior of volcanic eruptions (e.g.,b0035
Alidibirov and Dingwell, 1996;b0405
Ichihara et al., 2002).p0275 In relation to volcano seismology, there are some
aspects where bubbles play important roles. Asdescribed above, volcanic seismic signals are oftencharacterized by long-lasting low-frequency oscilla-tions with sharp spectral peaks (Figure 1), which arereminiscent of oscillations of some resonator filledwith material with a slow acoustic velocity. Sincethe existence of bubbles in magma drastically changesthe compressibility of magma, as pointed out byb0535
Kieffer (1977), these characteristics of volcanicseismic signals have been partly attributed to thepresence of bubbles in magma. The rapid temporalchanges of the observed spectral peaks (e.g.,
b0095
Benoitand McNutt, 1997;
b0680
Neuberg et al., 2000) have alsobeen considered to reflect the change in the state ofgas bubbles in volcanic conduits.
p0280 Bubbles in magma also act as an active source ofseismic and acoustic signals: the bursting and thecollapsing of bubbles at the lava surface precedingand/or accompanying volcanic eruptions generate
acoustic pressure signals which are sometimesaccompanied with seismic waves (e.g.,
b0910
Vergniolleet al., 1996;
b0770
Ripepe et al., 1996); the collapse of smallbubbles in hydrothermal f luids generates seismictremors (e.g.,
b0590
Leet, 1988); the movement of large-scale bubbles (slugs) in magma conduit causes anexchange of their momentum with the surroundingrocks and generates seismic signals that show domi-nant single force components (e.g.,
b0730
Ohminato et al.,1998;
b0760
Ripepe and Gordeev, 1999;b0160
Chouet et al., 2003,b0155
2005); the forced oscillations of bubbles by strainwaves can rapidly pump dissolved volatiles into bub-bles by a mechanism called ‘rectified diffusion’ andthe resultant pressure increase may trigger volcanicactivities (e.g.,
b0120
Brodsky et al., 1998;b0400
Ichihara andBrodsky, 2006).
p0285In spite of the difference in their generationmechanisms and resultant phenomena, all of thesephenomena are the manifestations of passive andactive processes in which the thermochemical energyand/or the gravitational energy of the volcanic f luidsare transduced into seismic/acoustic energy. Thequantitative understanding of the behavior of bub-bles, thus, is a critical key to elucidate the dynamicsof volcanic phenomena.
s01204.13.5 Observation and AnalysisAspects
p0290In addition to the conventional monitoring of activevolcanoes using short-period seismometers and geo-detic instruments, the digital era has brought newpowerful components to seismometry of active vol-canoes; broadband and array seismometries may betwo of the most notable developments.
s01254.13.5.1 Broadband Seismometry
p0295The first broadband seismic observation at an activevolcano was conducted by
b0810
Sassa (1935), who installedWiechert horizontal-component seismographs(pendulum period T0¼ 10.0 s) and vertical ones(T0¼ 4.6 s), Galitzin seismographs (T0¼ 8.0 s), andshort-period seismographs (T0¼ 0.55 s) at AsoVolcano. Summarizing the subsequent observationsin a period of several years, Sassa classified volcanicseismic signals of Aso into ‘eruption earthquakes’ andfour different kinds of tremors: periods of 0.2, 0.4–0.6,0.8–1.5, and 3.5–8.0 s. The observation of these tre-mors in the wide frequency band ranging from 0.2 to8.0 s clearly demonstrates the efficacy of broadband
TOGP 00073
16 Volcano Seismology
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seismometry at active volcanoes. Since Sassa’s pio-
neering work, until 1990s (when it has become much
easier to install portable broadband instruments),
there have not been many published reports on
attempts to observe long-period volcanic seismic sig-
nals except for a few cases. For example,b0835
Seidl et al.
(1981) installed broadband seismometers at Etna,
Italy, and observed 4–5 s period signals, which they
suggested as one of the fundamental peaks of the Etna
tremors.p0300 Examples of deployments of the current-generation
broadband seismometers are now plentiful, for exam-
ple, Sakurajima (b0530
Kawakatsu et al., 1992,b0525
1994) Unzen
(b0975
Yamasato et al., 1993;b0900
Uhira et al., 1994b) Aso
(b0505
Kaneshima et al., 1996;b0955
Yamamoto et al., 1999;b0520
Kawakatsu et al., 2000;b0595
Legrand et al., 2000) Satsuma-
Iwojima (b0735
Ohminato and Ereditato, 1997;b0720
Ohminato,
2006) Iwate (b0710
Nishimura et al., 2000); Miyake-jima
(b0585
Kumagai et al., 2001;b0285
Fujita and Ida, 2003), Usu
(b0960
Yamamoto et al., 2002) Bandai (b0715
Nishimura et al.,
2003) Hachijo-jima (b0560
Kumagai, 2006) Asama
(b0740
Ohminato et al., 2006), Japan; Stromboli (b0230
Dreier et al.,
1994;b0260
Falsaperla et al., 1994;b0685
Neuberg et al., 1994;b0160
Chouet et al., 2003), Italy; Semeru (b0360
Hellweg et al.,
1994) Merapi (b0370
Hidayat et al., 2000,b0365
2002), Indonesia;
Arenal (b0340
Hagerty et al., 2000), Costa Rica; Kilauea
(b0195
Dawson et al., 1998;b0730
Ohminato et al., 1998;b0045
Almendros
et al., 2002a), Long Valley (b0375
Hill et al., 2002;b0380
Hill and
Prejean, 2005), Mt. St. Helens (b0915
Waite et al., 2005), USA;
Erebus (b0785
Rowe et al., 1998,b0780
2000;b0070
Aster et al., 2003),
Antarctica; Popocatepetl (b0060
Arciniega-Ceballos et al.,
1999,b0065
2003;b0155
Chouet et al., 2005), Mexico; Karymsky,
Russia; and Sangay (b0445
Johnson and Lees, 2000),
Ecuador. A variety of long-period volcanic seismic
signals have been observed at different volcanoes.p0305 Besides the advantages already mentioned, the
long-period seismic wavefield observed with a
broadband seismic network may be used to directly
monitor the activity of a volcano.b0200
Dawson et al. (2004)
implemented the radial semblance method ofb0520
Kawakatsu et al. (2000; originally called ‘waveform
semblance’) for the near real-time monitoring of the
activities at the shallow magmatic conduit in Kilauea
Volcano, Hawaii (Figure 18). Similarly, the moment
tensor and single-force source real-time monitoring
should be possible using a grid-based method as
suggested byb0515
Kawakatsu (1998) and implemented at
the Earthquake Research Institute of the University
of Tokyo for the real-time seismicity monitoring, and
as recently realized at Stromboli Volcano (b0075
Auger
et al., 2006).
s01304.13.5.2 Array Analysis
p0310To locate the source of volcanic signals, various kinds
of methods have been applied to the analyses of
observed data. Classical hypocenter determination
methods using phase arrivals have been applied to
signals observed around active volcanoes and used to
predict the eruptions (e.g.,b0820
Scarpa and Tilling, 1996).
More advanced methods such as relative hypocenter
determination using the cross-spectral and the
double difference have also been applied to volcanic
earthquakes and have revealed vivid images of
volcanic activities like the transportation of magma
beneath active volcanoes (e.g.,b0355
Hayashi and
Morita, 2003).p0315Nevertheless, there still exist some specific issues
to be explored in the analyses of volcanic signals due
to complex source processes and complicated wave
propagations in inhomogeneous structures. Lack of
clear phases and relatively low dominant frequencies
of, for example, low-frequency events and volcanic
tremors make the direct application of the conven-
tional hypocenter determination means rather
difficult.p0320In the past decade, these difficulties have been
partly overcome by the use of seismic arrays. In array
observations, seismometers are placed at short distance
intervals (a few tens to hundreds of meters), and thus
East (km)
North (km)
Ele
vatio
n (k
m)
1.2
0.8
0.4
6
0 0
6
f0090Figure 18 Particle motions recorded by the network
during a 1 min long window of long-period signal. The
area with bright colors with high semblance values
indicates the source region. Adapted from Dawson P,Whilldin D, and Chouet B (2004) Application of near real-
time radial semblance to locate the shallow magmatic
conduit at Kilauea Volcano, Hawaii. Geophysical Research
Letters 31: L21606.
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Volcano Seismology 17
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the wave properties of the incident waves can beexamined by extracting coherent portion ofthe signals using the array data. Recent advance inportable instrumentation enables us to observevolcanic signals close to their sources with dense arraysand sheds new light on the understanding of the pro-cesses through the precise determination of locationand distribution of volcano seismic sources. For exam-ple, the MUSIC (MUltiple SIgnal Classification)technique (
b0825
Schmidt, 1986;b0320
Goldstein and Archuleta,1991) has been applied to the analyses of the arraydata observed at Kilauea, Hawaii (e.g.,
b0325
Goldstein andChouet, 1994;
b0045
Almendros et al., 2002a,b0050
2002b;Figure 19), and at Stromboli, Italy (e.g.,
b0180
Chouet et al.,1997), and revealed magmatic and hydrothermal activ-ities beneath the volcanoes. Other array analysistechniques have been also applied to constrain thesource locations and properties of volcanic signals(e.g.,
b0335
Gordeev et al., 1990;b0395
Ibanez et al., 2000).
p0325Seismic arrays have also been used to constrainthe shallow structure of volcanoes that is indispensa-ble for the quantitative volcano seismology. Here, thewavefield observed by a seismic array is treated as astationary stochastic one in time and space, and thestatistical correlation method originally proposed byb0005
Aki (1957) is applied to the data to determine thewave properties such as wave types and phasevelocities (e.g.,
b0270
Ferrazzini et al., 1991;b0620
Metaxian andLasage, 1997;
b0205
De Luca et al., 1997;b0170
Chouet et al., 1998;b0790
Saccorotti et al., 2001a).
s01354.13.6 Models for Volcanic SeismicSignals
p0330Although the basic governing physics, some of whichhave been discussed above, are common, and themethods of data acquisition and analyses employed
Hawaii
Kilauea
1 km
F21
Array F
Array E100 m
EF
D
East (km)
2
2
0
0 –2–2
1
0
0.5
250 m
100 m
Array D
D00 E00
caldera
F
E
N
N
N
N
D
Halemaumau
North (km)
Dep
th (k
m)
Halemaumau
(a)
(c)
(b)
f0095 Figure 19 Example of array observation. (a) and (b) Configuration of the array observation at Kilauea, Hawaii. (c) Source
location of the seismic signals obtained by array analyses. The arrows represent the apparent slowness vectors determinedby frequency-slowness analyses, and the gray region is the estimated source region. Adapted from Almendros J, Chouet B,
Dawson P, and Huber C (2002b) Mapping the sources of the seismic wavefield at Kilauea Volcano, Hawaii, using data
recorded on multiple seismic antennas. Bulletin of the Seismological Society of America 92: 2333–2351.
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may be the same, actual phenomena that we observedat volcanoes are different from one volcano toanother. To appreciate the diversity, some of thewell-studied volcanoes are reviewed. Since thenames of volcanic seismic events sometimes differfrom one volcano to another due to the differencein conventions as described in ‘Terminology’, in thissection, we keep common terminology locally used ateach volcano, and not necessarily follow the conven-tional classification of periods in earthquakeseismology.
s0140 4.13.6.1 Aso
p0335 Aso Volcano is one of the most active volcanoes inJapan. It has erupted in Strombolian style repeatedlywith intervals of 5–10 years, and recent activities takeplace at the youngest central cone which is composedof seven craters aligned northwest–southeast direc-tion with a length of 1 km. The current eruptiveactivities have occurred at the first crater located atthe northernmost of the chain of the craters.
p0340 Among the volcanic tremors observed byb0810
Sassa(1935), the most remarkable one is the volcanicmicrotremor of the second kind for itsextraordinary long-period (7.5 s) and its highly repe-titive occurrence. Observations using modernbroadband seismometers revealed that the fundamen-tal period of the long period tremor (hereafter LPT) is15 s (
b0505
Kaneshima et al., 1996;b0520
Kawakatsu et al., 2000), andthat the second-kind tremor of Sassa is the highermode of LPTs. The characteristics of LPTs are sum-marized as follows: (1) continually emitted from thevolcano regardless of surface activity; (2) spectra showseveral common spectral peaks which align withalmost equal spacing (15, 7.5, 5, 4 s); (3) decay fairlyfast and the duration is only a few cycles; and (4) oftenaccompanied with short-period tremors.
p0345b0505
Kaneshima et al. (1996) andb0520
Kawakatsu et al. (2000)deployed a broadband seismic network in 1994 andanalyzed the waveforms of the LPTs and thoseassociated with phreatic eruptions which ejectedcomposite of mud and water (Figure 6). Theyconcluded that the sources of both phenomena arelocated a few hundred meters southwest of the firstcrater and at a depth of 1–1.5 km below groundsurface. They further performed a source mechanisminversion of LPTs, and obtained a volumetric sourcemechanism which can be decomposed into anisotropic and a vertical tensile crack components(
b0595
Legrand et al., 2000). The detail of the tensile crackcomponent was further studied by
b0955
Yamamoto et al.
(1999) using the data obtained by a dense broadbandobservation. Analyzing the spatial distribution of thesignal amplitudes of LPTs (Figure 20), they revealedthe existence of a crack-like conduit whose strikeand width are almost the same as those of the chainof craters; the crack extends nearly vertically from adepth of 300–400 m below the surface to a depthof about 2.5 km, as illustrated in Figure 21. Thisobservation revealed that the chain of craters issimply a surface expression of a buried crack-likeconduit.
p0350The physical properties of the crack-like conduitwere studied by
b0950
Yamamoto (2005), who applied aboundary integral method to the oscillation of af luid-filled crack with slits at the edges, whichallow the fluid to escape, as an extension ofChouet’s closed crack model (
b0130
Chouet, 1986). Hedemonstrated that the spectral characteristics ofLPTs (mode frequencies, spacing, and attenuation)can be explained by the oscillation of a 25 m thickcrack-like conduit filled with gas–ash mixture. Theexistence of a crack-like conduit is also supported byother studies such as a ref lection study by
b0880
Tsutsuiand Sudo (2004), and the crack-like conduit is con-sidered as a subsurface path connecting a postulatedmagma chamber at a depth of around 5 km (
b0840
Sudo andKong, 2001) and the surface craters. The nature ofother short-period seismic signals with a period ofabout 0.5 s and a period of around 0.4–0.1 s has beenalso studied with modern digital data obtained bydense observations using short-period seismometers(
b0950
Yamamoto, 2005;b0850
Takagi et al., 2006).p0355Figure 21 schematically summarizes the system
beneath Aso Volcano that has been revealed by seis-mological analyses. Such a line of volcanic conduit
3600 s
30 s
1 μm/s
1 km
(a)
(b) (c)
f0100Figure 20 (a) A vertical-component broadband velocity
seismogram observed at a distance of 1.5 km. Spike-likesignals are LPTs. (b) Close-up view of an LPT. (c) Amplitude
variation of LPTs.
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Volcano Seismology 19
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s0145 4.13.6.2 Kilauea
p0360 The Hawaiian volcanoes have been produced by thehot spot which is presently beneath the Hawaiiisland. The volcanoes primarily erupt basalticmagma, and the relatively low viscosity of themagma yields gentle slope of the edifices. Kilauea isthe youngest and southeasternmost volcano on theHawaii island, and it is located at the splitting pointbetween the South-West Rift Zone and the East RiftZone. Owing to the high level of activities and easyaccess to it, Kilauea may be the best-studied volcanoein the world. A magma-rising system from mantlethrough narrow pipe-like conduit located beneath
the volcano, for instance, was detected by the ana-lyses of spatial distribution of VT earthquakes (e.g.,b0545
Klein and Koyanagi, 1989), and gives a rough imageof the deep magma transport and storagesystem beneath the volcano (Figure 22).
p0365The magma transport system at shallower depthshas been also studied using broadband seismic net-works and short-period seismic arrays. Performing themoment tensor inversions of broadband data,b0730
Ohminato et al. (1998) revealed that the source of thevery-long-period (VLP; 5–10 s) signals associated witha magmatic activity in 1996 is a crack-like magmapathway at a depth of 1 km, which acts like abuffer for surging magma from beneath summit ofKilauea to the East Rift Zone. Their analyses suggestthat �1000–4000 m3 of magma is injected into thecrack during a slow (1–3 min) accumulationphase, and it is ejected from the crack toward theEast Rift Zone during a rapid (5–10 s) deflationphase, which together produce a sawtooth-like displa-cement signal shown in Figure 23.
b0195
Dawsonet al. (1998) also analyzed VLP signals observed in1997, and concluded that the source location is almost
Crack-like conduit
Chain of the craters
E
WS
N
Reflector void
VT earthquakes
Short-period tremors (2 Hz)
Long-period tremors (15 s)Crack-like conduit
–SPT (2 Hz)
Fluid motion due tocrack resonance
–LPT (15 s)
High-frequency tremors (10 Hz)Craters
Magma chamber
–4 km
–3 km
–2 km
–1 km
0 km
1 km
Crack resonance
Cylindrical deformation
f0105 Figure 21 Image of crack-like conduit model. The crack is almost parallel to the chain of craters and the extension of the
crack meets the active fumarole at the surface.
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20 Volcano Seismology
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the same as that determined byb0730
Ohminato et al. (1998),although the source mechanism is different.
p0370 The source of long-period (LP; 1–2 s) signals, onthe other hand, is determined using seismic arraydata (e.g.,
b0795
Saccorotti et al., 2001b;b0040
Almendros et al.,2001), and the hypocenter of LP signals is locateddirectly above the source of VLP signals.
b0575
Kumagaiet al. (2005) analyzed the source mechanism of LPsignals, and imaged an expansion and contraction of anearly horizontal crack through the moment tensorinversions. Based on the f luid-filled crack model
developed byb0130
Chouet (1986), they further suggestthat LP signals represent a resonance of a hydro-thermal crack containing bubbly water and/orsteam, and that the heat from the underlyingmagma conduit (VLP source) may cause the pressur-ization of hydrothermal f luid in the crack and triggerthese LP signals. Other geophysical observations,such as a positive correlation between summit SO2
emissions and the shallow seismicity which waspointed out by
b0845
Sutton et al. (2001), also suggest theinteraction between magmatic and hydrothermalsystems at the shallow part of the volcano.
p0375These results indicate that the detailed quantita-tive analyses of volcanic seismic signals are criticallyimportant for a better understanding of volcanicprocesses, and such analyses may be crucial stepstoward eruption prediction and the assessment ofvolcanic hazards.
s01504.13.6.3 Miyake
p0380Miyake-jima is a basaltic stratovolcanic island in theIzu-Bonin arc, Japan, and has erupted quasiperiodi-cally with an approximate interval of 20 years; therecent activities took place in 1940, 1962, and 1983.The eruption in 2000 was expected in this sense;however, what actually happened was totally unex-pected: a new caldera was formed for the first time inthe past 2500 years (Figure 24). This caldera-forming process was recorded with contemporarygeophysical equipments.
p0385Figure 25 shows the seismicity observed duringthe 2000 Miyake-jima activity. The earthquakeswarm started right beneath the volcano, andmigrated to the southwest ((
b0885
Uhira et al., 2005), notwell-resolved in Figure 25), which may correspondto the subsurface migration of magma beneath thevolcano. The swarm activity next shifted to north-west of the island, indicating a major subsurfacemagma migration from the volcano to the outsidedike system. After a small summit eruption, thecaldera formation took place, and a number of low-frequency earthquakes are observed beneath thesummit area. It appears that sucking of magma fromthe magma storage system beneath the volcano to thenorthwest dike system, and the resulting magmavacancy in the volcano, are the direct causes of thecaldera formation (
b0300
Furuya et al., 2003).p0390During roughly 40 days of the caldera formation, a
number of peculiar bell-shaped long-period (50 s) seis-mic (velocity) signals, as well as steps in tiltmeters, areobserved once or twice a day (Figure 26). Based on
50 km
20°
(a)
(b)
19°
156° 155°
0 20 40 6060
40
20
0
Distance (km)
Dep
th (
km)
MAUNA LOA KILAUEAF (NW) F′ (SE)
f0110 Figure 22 Seismicity of Hawaii. (a) Hypocenters ofshallow earthquakes during the period 1970–84. (b) Cross
section through Kilauea and Mauna Loa for earthquakes
during 1970–83. The area of the cross section is that
delimited by the box in the top panel. Adapted fromKlein FW and Koyanagi RY (1989) The seismicity and
tectonics of Hawaii. In: The Geology of North America,
Vol. N, The Eastern Pacific Ocean and Hawaii, pp. 238–252.Geological Society of America.
TOGP 00073
Volcano Seismology 21
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the moment tensor inversion of the near-field and far-field broadband seismic records,
b0585
Kumagai et al., (2001)associated these long-period velocity pulses with infla-tions of a nearly vertical crack-like magma chamberdue to its pressure increase as response to therepetitive stick-slip-type falling of a piston-like masswithin the conduit, that results in the caldera forma-tion at the summit. Analyzing the signals of the samesequence of events recorded by the tiltmeters andbroadband seismometers deployed in the island,b0295
Fujita et al. (2004) arrived at a different conclusion.
They suggest that these events are caused by a cyclicexpansion of a subsurface sill-like magma plumbingsystem, and introduce a model based on a two-phaseflow instability, called a pressure drop oscillation asthe mechanism for the repeated activity. Eitherproposed model seems to explain a part of the obser-vations, but not all. Although the available data maynot be able to eventually resolve the difference, never-theless the 2000 Miyake-jima activity has provided usa rare opportunity to observe a caldera-forming eventwith contemporary geophysical instruments.
μmμr
adμr
adD
ispl
acem
ent (
μm)
East–west tilt at Uwekahuna
0 8 16 24 32 40 48Hours
HVO Vertical displacement
13121110Hours
987
0
Minutes
HVO Vertical15
0
0
GU9 Vertical
100
0
10
20
20
0
East–west tilt at Uwekahuna
15
–100
–15
–15
0 5 10 15
10
f0115 Figure 23 Tilt and broadband record observed around the summit caldera of Kilauea. Adapted from Ohminato T, Chouet B,Dawson P, and Kedar S (1998) Waveform inversion of very long period impulsive signals associated with magmatic injection
beneath Kilauea Volcano, Hawaii. Journal of Geophysical Research 103: 23839–23862.
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s01554.13.6.4 Stromboli
p0395Stromboli is one of the Aeolian islands of Italy, and has
been almost continuously in eruption for more than at
least 2000 years. In contrast to the widely used term
‘Strombolian eruption’ that is undiscerningly used to
describe a variety of volcanic eruptions, the activity of
Stromboli is characterized by short-term explosive
bursts of lava ejected into the air. Since they eject
relatively viscous basaltic lava, building up of the gas
pressure is required to fragment the magma, and
results in intermittent episodic explosions.p0400From the seismic point of view, such activities
generate a considerable number of explosion quakes,
persistent volcanic tremors, and long-period signals.
One of the most characteristic features of seismic
activities at Stromboli is the extremely wide fre-
quency contents of these signals (Figure 27).
Spindle-shaped signals superimposed on background
continuous tremor are generated by summit erup-
tions, and the nature of these short-period signals has
been extensively studied by many researchers (e.g.,b0220
Del Pezzo et al., 1974;b0215
Del Pezzo, 1992;b0760
Ripepe and
Gordeev, 1999;b0800
Saccorotti and Del Pezzo, 2000).
These studies shed new light on the seismic activity
of the volcano, which is mainly characterized by a
very shallow seismicity. Such very shallow origin of
the seismic signals is also confirmed by the analyses
of seismic array observations (e.g.,b0180
Chouet et al., 1997;b0800
Saccorotti and Del Pezzo, 2000), and the sources of
explosion quakes and tremors are located at depths
shallower than 200 m beneath the summit crater.
July 6, 2000 Aug. 30, 2000
f0120 Figure 24 Airborne SAR images of Miyake-jima Volcanobefore and after the 2000 caldera formation. Adapted from
Kumagai H, Ohminato T, and Nakano M, et al. (2001) Very-
long-period seismic signals and caldera formation at Miyakeisland, Japan. Science 293: 687–690.
0 5 km
34.4
34.3
34.2
34.1
34.0139.4 139.6139.2
f0125 Figure 25 The seismicity of the 2000 Miyake-jima activity.
Adapted from Sakai S, Yamada T, and Ide S, et al. (2001)
Magma migration from the point of view of seismic activity in thevolcanism of Miyake-jima island in 2000. Journal of Geography
110: 145–155.
12 Jul. 00:37
14 Jul. 02:12
25 Jul. 04:55
02 Aug. 15:15
50 s
f0130 Figure 26 Vertical-component near-field velocity
waveforms observed in the Miyake-jima island. Adaptedfrom Kumagai H, Ohminato T, and Nakano M, et al. (2001)
Very-long-period seismic signals and caldera formation at
Miyake island, Japan. Science 293: 687–690.
0 10 20 30 40 50 60 70 80
0 10 20 30 40 50 60 70 80
0 10 20 30 40 50 60 70 80
0 10 20 30 40 50 60 70 80
Thermal
Acoustic
rad
(o F)
prs
(Pa)
u/d
(10–5
m)
u/d
(10–4
m s
–1)
Seismic VLP
Seismic
20
40
60
–200
20
–2
0
2
–1
0
1
Time (s)
f0135Figure 27 Example of the synchronous occurrence ofseismic, acoustic, and thermal signals observed in
Stromboli. From Ripepe et al., unpublished data.
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Volcano Seismology 23
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p0405 Recent observations using broadband seismo-meters reveal the existence of VLP signals with aperiod of about 10 s which are accompanied withexplosions, and give additional constraints on theunderstanding of Strombolian activities (e.g.,b0685
Neuberg et al., 1994;b0160
Chouet et al., 2003). Analyses ofbroadband seismograms using the independentcomponent analysis method indicate that furtherlonger-period signals (30–40 s) are generated atalmost the same region as the VLP signals (
b0210
DeMartino et al., 2005).
p0410 Based on various field observations and laboratoryexperiments,
b0760
Ripepe and Gordeev (1999) andb0755
Ripepeet al. (2001) arrived at a model of Strombolian explo-sions that is a reminiscent of the analog experimentalmodel by
b0440
Jaupart and Vergniolle (1989), in whichdissolved gas bubbles are trapped at the roof of amagma reservoir as a foam layer and the periodicalcollapse of the layer causes ascent of gas slugsthrough a vertical conduit connecting the reservoirand the surface vent. The growth, flow, and burst ofthe coalescent gas bubbles observed in their labora-tory experiments well capture the characteristics ofsequential occurrence of very-long-period/short-period seismic signals, acoustic signals, and ther-mal/light emissions observed at Stromboli.Figure 28 shows such a direct link between gasf lux, magma volume flux, and seismicity, where thebalance between gas/magma flux and gas overpres-surization appears to determine the type of activities(i.e., effusive or explosive). The common trend in theVLP event rate and the SO2 emission rate during theeffusive phase suggests that the rate of gas f lux con-trols the frequency of foam coalescence and thedevelopment of gas slugs which generate VLP sig-nals; the decrease in SO2 and VLP rates coincideswith the increase in the explosion rate at the summitmeasured by the count of thermal transients and inthe amplitude of acoustic pressure; this may beunderstood in terms of the reduction of gas/magmasupplies that results in sealing of eruptive fracturesand the upward migration of magma in the conduit.
p0415 Performing detailed analyses of the broadbandnetwork data,
b0160
Chouet et al. (2003) concluded thatthe source mechanism of VLP events associatedwith Strombolian explosions consists of both momenttensor and single force components, which respec-tively correspond to the opening/closing of aninclined crack just beneath the active vents and themagma flow in the conduit caused by the piston-likerise of a gas slug. To investigate the behavior ofcoalescence and ascent of gas slugs in such an
inclined conduit,b0430
James et al. (2004) carried outlaboratory experiments of the two-phase f low invertical and inclined pipes, and demonstrated thatthe inclination of the conduit may play an importantrole in controlling the f low types and size/velocitydistributions of gas slugs in the f luid-filled conduit.
p0420In spite of the difference between models indetails, these results suggest that understanding ofthe bubble dynamics may be essential to elucidatethe source process of the seismic signals and thedynamics of Strombolian activities.
s01604.13.7 Other Volcano-Specific Issues
s01654.13.7.1 Explosion Quakes
p0425Explosion quakes are observed when explosive erup-tions occur (e.g., Figures 3 and 4). An explosivevolcanic eruption (vulcanian type) can be macrosco-pically understood as the def lation process of areservoir system beneath a volcano which may beobserved by geodetic (e.g.,
b0420
Ishihara, 1990) or broad-band seismic (e.g., Figure 6) means. The sourcemechanism of the accompanied explosion quakemay be represented by a combination of a singleforce and an implosive moment tensor (Figure 11).Since the pioneering work of
b0495
Kanamori et al. (1984),there are now a number of observations to quantifythe force mechanism and size of volcanic explosions(e.g.,
b0705
Nishimura et al., 1995;b0155
Chouet et al., 2005;b0740
Ohminato et al., 2006).p0430Explosion quakes are extensively studied at
Sakurajima Volcano, Japan; comparing seismic andvideo records,
b0415
Ishihara (1985) demonstrated that explo-sion quakes occur at a depth of 1–2 km beneath theactive crater about a few seconds prior to the explosiveeruptions at the summit which generate air shockwaves.
b0895
Uhira and Takeo (1994) showed that an explo-sive and implosive cylindrical moment tensorcomponent at the source depth is the dominatingsource mechanism for the explosion quakes. By care-fully analyzing borehole seismic network records,b0875
Tameguri et al. (2002) were able to decompose explo-sion quakes into two processes: the first one is anisotropic expansion followed by a contraction of acylinder at a depth of 2 km, and the second is anisotropic expansion and subsequent horizontal contrac-tion at depths of 0.25–0.5 km beneath the crater bottom,which occur about 1 s after the onset of the first andcoincide with the generation of the shock wave. Theplausibility of this second process was numericallydemonstrated by
b0700
Nishimura and Chouet (2003), who
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24 Volcano Seismology
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considered the behavior of a reservoir–conduit systemembedded in a homogeneous crust which is filled witha compressive fluid; it was shown that an abrupt pres-sure increase occurs just beneath the vent which cangenerate pulse-like Rayleigh waves as observed byb0875
Tameguri et al. (2002), and that the strength of the lidplayed an important role in defining eruption types.
p0435Scaling law of volcanic explosions.b0695
Nishimura(1998) showed that the far-field displacement due toan explosion is proportional to the area (square ofradius) of the crater vent, and obtained a scaling lawof volcanic explosion quakes, as similar to that ofearthquakes. Figure 29 compares the vent radii andestimated seismic magnitudes of the largest event for
VLPevent rate
Effusive
Explosive
SO2emission rate
Explosion rate(Thermal transients)
Tran
sitio
n
Infrasonicpressure
Thermo-acoustictime delayN
orm
aliz
ed c
umul
ativ
e tr
end
Jan 03 Feb 03 Mar 03 Apr 03 May 03 Jun 03 Jul 03 Aug 03 Sep 03 Oct 03 Nov 03
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(a) During eruption
(b) After eruption
NE SW
NE SW
Lava shield
Lava shield
High gas flux
Low gas flux
800 m
600–650 m
800 m
600–650 m
Dike
Dike
VLPsource
VLPsource
f0140 Figure 28 Example of the relation between gas flux rate and seismic/acoustic activities. During the lava effusion phase (model
A), few explosions at the summit were observed while the VLP seismicity well correlated with the SO2 emission rate. On the other
hand, in the explosive phase (model B), magma rose up in the conduit due to the sealing of eruptive fractures, resulting in the gasoverpressurization and the explosive activity at the summit craters. Adapted from Ripepe M, Marchetti E, and Ulivieri G, et al.
(2005) Effusive to explosive transition during the 2003 eruption of Stromboli Volcano. Geology 33: 341–344.
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Volcano Seismology 25
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Fstudied volcanoes by various investigators; it showsthat the magnitude is essentially proportional to thecross-sectional area of the vent for vent radii rangingfrom 10 to 600 m, as predicted by the theory, and thatthe initial excess pressure in the reservoir is of theorder of a few magapascals, a similar value as the stressdrops of earthquakes. This suggests that the size ofexplosion quakes may be estimated by knowing thevent radius, although it does not necessarily imply thatit is possible to predict the size of future eruptions, as itis not guaranteed that the size of present vent is thesame for the future ones.
s0175 4.13.7.2 Triggered Seismicity
p0440 It has been well known among local observers thatvolcanoes are ‘sensitive’, as seismicity there oftenshows changes apparently associated with externaldisturbances (tides or large earthquakes). Forexample,
b0815
Sassa (1936) documented diurnal andsemi-diurnal periodicities in the occurrence of vol-canic seismicity of Aso Volcano.
b0615
McNutt and Beavan(1981) also suggested a tidal correlation of seismicityat Pavlof Volcano (cf.
b0250
Emter, 1997).p0445 There are also convincing lines of evidence that
the passage of large amplitude seismic waves dyna-mically trigger seismicity, especially at volcanoes.b0385
Hill et al. (1993) documented the seismicity increasein Long Valley caldera, USA, following the Mw¼ 7.3
1992 Landers earthquake, California (alsob0330
Gomberget al., 2001;
b0750
Prejean et al., 2004).b0110
Brodsky and Prejean(2005) made a systematic survey of remotely trig-gered seismicity at Long Valley; showing theevidence for a frequency-dependent triggeringthreshold, they concluded that long-period (>30 s)waves are more effective at generating local seismi-city than short-period waves of comparablemagnitude; among the several proposed mechanismsfor such remote dynamic triggering, they suggestedthat the mechanism that invokes f luid f low through aporous medium can produce such a frequencydependence of triggering threshold (�5 kPa). Thismechanism also seems to explain why volcanoes aresensitive to external disturbances, as unclogging frac-tures to drive f luid f low (
b0115
Brodsky et al., 2003) may beeasily initiated in volcanic/geothermal areas wherethe constant precipitation from volcanic f luids gen-erate fragile blockages in fractures and faults.
b0930
Westet al. (2005) observed periodically (20–30 s) triggeredseismicity at Mt. Wrangell, Alaska, when long-periodRayleigh waves generated by the Mw¼ 9.0 2004Sumatra earthquake swept across Alaska, and con-cluded that the extensional stresses of�25 kPa due tothe Rayleigh waves triggered shear failure on normalfaults, which may be also inf luenced by the afore-mentioned f luid pumping mechanism. Theseobservations suggest that the presence of amplef luid in volcanic systems makes them ideal experi-mental field for elucidating the earthquake-triggering mechanism.
s01804.13.8 Concluding Remarks
p0450One of the ultimate goals of volcano seismology is toextract quantitative information about the volcanicfluid transport systems beneath active volcanoes andto utilize such information for forecasting short- andlong-term eruptive behaviors. As described in thischapter, macroscopic behaviors of dynamic interactionsbetween volcanic fluids and volcanic edifices have beenextensively studied in volcano seismology, and signifi-cant progress has been made; the observational/numerical/experimental studies have established firmfoundations for explaining the macroscopic processesresponsible for generating observed volcanic seismicsignals and other related volcanic activities.Considering the rapid progress in technologies for thefield observations and numerical simulations, the futureof these lines of studies seems to be promising; theaccumulation of high-quality digital data at different
8
2
3
4
5
6
7
9
101
11
Radius (m)
Sei
smic
mag
nitu
de
–2
–1
0
1
2
3
4
5
6
100 101 102 103
.1 MPa
1 MPa
10 MPa
f0145 Figure 29 Relation between the seismic magnitude of
eruption quakes and the observed vent radius. Numbersfrom 1 to 11 indicate representative points of different
volcanos. Lines show theoretical relation for assumed initial
excess pressures. Adapted from Nishimura T (1998) Sourcemechanisms of volcanic explosion earthquakes: Single
force and implosive sources. Journal of Volcanology and
Geothermal Research 86: 97–106.
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volcanoes around the world will further bring significantadvances in volcano seismology, as well as volcanologyin general; the increase in computer power remarkablyextends our capability to simulate complicated dynamicinteractions between phases, and enables us to identifyand separate distinct signatures of volcanic seismicsignals; the recent advance in high-sampling, high-precision GPS and InSAR observations may also helpour understanding from the observational aspect.
p0455 Nevertheless, our understanding of the elemen-tary physics of multiphase systems, such as thedynamics of bubbles in magmatic f luids and theirf lows, is still immature, although such understandingfused with the established studies are indispensablefor the elucidation of the whole volcanic f luid sys-tems. For instance, recently
b0435
James et al. (2006)experimentally demonstrated that the geometry ofthe conduit is the key parameter governing the f lowpattern of ascending slugs in viscous magma whichact as a trigger/source of volcanic seismic signals.Such a parameter of the conduit system may beretrieved by acoustic measurements as demonstratedby
b0305
Garces et al. (2000), and the integration of thesestudies will provide a great feedback to volcano seis-mology. Such studies on the modeling of physicalprocesses, in cooperation with multiparameter obser-vations and numerical simulations, may bring a newboost to volcano seismology in the near future.Volcano seismology, thus, remains to be one of themost challenging fields in Earth science as an inte-grated science of multidisciplinary researches onmultiscale/multiphase systems.
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Relevant Website
http://wwweic.eri.u-tokyo.ac.jp – Grid-Based Real-Time
Determination of Moment Tensors (GRiD MT),
Earthquake Information Center.
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