8/11/2019 Array Tracking of the Volcanic Tremor Source at Deception Island

1/4

GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 23, PAGES 3069-3072, DECEMBER 1, 1997

Array tracking of the volcanic remor source at Deception sland,Antarctica

J. Almendros , J.M. bfifiez '2,G. Alguacil '2E. Del Pezzo , and R. OrtizAbstract. We have ound experimental vidence which shows hatthe volcanic remor ecorded t Deception sland South ShetlandIslands, ntarctica) s a superposition n time of overlapping ybridevents. We studied ata rom a small aperture eismic rray. Dataanalysis or tremor nd hybrids ncluded: 1) spectral nalysis; 2)apparent lowness nd back-azimuth etermination y using hezero-lag cross-correlation ethod; nd (3) polarization nalysis.Both types of events hare hese ommon eatures: a) two domi-nant spectral ands t frequencies -3 Hz (the most energetic) nd4-8 Hz; (b) several oherent hases with the same back-azimuth othe source nd apparent lowness long he whole signal; c) in thehigh frequency and, he apparent lowness s very low (around0.17 s/km), ndicating he propagation f body waves; d) in the owfrequency and, he apparent lowness s high (around 1.6 s/km),consistent ith the presence f surface aves; nd e) clear P-waveonset ollowed by a complex attern f Rayleigh waves. Therefore,both types of events re strongly elated because hey share hesame ource egion, he same wave-propagation roperties, nd hesame wave composition. Moreover, several arrivals, hat resemblea single hybrid event, have been ound along he tremor signals.Due to these reasons, we hypothesize hat volcanic tremor ofDeception sland is a superposition f hybrid type events. Thesource of both ypes could be the interaction etween haw waterand hot materials n a shallow aquifer.

Introduction

Among the seismic signals generated y volcanoes, olcanictremor s the most characteristic nd complex. Several studies avebeen performed o understand he source mechanism and thecomposition f volcanic remor. Many models have been proposedto explain he volcanic remor sources elated o different kinds ofvolcanic activity (Aki et al., 1977; Julian, 1994; McNutt, 1986;Schick, 1992; and Chouet, 1996). Spectral nalysis hows hat thevolcanic tremor spectrum s mainly dominated by one or morepeaks at frequencies elow 3 Hz, independent f the type ofvolcanism. Many researchers ave ound hat hese peaks must bedue to a source ather than a path or site effects Fehler, 1983;Ferrazzini et al., 1991), considering hem as governed by thedimension of the plumbing system. However, other authors(Goldstein nd Chouet, 1994) suggested hat path effects ould bealso responsible or some of the observed spectral eatures.Although he majority of the studies ave shown hat the tremorwavefield s composed f surface waves McNutt, 1986; Gordeev

Instituto ndaluz e Geofisica, niversidad eGranada, pain.2Departamento e Fisica Te6rica y del Cosmos, niversidad e

Granada,Spain.3Dipartimento i Fisica, niversita i Salerno, taly.4Departamento eVolcanologia, useo acional eCiencias aturales

- CSIC, Madrid, Spain.

Copyright 997by he American eophysical nion.

Paper number 97GL03096.0094-8534/97/97GL-03096505.00

et al., 1990; Ferrazini et al., 1991), body waves have also beenshown o exist Goldstein nd Chouet, 994). Polarization nalyses(Seidl and Hellweg, 1991), waveform studies Fehler, 1983), andother echniques Chouet, 1996), have pointed out that volcanictremor consists f many simple overlapping avegroups adiatedby time-discrete ources.

The present tudy was performed t Deception sland 62 59' S,6041' W), an active olcano ocated t the spreading entre of theBransfield Strait back-arc on the southwestem side of the Scotia Sea

region, Antarctica Klepeis and Lawver, 1996). A small apertureseismic array was deployed n an almost lat zone close o theSpanish tation. t was composed f 15 Mark L25B seismometerswith extended esponse lat down o 1 Hz and 3 three-componentMark L4C seismometers ith extended esponse etween .1 and48 Hz, in a triangle-shaped onfiguration bout 00 m wide Figure1). Each channel perated ith GPS ime, with sampling requencyof 200 sps and with ocal digital ecording n a PC (Alguacil et al.,1996). This array was n operation uring wo summer ield surveys(one from December 1994 to March 1995 and another fromDecember 1995 to March 1996) in order to monitor the seismicactivity. Data records ontain a wide variety of seismic ignals,ranging rom ectonic arthquakes f intermediate epth Ibfifiez etal., 1997) and ocal earthquakes Vila et al., 1995) to long-periodevents, hybrid events following the classification f Lahr et al.,1994) with a high-frequency irst onset more pronounced han hefirst onset of the long-period events, and volcanic tremor. Adetailed tudy of the whole seismic ctivity s reported n Ibfifiez etal. (1996).

In this paper we study several pisodes f volcanic remor andhybrid events, hat occurred n a three-days ime interval. Threetechniques ere used: 1) spectral nalysis n order o determine hespectral eatures; 2) ray parameter stimations y using arraymethods; nd 3) polarization tudies o investigate he nature of thewave ield. We compare he results btained or the volcanic remorwith those obtained or the hybrid events, nd analyze he possiblerelationship etween he sources f both ypes of events.

Data analysis

We have nalyzed, mong he 92 events ecorded uring he 3-days period from January 9 to 21, 1996), 21 digital ecords fvolcanic remor and 20 hybrid events, elected n the basis of thebest ignal o noise atio. A thermal-paper rumcontinuous onitorstation which operated lose o the array site ensures hat thesetremor pisodes re he most energetic art of a continuous remorlasting from minutes o some hours. The recorded remors startabruptly enough o trigger he STA/LTA detection lgorithm,sometimes ith a clear onset, ut due to the limited duration of therecords 150 s), we have vailable nly ime imited samples f thewhole remor ignal. ybrid vents ave durations anging rom 40to 70 seconds, ith amplitudes n the seismogram imilar o thosefor volcanic remors. rray-averaged pectra nd spectrograms feach event were obtained y FFT spectra f 1024-point movingwindows shifted 512 points each step along he whole signal(Figure ). They eveal hat he maximum pectral eaks re n therange -3 Hz, and hat a significant mount f energy s also presentin the 4-8 Hz band. There s only one high-frequency eak n the

3069

8/11/2019 Array Tracking of the Volcanic Tremor Source at Deception Island

2/4

America: ' Scotia .,Sea -:- 'Port .Drakeassage .. Foster

'. Peninsula / ,. ' ': : .,

.......... :': a .StatiOn

....... ' :.0-200 400.: . .

Figure 1. Map of Deception sland howing ts ocation n the Scotia earegion, the array site close to the Spanish Station and the ar rayconfiguration, almost triangular, with a densely covered center. (,)Vertical Mark L25B stations. x) Three-component Mark L4C stations.

spectrogram f hybrids, ust at the beginning, hile or tremor hereare several sporadic igh-frequency ursts along he signal. Thelow-frequency eaks ollow in time the high-frequency nes, andare continuously resent n the spectrograms f both kind of events.The peaks at low frequency shift from one tremor episode oanother, as well as inside the same remor event. Moreover, thespectra of tectonic ocal events Ibfifiez et al., 1997) do not showpredominant eaks at the same requency and. Therefore, we areconfident hat hese ow frequency eaks re more related o sourceprocesses han site or path effects.

On the basis of the results rom spectral nalysis, we filtered heseismograms n two bands 1-3 and 4-8 Hz) using a 5-poleButterworth ilter. Then, we applied he zero-lag cross-correlationmethod Frankel et al., 1991) o the filtered seismograms n orderto determine he apparent lowness cross he array and he back-azimuth o the associated ource. We used a moving window 80points ong or the high requency and and 200 points ong or thelow frequency and) sliding n time one half window ength eachstep, and beginning t the pre-event eismic oise. For each stepand frequency band, the maximum averaged cross-correlation(hereafter, MACC) was calculated o obtain back-azimuth andapparent lowness or both ype of events. rror imits are discussedin Del Pezzo et al. (1997), who show wo main sources f error: onedue o the array geometry, ime sampling nd grid search, nd heother due to the seismic noise and the lack of coherence of the

signal across he array. A joint evaluation f both provides s anuncertainty orrelation evel hat s subtracted rom MACC, leadingto the determination f an error ellipse n the slowness lane. Theaverage alues of errors re 10 n back-azimuth nd 0.02 s/km and0.2 s/km respectively or high- and low-frequency n apparentslowness. or the pre-event eismic oise, he averaged MACCvalues re 0.15 (%) for the ow frequency and and 0.10 (c h ) forthe high requency and. We selected olutions ith MACC valuesgreater han hree imes % and ive times o,, respectively. n Figure3 we show histograms f all these elected olutions or both ypeof events nd frequency ands.

Two distinct anks of apparent lowness re observed or the wo

frequency bands. At low frequency, he calculated apparentslowness ange between and 2 s/km apparent elocities etween0.5 and 1 km/s) with a dominant value of 1.6 s/km. At highfrequency, e observe pparent lowness alues own o O. 12 s/km(velocities up to 8 km/s) with a dominant value of 0.17 s/km.

Apparent lowness alues t ow requency recompatible ith thepresence f surface aves n a low velocitymedium e.g. a volcanicenvironment), hile the higher apparent elocity at high frequencyindicates he presence f body waves. A similar esult was alsoobserved y Goldstein ndChouet 1994) n the shallow remor ofKilauea volcano Hawaii). Also two main directions or the back-azimuth o the source were obtained or the two frequency ands.We obtained most part of the solutions etween 80 and 220 clockwise rom north t ow frequency, ndbetween 20 and 140 ,

mainly around 130 , at high frequency. his diference n back-azimut s an open question hat have o be explained n furtherstudies. t can be easily nferred rom Figure that both remor ndhybrid vents hare he same ack-azimuth ndapparent lownessdistributions or both requency ands. igure 4 shows hat MACCvalues re also dependent n he window position long he signal.At low frequency, olutions with high MACC are uniformlydistributed long he signal. At high frequency, he pattern orhybrids s different rom hat or remor. While hese est orrelatedarrivals re ocated nly at the beginning f the signal or he hybridevent, he good olutions ppear uddenly everal imes long hesignal or he volcanic remor, oincident ith he arrival f high-frequency ave packets nd herefore ith high-frequency urstsin the spectrogram f the signal.

Furthermore, ow-MACC solutions or back-azimuth nd appa-rent slowness n the high frequency and are sparse n a broadrange overing 60 in azimuth nd 0.12-0.75 /km n apparentslowness 1-8 km/s n apparent elocity). his sparse is, ributionof low-MACC solutions s consistent ith the presence f wavepackets cattered yramdomly istributed eterogeneityDelPezzoet al., 1997).

To study hewave composition f tremor, olarization nalysiswas performed sing article motion lots, hecking he resultswith polarigrams nd products f vertical, adial and ransversecomponents see .g. Goldstein ndChouet, 994) nd polarizationfilters Montalbetti ndKanasevich, 970). he analysis eveals he

80 . Hybridvent' ' 60 ......... Volcanic remor

E ,.,.,, --- Seismicoise,., 40 -',.,,,.[ 20 /5 .2 4 6 8 10

frequency (Hz), , , ,. , ,

8

Volcanic tremor

20 40 100 120 140

10

, , Hybrid

2

60 80

time (s)

20

Shallowearthquake

20 40 40 60 80

time (s) time (s)

8O

120

140

,20lOO.. -:' 80 o-

Figure 2. (up) Array-averaged pectra f volcanic events and seis-mic noise. bottom) rray-averaged pectrograms f a volcanic re-mor, a hybrid event, and a shallow ocal ectonic arthquake epor-ted for a comparison. o peaks ppears n the ow-frequency andfor tectonic vents where hybrids and remors are mainly peaked.

8/11/2019 Array Tracking of the Volcanic Tremor Source at Deception Island

3/4

high requency low requency

4o

Hybrid

events 0

6oybrid4o

2o

lOO

I lOO 50rerno 50o

I'1'1'1'1'1 ' I '

0 1 2 3

back-azimuth distribution apparent slowness s/km)

Figure 3. Bar plots of the high-MACC solutions or the analyzedhybrids (top) and tremor (bottom). Left column: distribution f

back-azimuths rom the array center. Right column: istribution fapparent lowness cross he array site. The vertical axesof ightgraphs how he number f MACC solutions ver 3oi. or the owfrequency and and 50. for the high requency and see ext forexplanations).

Hybrid event11J' ' '

' 0 ' I , ,,',',,

-10000 20 40 60 0

time (s)

Tremor episodeI,I II I, , ,I,,,t,,I

20 40 60 80 100 120 140time (s)

o o o o o o o o o oD o o o 02.0 o.., o^. oo oo o_h; o-e-oo, 0o owo

o 4 ooo o o o oae0 .....' o ' ' '

o o o o o o27o. $ .... o180

go o o O Oo Zo oOo

0 20 40 0 0 20 40 0 80 100 120 140time s) time

0.8 ..........o_o o o Oo o oooo o o o

o

go oo o . .. .... m ,

ae0 e 8ooog& - ' - [o' o'o270 oo o 8 oooo o o o.o.oeo

:, ., o o

oo o0 20 40 60 0 20 40 60 80 100 120 140

time s) time s)

Figure . Whole nfiltered eismogramtop), nd plots f appa-rent slowness nd back-azimuth lockwise rom North versus imein he 1-3Hz frequency and center) nd n the 4-8Hz frequencyband bottom) f a hybrid vent left column) nda volcanic remor(right column). Time and amplitude cales re the same. Smallcircles re plotted or MACC below %. 5o.), and illed circlesfor MACCover o L 5o.), where %. o.) is the averaged ACCfor the pre-event oise t the ow (high) requency and.

presence f a complex attern of surface waves at low frequency,mainly composed of Rayleigh waves. At high frequency hecorrelated rrivals are composed y highly inear, almost adiallypolarized wave packets with durations rom 0.5 to 2.5 seconds,possibly P-waves. Clear shear wave phases were not observedinside the best correlated intervals. The above results are valid for

both remor nd hybrid events. n Figure 5 we show an example fthe high frequency article motion pattern or a volcanic remorsample and for a hybrid event. For both we show he first onset,

because his is the only portion of the seismogram n which thewaves re ess erturbed y other wave arrivals. t can be observedfrom the plots hat he ground motion s strongly olarized longthe radial component, with little contribution of SV and SHcomponent. his esult ndicates he possible resence f a P-wavein both cases.

Conclusions and Discussion

The array analysis f volcanic remors t Deception sland howsthe presence f well correlated ave packets omposed f surfaceand body waves coming from the same back-azimuth. Thepolarization nalysis eveals hat the body waves are mostly P-

VOLCANIC TREMORSH

SV

0 0.:5 1.0 I .:5 2.0 2

$H

Time (sec)

RAD

RAD

HYBRID

'0 0 .:5 1.0 I .:5 2.0 2.:5

Time (sec)

$v

RAD '

RAD

Figure 5. Comparison f the particle motion of tremors with thatfor hybrids. We show he irst 2 seconds f the high-frequency,filtered, otated, hree-component eismograms, nd he corres-ponding article motion plots.

8/11/2019 Array Tracking of the Volcanic Tremor Source at Deception Island

4/4

3072 ALMENDROS ET AL.: ARRAY TRACKING OF THE VOLCANIC TREMOR SOURCE

waves, and that surface waves are mainly Rayleigh waves. Thecomparison etween ybrids nd remor suggests hat both kind ofevents share he same source egion and mechanism, nd weconclude hat tremor s composed y hybrid-type vents whichoccur n temporally ense warms. his conclusion ests n: a) thespectral imilarity of volcanic remor and hybrids; b) the similarpolarization attern; c) the same back-azimuth istribution t low

and high requency; d) the same pparent lowness alues or bothfrequency ands; e) the existence f several igh-frequency urstsin the remor pectrogram hat esemble he spectrogram f a singlehybrid event; nd f) volcanic remors how well-correlated igh-frequency ave packets long he seismogram ith the same back-azimuth and apparent lowness ound or hybrids.

Although the real position of the source s not determined ndepth and distance, he stability of the solutions eveals hat thesource ocation s the same during he analyzed warm. Everyspatial change n the position of the source would in fact bereflected y a change n the back-azimuth nd apparent lowness.Therefore, hybrids and volcanic tremor share he same sourceregion. At the present, here s not an active vent on DeceptionIsland, and therefore we cannot directly relate he source withtransport of magma and other luids n an active channel.

The existence f recent ruptions e.g. 1842, 1912, 1917, 1967,1969, 1970), he special weather onditions, he presence f manyglacials hat n summer rovide a huge quantity of thaw water, andthe existence f shallow nd confined ater-saturated ayers Martiand Baraldo, 990) make us o suppose hat he most ikely sourcemechanism involves the interaction between water and hot

materials. t should e pointed ut hat he high-frequency bservedin tremor seismograms ay be detected only close o the sourcebecause f the high seismic ttenuation f most volcanic areas Qas ow as 11, see e.g. Vila et al., 1995). Therefore, he source mustbe located very close o the array site. The average ack-azimuthdirection for high frequency solutions of the swarm source stoward the Crater Lake (see Figure 1). This back-azimuth ine

crosses very recent racture system about 2 km southward heCrater Lake, where a strombolian ruption ook place in 1842(Gonzfilez-Ferrfin, 995). We suppose hat this fracture system sthe most ikely source egion of the swarm. The possible resenceof high emperature aterials n the vicinity of this racture ystemcould ead o abrupt water-steam hase hanges roducing steppressure hat generates hybrid event. The tremor episodes ouldoccur when he phenomenon f water-steam hase hange eachesa critical stage.

Acknowledgements. This paper has been partially supported y theprojects NT94-0854-C02 nd ANT95-0994-C03, y Grupo de nvestigaci6nen Geofisica .A.4057 and by Italy GNV (Grupo Nazional di VulcanologiaCNR). We gratefully cknowledge r. Jos6 Morales and hree annonimousreviewers or their useful omments; panish rmy and Navy for providingus with logistic help during he antarctic urveys n the Gabriel de CastillaStation; and all the participants n the two surveys at Deception sland,specially o Mr. Rafael Abella and Dr. Manuel Berrocoso, or their help andcooperation.

References

Aki, K., M.Fehler, S.Das. Source mechanism of volcanic tremor: Fluid-driven rack models nd heir application o the 1963 Kilauea eruption,Journ. Volcan. Geotherm. Res., 2, 259-287, 1977.

Alguacil, G., R. Ortiz, J.C.Olmedillas, .Del Pezzo, J.M.Ibfifiez, Desarrollode arrays sismicos ara a monitorizaci6n e la actividad ect6nicavolcfinica n a Antfirtida, VI Spanish imposium nAntarctic Studies,Madrid, 1996.

Chouet, B., Long-period olcano seismicity: ts source nd use n eruptionforecasting, ature, 380, 309-316, 1996.

Del Pezzo, E., M.Larocca, J.M.Ibfifiez, Observations f high-frequencyscattered waves using dense arrays, Bull. Seism. Soc. Am., in press,1997.

Fehler, M., Observations of volcanic tremor at Mount St. Helens volcano,.Iourn. Geophys. Res., 88, 3476-3484, 1983.

Ferrazzini,V., K.Aki and B.Chouet, Characteristics of seismic wavescomposing awaiian olcanic remor and gas-piston vents observed ya near-source rray, dourn. Geophys. es., 96, 6199-6209, 1991.

Frankel, A., S.Hough, P.Friberg nd R.Busby, Observations f Loma Prietaaftershocks rom a dense array n Sunnyvale, California, Bull. Seisin.Soc. Am., 81, 1900-1922, 1991.

Goldstein, . and B.Chouet, Array measurements nd modelling of sourcesof shallow olcanic remor t Kilauea volcano, Hawaii, dourn. Geophys.Res., 99, 2637-2652, 1994.

Gonzfilez-Ferrfin, ., Volcanes de Chile, 640 pp., Instituto GeogrfificoMilitar, Santiago de Chile, 1995.

Gordeev, E.I., V.A.Saltykov, V.I.Sinitsin and V.N.Chebrov, Temporal andspatial characteristics f volcanic remor wave fields, riourn. Volcan.Geotherm. Res., 40, 89-101, 1990.

Ibfifiez, J.M., E.Del Pezzo, R. Ortiz, J.Morales, G.Alguacil, J.Almendros,A.Garcia, F.Vidal, Dos aftos de monitorizaci6n de la actividad sismicade a Isla Decepci6n usando rrays sismicos, VI Spanish Symposium nAntarctic Studies, Madrid, 1996.

Ibfifiez, J.M., J.Morales, G.Alguacil, J.Almendros, R.Ortiz, E.Del Pezzo,Intermediate-focus arthquakes nder South Shetland slands, ntarctica,Geophys. es. Lett., 24, 531-534, 1997.

Julian, B.R., Volcanic tremor: nonlinear excitation by fluid flow, riourn.Geophys. es., 99, 11859-11877, 1994.

Kepleis, K.A., L.A.Lawver, Tectonics of the Antarctic-Scotia plateboundary ear Elephant and Clarence slands, West Antarctica, iourn.Geophys. es., 101, 20211-20231, 1996.

Lahr, J.C., B.A.Chouet, C.D.Stephens, .A.Power, R.A.Page, Earthquakeclassification, ocation, and error analysis n a volcanic environment:implications or the magmatic system of the 1989-1990 eruptions tRedoubt volcano, Alaska, riourn. Volcan. Geotherm. Res., 62, 137-151,1994.

Marti, J., A.Baraldo, Pre-caldera yroclastic eposits f Deception sland(South Shetland slands), Antarctic Science, , 345-352, 1990.

McNutt, S.R., Observations nd analysis f B-type earthquakes, xplosionsand volcanic tremor at Pavlov volcano, Alaska, Bull. Seism. Soc. Am.,76, 153-175, 1986.

Montalbetti, .F., E.R.Kanasevich, nhancement f teleseismic ody phaseswith a polarization ilter, Geophys. . R. Astr. Soc., 21, 119-129, 1970.

Schick, R., Volcanic tremor: seismic signals of (almost) unknown origin,in Volcanic Seisinology IAVCEI Proceedings n Volcanology 3),P.Gasparini, . Scarpa, K.Aki, 157-167, Springer-Verlag, erlin, 1992.

Seidl, D., M.Hellweg, Volcanic remor ecordings: olarization nalysis, nVolcanic remor and magma low (Scientific Series of the nternationalBureau 4), R. Schick, R. Mugiono, 31-47, Forschungszentrum lichGmbH (KFA), 1991.

Vila, J., A.M. Correig, J. Marti, Attenuation and source parameters tDeception Island (South Shetland slands, Antarctica), Pur. Appl.Geophys., 44, 229-250, 1995.

J.Almendros, .M.Ibfifiez nd G.Alguacil. nstituto ndaluz de Geofisica,Universidad e Granada. Campus e Cartuja, s/n. 18071 Granada, pain.(viking@platn'ugr'es' banez@gea.ugr.es, lguacil@gea.ugr.es,

E.Del Pezzo. Dipartimento di Fisica, Universita di Salerno, 84081Baronissi, alerno, taly. (delpezzo@axpgeo.csied.unisa.it).

R.Ortiz. Museo Nacional de Ciencias Naturales CSIC, Departamentode Volcanologia, Jos6 Guti6rrez Abascal 2, 28006 Madrid, Spain.(mcnor72@pinar csic.es).

(Received: December 2, 1996; evised: une 20, 1997;accepted: ctober 3, 1997)