:1. . f Geophysics

I P J Corporation

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SEISMICITY REPORT

ON THE PAHOA ·PROSPECT

HAWAII COUNTY

HAWAII

�1\1«4 1-f •t'AuA

MICRO GEOPHYSICS CORPORATION

607 Tenth Street, Golden, Co 80401 303/279-0226

� .=.. � �r

SEISMICITY REPORT ON PAHOA PROSPECT

HAW AI I COUNTY

HAWAII

ABSTRACT

For the purpose O·f evaluating the geo�hermal potential of

a prospect Southeast of Pahoa, Puna District, Hawaii, a .hi.gh

gain (50-500K) , high-frequency (5�30hz) seismic array with a

detection threshold below magnitude 0.5 was operated for thirteen

days between December 27, 1974 and January 9, 1975. Forty-two

local microearthquakes were recorded, nineteen of which were

locatable.

The majority of located events were placed seaward of the

East Rift of Kilauea Volcano near previously recorded seismic

activity. The level of seismicity .recorded here. agrees with the

seismicity reported by the Hawaiian Volcano Observatory. The

location of these events and first motion studies suggest a

seaward dipping normal fault along the East Rift. A Poisson's

ratio anomaly appears to ·be associated with the active area. The

data supports the contention that this area has a high potential

for production of geothermal steam.L

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INTRODUCTION

LOCATION MAP

AoAHU . �w r:::::::;,�AUI

HONOW

·' . . . . 0 :::.: . .

SCALE

0 100

KILOMETERS

G) 0

LONG 156W

LAT 20°N

FIGURE 1

INDEX MAP

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PACIFIC OCEAN

;;WAll

z I

: �,._,·"' I ,,.3d ;-r-

SCALE

10 20

KILOMETERS

FIGURE 2

A geothermal prospect of approx­

imately 375km2 (150 mi2) extent in the

District of Puna on the Island of

Hawaii (Figures 1 and 2) was surveyed

for microearthquakes. Microeart;hquake

activity is felt by many to be a nee-

essary but not sufficient ingredient of

a commercial geothermal occurrence

(Lange and Westphal, 1969; Hamilton and

Muffler, 1972; Ward, 1972; Hamilton,

et al 1973). The area of this survey

is part of the East Rift system of

Kilauea Volcano. Recent volcanic

eruptions have occurred within the

boundaries of the survey in 1955 and

1960 and the entire region is composed

of Quaternary volcanic flows and ash.

In a region of current vulcanism,

such as the Pahoa prospect, the existence

of a concentrated region of microearthquake

activity is evidence for the existence of

permeable channels·which provide excel-

lent targets for possible geothermal

exploitation.

The Hawaiian Volcano Observatory

(HVO) maintaips a dense network of

permanent seismograph stations as part

of their program of monitoring volcanic

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activity. This network records $everal

thousand events per year. They current-

ly report events generating felt reports

(approximately m>3.5) to the World Wide

Standard Seismograph network for inclus­

ion in the NOAA hypocent�r data file.

(Elliott Endo, HVO, personal com:tnunica-

tion). Three hundred eighty three events

HIS'I'ORI_CAL SEISMICITY

HAWA'II

I ...

\I . • • ' •

.. .. . . .

. . �· . . �- ·'

... ��� ·. .

.:·\.... .:. .

. ·.

: . . .:. I •· "• • ••••• f. "•( • • �: �> ; -:""��;-)_. ·. :.�.�, . :·.

,.-e:·PICE N TER

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within 50km of the survey origin, listed 0 tO ICM

in the NOAA hypocenter data .file, are

shown on the historical seismicity map in

Figure 3. The coverage is fair for the

I period 1935 to 1960 and good from 1960 to

I present. The staff of the HVO have

regularly, since 1962, written U.S.G.S.

. ...

..

.

-----�:__RADIUS a

FIGURE 3

Professional papers titled "Hawaiian Seismic Events of 19 __ II

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I Epic�nter maps of earthquakes with magnitude greater than 2.0 are

given. These reports have shown approximately 10 events per year

I near the location of activity recorded in this survey. Occasional

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swarms have also been recorded in this area. For example, five

hundred ninety six events were recorded "near Pahoa" from August 27

to September 25, 1964 (Koyanagi and Okamuri, 1966).

The object of this survey was twofold, first to verify and

improve the location of the active area reported by the HVO and

second to maintain dense station coverage with enough azimuthal

control to develop a meaningful fault plane solution. The field

work was conducted according to the following schedule:

/

Dec 27 - Field party arrived in Hilo� installed 5 stations.

Dec 28 - Installation complete1 recording begun.

Dec 29 - Client inspection of seismograph stations.

Jan 10 - Recording discontinued.

Jan 11 - Field party and equipment arrived in Golden.

INSTRUMENT RESPONSE u

� z u '-4 2:10 0 .

:z <! (.9 >-� 3 U10 9 w >

1 --'-

0.1

1d 2 u " 2 6 u10

z

<! (9 � z 5 �10 w

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0 Qj

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� -j

-,

at 96 db

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v I I I 11 �1NFILTERE91( �

+ �\·. � .

+-� .. '

I =: I FILTERED HI =30hz-r-- -

LOW= 5hz I I

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VEL9C IT( RESPONSE1 I I _l_ _l I

1.0 10 100 FREQUENCY- HERTZ

lll � I i � ·

��ir--UNFILTERE

U� J

yJ2:rFILTERED Hl·30;:-I ' . LOW• 5hz-

-DISPLACEMENT

RESPONSE -

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1----

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10 10 100 FREQUENCY-HERTZ

FIGURE 4

1�

3 10

107

106

105

---

INSTRUMENTATION AND OPERATIONAL SUMMARY

Eight Sprengnether Instrument Co. MEQ-800-B portable_seismic

systems were used for this survey. Each system consists of a

Mark Products model LC-4, 1-hz natural.-frequency vertical seismo-

meter, gain-stable amplifer, in,tegral timing system, and smoked

paper recording with 0. OSnun sty.lus width and 120nun/min recording

speed. The frequency characteristics of the inst�ument are

summarized in Figure 4. G.ain changes are by 6db steps from the

arbitrarily assigned level of +96db plotted in the figure. +96db

is a typical gain in the western continental United States.

Clocks were synchronized daily with WWVH . Clock drifts

between synchronizations were below expected record reading errors,

therefore no corrections were necessary. Records were read to·

+0.03 sec for P arrivals and +0.10 sec for S-P times. Amplitudes,

peak-to-peak, were read to the nearest millimeter, and durations

to the nearest 0.5 sec.

Table 1 is a detailed operating

summary for the stations whose

locations are listed on Table 2 and

which are shown on the map in

Figure 5. All stations of this

survey were by necessity placed

on recent volcanic flow material.

STATION

HAWAII

LONG 155•

LAT 19°30'.tll + ORIGIN

LOCATIONS

I N

This material provides uniform, OCEAN

but noisy sites for seismograph

stations and is the major factor

limiting gains to 50-500 thousand.

0 10

KILOMETERS

FIGURE 5

20

TABLE 1

Opera tat i ng Sc-hedule of Seismographs

DEC. I 74 I JAN. '75 2 7 J 2 8 I 29 . I 3 o I 31 I 1 1· · 2 I 3 I 4 I 5 I 6 ! 7 I --l?) 1 9 1

•

I G 59

G 59 JG120 I G 59

1 G 240

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G 240 I

G 240 _.,.E��-L-'------- G240

G 240 G120 G240

1 G 240 1G1_�0 1(.;240, G 120

1 G 950 G480

G 24.0 1e:.120 1 G 240

I G :20

L10

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G GAIN/1000 AT 20 HERTZ L Lm'l-CUT FILTER SETTING (l:IZ) FILTERS 5-30 HZ UNLESS NOTE))

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TABLE 2

STATION LOCATIONS

St?ttion x/km :i/km z/km ·

1 18.77 +1.83 0.012

2 15.32 -0.48 0.050

3 8.36 -1.25 0.198

4 11.95 -3.42 0.134

5 15.14 -3.56 0.049

6 3.76 -9.04 0.287

7 8.23 -4.62 0.280

8 7.67 -7.44 0.122

9 8.61 +1.20 0.125

10 9.35 -2.03 0.207

11 5.52 -6.74 0.304

Coordinates of ,stations are in kilometers from 1550W, 19°30' West 1 + X is East 1 + Y is North, Z is distance above mean sea level.

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OBSERVATIONS AND ANALYSIS

Events were regarded as seismic in origin if they appeared

on one record with the characteristic signature of an earthquake.

Local events were those with S-P times of less than 4 seconds.

Several hundred regional events from Kilauea Volcano were recorded.

These events were identified as such by their S-P times arid no

lf) 1-z w > w lL 0 0::: w co L: ::) z

further analysis was done.

LOCAL EVENTS BY HOUR OF

OCCUR REN,CE

LOCAL MIONlC,HT

+

TIME- HOURS

FIGURE 6

10 LOCAL EVENTS BY DAY

l/) ....... z w > w

5 I.J.._ 0 a:: w CD 2 :::::> z

uc

C.� ��

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�!!!!i!!!i!i!!!f"·"· I �

.-;���r ·:���i�iif::::;�;�;!;�;�;�;�;�=� �::::::

27 28 2, 30 31 I , 2 3 • s • 7 s �

DEC '74 JAN '75

DATE

FIGURE 7

Figure 6 shows local events by

hour of occurrence. The figure shows

no unusual concentration of events

attributable to cultural activity,

however it does indicate that 10 to 15

events may have been missed during the

daytime due to high cultural-noise levels.

Figure 7 shows local events by

ciay of occurrence. The low number of

events recorded January 1 and 2 is

felt to be caused by high noise levels

from harmonic tremor and earthquakes

associated with the Deceit1ber 31 eruption

of Kilauea Volcano. There is no

evidence that the seismicity of this

area would be affected by a volcanic

eruption more than 25km distant.

Local events timed on fotir or

more stations were located using a

generalized inverse program. Details

of the precise velocity structure in

"bhe project area are not known, although

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refraction studies have been done on

the Island of Hawaii (Hill, 1969) �

Figure 8 illustrates two velocity

models based on this refraction

data. This .model is based on widely

0

2

VELOCITY MODEL THE EAST RIFT

THE ISLAND OF HAWAII ADAPTED FROM D.P. HILL

VELOCITY (KM/SEC)

I 2 4 6

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MODEL

8

spaced refradtion shots and thus

__ .;;.-; I ALTERNATIVE

._ __ - ·-

does not give any indication of

small scale velocity variations

which are presumably present in

this region.

Using the �ethod of Peters and

Crosson (1972) error prediction maps

(Figure 9) were prepared in the

region S<X<lO, -lO<Y<O, at depth =

4

� �

�6 a.. w 0

8

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FIGURE 8

2km, using a Skill/sec heilf space and assuming a timing error of

.030 sec and a velocity error of O.Skm/sec. These values where

chosen for computational ease and the values shown are relative.

The shape of these curves will be similar for any velocity model,

although the values will change. The actual uncertainties in the

region of the minimum are estimated to be +1 km in X and Y and + 2

km in z.

The locations of the nineteen

located events are shown in Figure

10, and Table 3. Magnitudes for the

located events were computed using

the equation: M=3.4 - log G+ log A

+ 2.4 log R, where;

G = 20 hz gain A = Maximum peak to peak trace

displacement R = hypocenter station distance

EPICENTER

ORIGIN o

EP1CENTE�

ST.TlON'

®o HAWAII

FIGURE 10

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Seismograph

jl II 10

.s

1.0

PREDICTED UNCERTAINTIES IN X

•

G •

•

•

PREDICTED UNCERTAINTIES IN Z

Error prediction Pahoa, Hawaii

coo'rdinates in km, time in seconds

FIGURE 9

0.5

1.0

PREDICTED UNC.ERTAINTIES IN Y

PREDICTED UNCERTAINTIES IN T

I

I TABLE 3

LOCATED EVENTS

I Station

I Number Date Time X y H M

1 362 0751 11.3 + 1.5 1.1 0.9

I 2 362 0752 11.9 - • 8 ' 1.1 0.4

3 363 1014 11.8 - .8 4.0 0.6

I 4 365 0624 15.2 - 3.5 3.4 1.1

I 5 365 0716 - 2.0 - 9.8 8.4 1.5

6 365 0740 11.5 - 6.2 5.2 0.8

I 7 365 1235 11.8 . - 1.5 1.0 - 0.1

8 003 724 1.4 - 6.5 4.5 0.8

I 9 003 1118 6.6 - 3.7 1.0 - 0.2

I 10 004 1355 13.8 - 1.5 1.3 - 0.3

11 004 2328 9.4 - 4.6 2.1 0.0

I 12 006 0331 3.1 - 8.7 5.2 1.5

13 006 0448 11.4 - 1. 5 1.0 0.3

I 14 006 0810 10.4 - 4.0 1.7 0.7

I 15 006 0937 11.9 - 6.5 , 7.3 0.8

16 006 1124 12.2 - 3.0 1.9 0.4

I 17 007 0340 10.2 - 4.9 .9 0.3

18 007 0422 19.4 2.6 1.0 1.4

I 19 007 1339 9.2 - 5.3 1.0 0.4

I Coordinates are in kilometers from 155°W, 19°301 North,

I + X is East, + Y is North, H is the depth below mean sea level. Time is UTC; date is Julian day.

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I 20

I Ul10 � 9 w > 6 w

II � 4 Ill 2 2

II �

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RECURRENCE CURVE

I 2's PER YEAR --T�----05 0.0 OS 1.0 1.5 20

MAGNITUDE

FIGURE 11

I FIRST

MOTION N

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0

----

0 0

0

•

• 0

d'. •

o Dilitation

• Compression

FIGURE 12

o.

Tbe magnitude was computed for all

arrivals and the average for each

event is reported in Table 3. A

cumulative recurrence curve for these

events is shown in Figure 11.

Figure 12 is a tipper-.hemisphere

composite fault plane solution. Sucb

a plot can be used to establish the

fault mechanism, the probable fault

plane, and the regional stress pattern

in the area. The first motion areas

are reasonably well separated by two

planes each with an East-West strike

and dips of 30° South and 60° North.

Many of tbe events recorded in this

survey showed very high S move-outs.

Figure 13 is the day, 006, 0331 UTC

e�ent illustrating this effect.

Note that there is another event

at 03.36 which is especially prominent

on record 4. When P times were plotted

against S-P times (a Wadati diagram)

these events did not form a straight

line (Figure 14). Model calculations

indicate that the most straight for-

ward means of explaining this data

is by a lateral variation in the body

wave velocity ratio. To check this

hypothesis, S-P times were plotted

MICROEARTOUAKE 03h 3f'DAY006UTC

STATION GAIN

6

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9 78db /487,000

7 72db /243,000 .

1 0 7 2db

4

/243, 000

7 2db /243,000

,...... .:.:-::::._ , .... ,11 . .

1 1 ...... --- ..

. �- ·iii:;; .. ,:";�_;;;_ .. :.:.,;;._. .. .,. _ _ II. 66db / .. � ...

I 122 000 - :_ - · -., �g;;hh J If.t!t·-' . • 'flfi"f -�---.-.:·,·t�.A .. �- ;..,...,.

FIGURE 13

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versus P times and the Wadati slope

taken from the line between the origin

time and data point for the individual

station. Two assumptions are,required

to justify this procedure: (1) that

the computer generated origin time is

accurate and, (2) the Poi$son's ratio

for the entire hypocenter-to-sta�ion

path is constant. While neither of

these assumptions are perfectly valid,

WADATI DIAGRAM

\5+----+--

� 10'+-----w � 1-a.

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(/)

1.0 15 P ARRIVAL TlME

(SEC)

FIGURE 14

20

it is felt that useful information may be developed from this technique.

The Poisson's ratios were plotted along the epicenter�to-station

rays and the resulting map was subjectively contoured as shown in

Figure 15. This figure shows a closed Poisson's ratio high approx-

POISSONS RATIO

� 35

" � .30 " ea

• 7

.9

"'--.21

.40

POISSON'S RATIO CONTOURS

FIGURE 15

Seismograph e2

..

t t

..._

0 ./fKM

«""�"'

<;__'?-� cF

imately surrounding.the active area. The uncertainty in each

determination is estimated to be about +0.05, bu� an area of

values clearly below 0.30 surrounds the closed high which

contained values as high as 0.44.

INTERPRETATION

The located events of this survey concentrate primarily

along and to the south of the Kilauea East Rift. Figure 16 shows

the epicenters and Poisson's ratio anomaly superimposed on the

topography. The composite fault plane solution is definitely

that of a normal fault and can not be inteq�reted as a thrust

or strike slip fault.

The hypocentral depth cont�ol is not precise enough to

predict a fault plane, but a clear trend of greater depth to

the south is present. Based on the observed hypocentral distrib­

ution, tbe southward dipping plane is determined as the fault

plane. It is also very difficult to envision a normal fault

located to the south of the rift system dipping into the island.

The interpretation of a seaward dipping normal fault is there­

fore given with considerable confidence.

Due to the strong velocity contrasts present in this area,

the uncertainty in the orientation of the fault plane is approx­

imately 20°. 'Based -on the- sharp easte-rn-bounda-ry of. -the epicenters.

it was anticipated· that a transverse strike slip fault might ex-ist

along the_ epicentral, boundary • cNo events were found whose .first

·motions cou-ld be interpreted as =agreeing with such a mechanism

and ·no adequate explanation has ·been found- for the- lack o.f:

seismici-ty along the eastern tip of the rift.

Interpretation of the Poisson's ratio high reported in this

survey is more speculative. Given the assumptions stated above,

a clear anomaly in velocity r(itio is present in this area. The

simplest explanation is that of a change in rock type, but no

subsurface geological information is present to confirm or deny

t'lj H (j) c::::

m ....... "'

aoo'

�35 ;t ,0;:�00 �30�¥· :�-� · 6 · 5r,� so 700 I

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�

�· - �

o o� -�� ��----

:fr 200�NTOUR I·NTER .

100 VAL=100'

(

- -

t �

- -

k(()

opicenter

1KM

�'f"-� cP <(��-0

- - -

this interpretation. It should be noted that 1:0e<:ks of high,

Po.ts·son's ratio. tend to ha:ve less. strength and to .have higher

attenuation than rocks with low Poisson's ratio.

Another interpretation is based on observations that large

scale fracturing of rock can increase the Poisson's ratio by 0.1

(Kumamoto, 1972; O'Connell, 1974). If the rock throughout this area

is relatively homogeneous and if a source of heat is present in the

area, an area of increased fracturing on a large scale, as defined by

this anomaly, would be a favorable target for geothermal exploitation.

It is interesting to note that.the present tendency in earth­

quake prediction measurements is to look for time-dependent, low

(down to 0.1) Poisson's ratio anomalies. There was no evidence

that this anomaly was time variant during the course of this survey.

In terms of the diffusion model presently accepted in the United

States, the Poisson's ratio low corresponds to an opening of micro­

fractures which are dry. As these fractures fill with fluid, the

Poisson's ratio increases, often to a value above its long-term

average, just before failure. Thus fluid filled fractures may have

a Poisson's ratio higher than the value expected for the undisturbed

rock, but this interpretation is dependent on acceptance of the

diffusion earthquake prediction model. A body of evidence does

exist (Walsh, Coombs, 1975) that indicates that vapor filled cracks

would cause a low Poisson • s ratio. Th_l::s_!�

_ig�

-�.9-

��-�on

_�_::�ti�

-�ay

be evidence.against the existence of steam-filled fissures at de:eth.

In s\immary, a velocity ratio anomaly is present in the area

outlineq in Figure 15 and a·possible interpretation of this anomaly

is that it is due to fluid filled fractures. If a source of heat is

present, such an area is a favorable target for geothermal exploration.

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CONCLUSIONS

1. The seismicity recorded by this survey is at g. high

level (2-4· magnitude zero events per day) and is consistent with

the historical �eismicity recorded in this area for over 10 years.

2. The events detected have a common gravity mechanism �nd

a similar geographic expression (East-West strike, probably 30--40°

South dip).

3. The high level of seismicity indicates that extensive

fracturing of the rock has taken place in this area and that good

permeability is probably developed along the fault systems. If

the existence of a heat source can be established by other geo­

physical methods, the area has a good potential for the production

of commercial earth steam.

RECOMMENDATIONS

1. The existence of an adequate heat source in the area is

necessary before an extensive drilling program is undertaken.

Other. geophysical methods, such as electrical resistivity, should

be applied to confirm or deny the presence of a heat source, and

thereby confirm or deny the existence of a drillable target.

2. A program of monitoring of the earthquake risk engendered

by the injection or wi.thdrawal of fluids is necessary i:f a geo­

thermal field is established. Such a risk evaluation program

should be appended to the program evaluating the risk of damage

to geothermal wells and power plants by the frequent lava flows

across the area •

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REFERENCES

Brune, J.N., Allen, C.R., 1967, Microearthquake: Survey of the San .Andreas Fault System in Southern California, B.S.S.A., vol.57, no.2,.pp.277�296.

Coffman, J., C.A. vonHake, 1973, Earthquake History of the u.s. ,

u. S. Department of Commerce, NOAA, Environment data service.

Hamilton, R.M., L.J.P. Muffler, 1972, Microearthquakes: Geysers Geothermal Area, Califo:t:nia, Journal of Geophysical Research, Vol.77, no.ll, pp.2081-2086.

Hill, D.P., 1969; Crustal Structure of the Island of Hawaii from Seismic-Refraction Measurements, BSSA, v.59, no.l, pp.lOl-130.

Kinoshita, W.T.,· 1963, Earthquake and Deformation in the Koae Fault Zone, Kilauea Volcano, Hawaii, USGS Prof. Paper, 575-C, pp.Cl73-176.

Kisslinger, C. arid Engdahl, E.R., 1973, The Interpretation of the Wadati Diagram with Relaxed Assumptions, BSSA, vol.63, no.5, pp.l723"-1736.

----,1974, Semyenov Prediction, Test of the Semyenov Prediction Technique in the Central Aleutian Islands, Tectonophysics, vol.23, pp.237-246.

Koyanagi, R.Y., 1964, Hawaiian seismic events during 1962, in Geological Survey Research. 1964; U.s. Geol. Survey Prof. Paper 475-D, p.D�l2-Dll7.

_ _: __ , 1968, Hawaiian seismic events during 1965, in Geolog.ical Survey Research 1968; U.S. Geol. Survey Prof. Paper 600-B, p.B95-B98.

----,1969a, Hawaiian seismic events during 1967, in Geological Survey Research 1969; u.s. Geol. Survey Prof. Paper 650-B, p.Bll3-Bll6.

----,1969b, Hawaiian seismic events during 1967, in Geological Survey Research 1969; u.s. Geol. Survey Prof. Paper 650-C, p.C79-C82.

----,1969c, Hawaiian seismic events during 1968, in Geological Survey Research 1969; u.s. Geol. Survey Prof. Paper 650-D, p.Dl68-Dl7L

Koyanagi, R.Y., and Endo, E.T., 1965, Hawaiian seismic events during 1963, in Geological Survey Research 1965; u.s. Geol. Survey Prof. Paper 525-B, p.Bl3-Bl6.

----,1971, Hawaiian Seismic events during 1969, USGS Prof. Paper 750�C, pp.Cl58-Cl64.

Koyanagi, R.Y., and Okamura, A.T., 1966, Hawaiian seismic events during 1964, in Geological Survey Research �966; U.s.· Geol. Survey Prof. Paper 550-C, p.Cl29-Cl32.

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Koyanagi, R.Y., Swanson, D.A., and Endo, E.T., 1972, Distribution of Earthquakes related to Mobility of the Soutb Flank of Kilauea Volcano, Hawaii, USGS �ref. Paper, 800-D, pp.D89-D97.

Kumamoto, L.H., 1973, Redundant Strain Observations in Southern. Nevada: Strain Steps from the Handely Nuclear test, unpublished MSC Thesis #Tl457, Colorado School of Mines, Golden, Colorado.

Lange, A.L., et.nd Westphal, W.H., 1969, Microearthquakes near the Geysers, Sonoma County, California, JGR, vol.74, pp.4377.

Malahoff, A. and McCoy, 1967, The Geologic Structure .of the Puna Submarine Ridge, Hawaii, JGR, vo1.72, no.2, pp.541-548.

O'Connell, R.J., and Budiansky, B., 1974, Seismic Velocities of Dry and Saturated Cracked Rocks and Precursory Velocity Changes, Ees, 1974.

Peters, D.C. and Crosson, R.S., 1973, Application of Prediction Analysis to Hypocenter Determination using a local array, BSSA, v.62, no.3, pp.775-788.

Sanford, A.R., and Singh, s., 1968, Microearthquake Activity, Minimum Reco.rding Time for Determining Short-term Seismicity Forms, BSSA, vol.SB, no.2, pp.639-644.

Ward, P.L., Microearthquakes: Prospecting Tool and Possible Hazard in the Development of Geothermal Resources, Geothermics, vol.l, no.l, pp.3-12 (1972).

Ward, P.L., and Bjornson, s., 1971, Microearthquakes, Swarms and Geothermal Areas of Iceland, Journal of Geophysical Resources, vol.76, no.l7, pp.3953-3982.

Ward, P.L. and Jacob, K.H., 1971, Microearthquakes: In the ·

Anuachapan Geothermal Field, El Salvador, Central America, Science, vol.l73, pp.328-330.

Ward, P.L. and Gregersen, 1973, Comparison of Earthquake Locations determined with data from a network of stations and small tripartite arrays on Kilauea Volcano, Hawaii, BSSA, vol.63, no.3, pp.679-711.