Seismic Monitoring of the Olkaria Geothermal Area, Kenya Rift

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Ž .Journal of Volcanology and Geothermal Research 95 2000 197–208

www.elsevier.comrlocater jvolgeores

Seismic monitoring of the Olkaria Geothermal area, Kenya Riftvalley

Silas M. Simiyu a,), G. Randy Keller b

aKenya Electricity-Generating Company, Olkaria Geothermal Project, PO Box 785, NaiÕasha, Kenya

b Department of Geological Sciences, UniÕersity of Texas at El Paso, El Paso, TX 79968, USA

Received 14 April 1999; received in revised form 21 July 1999; accepted 21 July 1999

Abstract

Seismic monitoring of the Olkaria Geothermal area in the southern Rift Valley region of Kenya has been carried out

since 1985. The initial purpose of this effort was to determine the background level of seismicity before full exploitation of

the geothermal resource was started. This monitoring began with one seismic station. However, since May 1996, a seismic

network comprising six stations was operated and focused mainly on the East Production Field. During the 5 months of

network recording up to mid-September 1996, more than 460 local events originating within the Olkaria Geothermal areaŽ . ŽT yT -5 s were recorded, out of which 123 were well-located. Also, 62 events were recorded at regional distances 5

s p

. Ž .s-T yT -40 s , and 44 events at teleseismic distance T yT )40 s . During this period, the local microseismicity wass p s p

found to be continuous with swarms occurring every 4–5 days. Duration magnitudes based on the coda length did not

exceed 3.0. Preliminary spectral analysis shows three kinds of seismic signals, with only the first type displaying

well-defined P- and S-phases. The seismicity is mainly concentrated in the central area of the recording network, and the

linear alignments in the epicenters are striking. A prominent alignment occurs along the Ololbutot fault zone extending fromthe northern end of the greater Olkaria volcanic complex to the south near the southern terminus of Hell’s gorge. Two other

prominent alignments occur along NW–SE trends that coincide with fault zones which have been detected by geological and

gravity studies. Consequently, they are interpreted to be associated with fluid movement in the geothermal field. These

preliminary results suggest that seismic monitoring will be useful to both monitor the field during production and to help site

additional wells. q2000 Elsevier Science B.V. All rights reserved.

Keywords: Kenya Rift; geothermal exploration; seismic fault mapping; hydrothermal activity

1. Introduction

The Kenya Rift Valley is the classic example of an active continental rift. It is part of the eastern arm

of the East African rift zone, and its geologic evolu-

)

Corresponding author.Ž . E-mail addresses: [email protected] S.M. Simiyu ,

Ž [email protected] G.R. Keller .

Ž .tion has been reviewed recently by Smith 1994 andŽ .Smith and Mosley 1993 . The Olkaria Geothermal

field is located within the central Kenya Rift Valley just south of Lake Naivasha and covers an area of 352 Ž .km Fig. 1 . It is located in one of the large

Quaternary axial volcanic centers that occur alongŽ .the Kenya Rift Valley Fig. 1 . To date, it is the only

center to produce geothermal energy, but exploration

in the region is active. Geothermal exploitation at

Olkaria started in the early 1980s. Since then, more

0377-0273r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .P I I : S 0 3 7 7 - 0 2 7 3 9 9 0 0 1 2 4 - 9



( )S.M. Simiyu, G.R. Keller r Journal of Volcanology and Geothermal Research 95 2000 197–208198

Fig. 1. Index map of the Kenya Rift Valley showing location of the Olkaria Geothermal area and other significant geothermal areas in the

Rift Valley of Kenya. Also shown are the rift valley lakes.

than 90 wells have been drilled, and a power plant

has been constructed that produces 45 MW, which is

6% of the national electric consumption.

The Olkaria region, an area dominated by young

volcanic rocks and where igneous activity occurredŽin the recent past Clarke and Woodhall, 1987; Mu-

.chemi, 1994; Mungania, 1995 , lies in the southern

part of the Kenya Rift Valley just SW of Lake

Ž .Naivasha. Surface mapping by Naylor 1972 identi-

fied the Olkaria volcanic area as a remnant of an old

caldera complex that has been subsequently cut by

N–S normal faulting. The faults provided the loci for

later eruptions of rhyolitic and pumice domes. Areas

of altered and warm ground are extensive throughoutŽ .the Olkaria area Glover, 1972 . Surface manifesta-

tions show a close association with dominant near



( )S.M. Simiyu, G.R. Keller r Journal of Volcanology and Geothermal Research 95 2000 197–208 199

N–S faults and the ring domes. The youngest lavaŽflow at Ololbutot is about 180 years BP Clarke et

.al., 1990 .

Relatively few seismic stations have been located

in East Africa, and so our knowledge of seismicity inŽ .the region is limited e.g., Fairhead and Stuart, 1982 .

Temporary seismic networks have been operated in a

few areas of the Kenya Rift Valley region and

document the presence of local centers of seismicŽ .activity Tongue et al., 1992 . In the first seismic

monitoring at Olkaria, both passive and active seis-

mic experiments were conducted by the U.S. Geo-Ž .logical Survey Hamilton et al., 1973 using an

eight-station network. Explosions were detonated in

the area to determine a velocity model and station

time corrections. Only earthquakes with body wave

magnitudes less than 2 were recorded, and the epi-

centers were mainly located along the N–S-trending

Ololbutot fault zone. Time distance plots were alsoconstructed and indicated that crystalline basement

rocks with a P-wave velocity of 6.38 kmrs underlie

the Olkaria area.

Large-scale seismic refraction profiles were

recorded across the region by the Kenya Rift Interna-Ž .tional Seismic Project KRISP in 1985 and 1990.

These data provide a good picture of the overallŽcrustal structure of the region Henry et al., 1990;

.Mechie et al., 1994 . The variations in crustal struc-

ture observed across the rift are abrupt but generally

Ž .what one would have expected Maguire et al., 1994 .However, variations in crustal structure along the rift

are surprisingly large, generally correlating with theŽelevation of the Rift Valley floor Keller et al.,

.1994 . At Lake Naivasha and Olkaria, the elevation

is about 2 km and the crust is about 35 km thick,Ž .while to the north at Lake Turkana 3.58N latitude

the average elevation is about 0.5 km and the crust is

only 22 km thick. The main source of the difference

in crustal thickness is the presence of an ;8-km-

thick layer at the base of the crust. This high-velocityŽ .;6.8 kmrs layer has been interpreted as under-

plated material which is the source of the flood

phonolite which covered much of the central portionŽ .of the rift Hay et al., 1995 . Another result of the

KRISP effort was the lack of evidence for an exten-Žsive axial dike along the Rift Valley Henry et al.,

.1990 . An axial dike was a feature common to manyŽearly models of crustal structure in the rift Swain et

Table 1

P-wave velocity model developed from the KRISP 85–90 seismicŽ .refraction data by Simiyu and Keller in review : This velocity

model was used in the location of earthquake hypocentres. Also

given are the layers thicknesses and lithology estimated from drill

hole data, geology and seismic velocity.

Layer Thickness P-wave LithologyŽ .number km velocity

1 0.2 2.0 Pyroclastics

2 0.8 3.7 Trachytesrtuffs

3 1.5 4.2 LavarintrusivesŽ .4 4.0 5.0 Fractured granite ?

5 )6.5 6.0 Crystalline basement

.al., 1994 . For the Olkaria region, the picture of

crustal structure provided by the KRISP effort has

been refined by more detailed analysis comple-

Žmented by gravity data Simiyu, 1996; Simiyu and.Keller, in review and indicates that the Olkaria area

is underlain by a five-layer upper crustal structureŽ .Table 1 . This interpreted model shows a structure

with velocities higher than the average upper crustalŽvelocities within the rift Henry et al., 1990; Mechie

.et al., 1994 , suggesting a localized igneous intrusion

associated with the volcanic field.

2. Data acquisition and processing

Monitoring of the local seismicity by the Kenya

Electricity-Generating Company started in early 1985

with one analog MEQ 800 seismic system equipped

with a vertical component sensor. Data collected by

this station up to early 1996 have been processed andŽ .discussed by Mariita et al. 1996 . These data show

that most of the events at Olkaria have body waveŽ .magnitudes m of about 2, but obviously, moreb

station coverage was needed to locate events.

From May to September of 1996, there was con-

tinuous seismic monitoring using six instruments.

The objectives of this effort were to determine the

locations and nature of the earthquakes in order to:Ž .1 Delineate zones of high fracture permeability

that may channel hot fluids from a deep heat source

to shallow levels;Ž .2 Get a better definition of the reservoir, its

recharge and the associated structures;




Ž .3 Evaluate the main characteristics of the local

seismic activity to define the initial state of seismic-

ity before the whole geothermal area is under full

exploitation; andŽ .4 Delineate zones of high microseismic activity

and use this as a basis for future seismic network

planning.

The network was designed to cover the geother-

mal production area with instrument locations close

enough to detect events of low magnitude. This

paper presents a qualitative description of the seis-

micity recorded from May to September of 1996 and

of its nature and relationship to specific geologic

features. It is expected that with more data and

refinement of the velocity model used in the loca-

tions, the patterns of seismic activity will become

clearer and interpretations may change.

The seismic network shown in Fig. 2 was de-

signed to cover an area of about 8 km across,Ž .centered on the East Production Field EPF . Thus,

Ž .for shallow earthquakes depth-6 km , the outer

stations would record waves refracted through base-

Fig. 2. Tectonic map of the Olkaria Geothermal field area. Locations of seismographs in the networks are shown by triangles. Solid lines

show major faults and some of the production wells are shown as circles. Field divisions are shown as shaded areas; EPF, East Production

Field; OWF, Olkaria West Field; NEP, Northeast Production Field.




ment rocks and the inner stations would record direct

waves traveling through the shallow layers. Given

the logistic constraints, we felt that this was an

optimal station configuration for epicentral and focal

depth determinations for the area. However, we rec-

ognize the need to have a closer-spaced network in

order to allow determination of hypocenters for

smaller earthquakes in localized areas.

The seismograph network comprised of six sta-

tions including four MEQ-800 drum recorders and

two state-of-the-art, digital Refraction TechnologyŽ .RefTek instruments. The RefTek instruments were

Ž . Ž . Ž .Fig. 3. A Histogram of local events T yT -5 s recorded in the Olkaria area from May to September 1996. B Duration magnitudes of s p

Ž .local events T yT -5 s recorded between May and September 1996.s p




equipped with GURALP three-component broadband

sensors and digital GPS timing systems. Using the

GPS, output through a laptop computer provided the

timing on the MEQ-800s. Station locations were

determined using a hand-held GPS system which

also provided grid coordinates that were converted to

geographical coordinates for use in the hypocenter

determination computer program.

The MEQ-800 instruments recorded continuouslyŽ .at high speed 30 mmrmin . The analog records

were scanned to identify earthquakes and to note

their time of occurrence. The events identified were

photographically enlarged in order to provide accu-

rate picks of the arrival times. The P- and S-waveŽ .arrival times T and T , respectively read from thep s

sections were entered manually into the PCSUDS

program that included the HYPO71 package forŽepicenter and hypocenter location Lee and Lahr,

.1975 . The RefTek data were downloaded directlyfrom the instruments onto a PC and converted from

the RefTek format to the PCSUDS format using the

package provided by the instrument manufacturers.

The data were processed, events were identified,

located and then classified into three categories:Ž . Ž .1 Local events T yT -5 s ;s p

Ž . Ž .2 Regional events 5 s-T yT -40 s ; ands p

Ž . Ž .3 Teleseismic events T yT )40 s .s p

A histogram for local events is shown in Fig. 3.

The daily rate of local seismicity ranges, on average,

from zero to five microearthquakes per day withŽ .small peaks greater than five events over periods of

about 4–7 days, corresponding to swarms of events

with linear alignments in space and time.

In the absence of any specific magnitude formula

determined for the southern Rift Valley of Kenya,Ž .we used the formula given by Lee and Lahr 1975

to derive duration magnitudes:

M s0.97q2log c q0.00325d ,Ž .

Ž .where d is the epicenter distance km and c is theŽ .duration of the event coda s .

The daily average magnitudes of local events are

also shown in Fig. 3. The continuous character of

seismicity is clear. There were no duration magni-

tudes greater than 3.0, but magnitudes around 2.0

occurred regularly. Negative magnitudes were also

detected in areas with very low noise levels. Epicen-

ters and hypocenters for events with clear P- and

S-wave onset were determined with the computerŽ .program, HYPO71 Lee and Lahr, 1975 . The five-

Ž .layer model Table 1 was used to determine the

locations and was based on the calibration shots of Ž .Hamilton et al. 1973 and the KRISP 85 and 90

Žseismic refraction results Simiyu, 1996; Simiyu and.Keller, in review .

The mean residual errors for each station showed

that the P-wave residual errors were rather high forŽ .the four MEQ-800 stations A, B, C and D because

the picks were less accurate from single-component,

analog, vertical records. The S-wave picking errors

were also very large on the single-component verti-

cal records. The residual errors are low on the RefTek

instruments since the P- and S-wave arrival times

were more accurately picked on the broadband,

Ž .three-component data Table 2 . The mean locationerrors that were estimated by HYPO71 for the 123

local events are as follows:

Horizontal error: 0.32 km;

Vertical error: 0.67 km; and

RMS error in arrival times: 19.35 ms.

Based on waveforms and spectral content, three

general types of seismic signals were observed dur-Ž .ing this study Fig. 4 . For most of the events, the

amplitudes are higher for the horizontal componentsthan for the vertical components. For all compo-

nents, the amplitudes are higher for station B than

any other station.

Table 2

Mean residual errors calculated by HYPO71 for each station

showing the number of observed events used and the residual in

milliseconds

Station P-res. Number of S-res. Number of Ž . Ž .ms observations ms observations

A 2.3 84 12.5 76

B 4.7 99 45.3 87

C 57.2 110 62.4 100

D 53.4 74 58.5 67

E 1.24 88 1.11 81

F 0.78 92 1.9 84




Fig. 4. Waveforms and spectral signatures of the three types of seismic events observed in the Olkaria Geothermal area.

2.1. Type 1

Signals with well-defined P- and S-phases and

spectra that are characterized by one corner fre-

quency. They have a monochromatic character start-

ing with a weak emergent phase followed by a phase

of greater amplitude but at about the same frequency.

A strong dominant frequency is observed at 2 Hz

and smaller frequency peak is noticeable at 4 Hz.

These types of signals are from deep events and areŽprobably related to volcano-tectonic activity Ward,

.1972; Ward and Bjornson 1971 .




2.2. Type 2

These signals with characteristics between Types

1 and 3 have a more complicated shape with two

phases. The first phase is a low-frequency signal

followed by a second phase, which is more enriched

in high frequencies. The dominant frequency is 2 Hz

for the first phase and secondary frequency peaks

occur at about 5 and 8 Hz for the second phase. We

are presently unable to infer a process for the genera-

tion of these events.

2.3. Type 3

Events of the third type lack clear phases after the

first arrival and have spectra characterized by one

dominant frequency. These events have higher fre-

quencies than Types 1 and 2 events and the onset of

the signals is relatively sharp with a dominant fre-quency at 5.5 Hz. These events have relatively shal-

low hypocenters, cluster along known fault zones,

and are possibly related to fluid movement within

the reservoir.

3. Interpretation

Seismic studies in active tectonic and volcanic

areas show that some high temperature geothermal

fields are characterized by a relatively high level of microearthquake activity than the surrounding areasŽe.g., Combs, 1975; Palmason, 1975; Albores et al.,

.1980; Foulger et al., 1989, 1997 . There are also

other geothermal fields that are characterized by lowŽearthquake activity McEvilly et al., 1978; Sherburn

.et al., 1993 . However, numerous studies show that

recent intrusions are associated with high levels of Žearthquake activity e.g., Mt. St. Helen: Fehler, 1983;

Lees and Crosson, 1989; Nevado del Ruiz, Colom-

bia: Zollweg, 1990; Hengill and Krafla, Iceland:

Foulger et al., 1989; Stromboli, Italy: Ntepe and

Dorel, 1990; Casa Diablo: Stroujkova and Malin, in.review . Spectral analysis of individual events from

these studies shows that they are characterized by

unique, low-frequency source mechanisms. The

events are often emergent, lack clear phases, and

contain several characteristic frequencies. These

events gave information on the dimensions of their

associated magmatic and hydrothermal systems. In

practical terms, these are the features that control the

potential energy present in a given geothermal field.

It has also been shown that the depth distribution of

events in a geothermal field is mostly controlled by

the temperature regime at depth and penetration of Žwater into hot rocks Lister, 1974; Palmason, 1975;

.Chen and Molnar, 1983 .

Regional stress analysis in the southern KenyaŽ .Rift Strecker and Bosworth, 1991 and seismic stud-

Ž .ies Fairhead and Stuart, 1982 suggest that stress

along the rift floor is released by a high intensity of

microseismic activities in geothermal areas but by a

few large earthquake sequences along the rift bound-

ary faults. In many cases, the location of a geother-

mal system coincides with an area where regional

stress is being released at a different rate to theŽ .surrounding areas Foulger et al., 1989, 1997 . The

differences in the seismicity of different geothermalareas reflect differences in their regional tectonic

set-ups. In seismically active geothermal areas such

as Olkaria, the location of seismic events can provide

data necessary to determine the location of active

fault zones that function as subsurface conduits for

geothermal fluids. The focal depths are used to

predict the depth of circulation in a geothermal

system as well as the depth to the brittle–ductile

transition zone that is directly related to the geother-

mal gradient.

In this study, seven linear alignments of epicen-Ž .ters were identified Fig. 5 . Our interpretations of

the origins of these alignments are based, in large

part, on their correlation with geologic structures

identified during the numerous geological and geo-

physical studies that have been carried out in the

area by the Kenya Electricity-Generating Company’s

geothermal exploration project staff.

The first linear alignment of events trends N–SŽ .approximately following grid line 198 I, Fig. 5 and

Ž .is coincident with the Ololbutot fault zone Fig. 2 .

These events extend completely across the studyarea, are shallow, and have magnitudes varying from

1.2 to 2.5. The spatial correlation with the Ololbutot

fault suggests that the activity within this zone is due

either to the movement of geothermal fluids within

the fault zone or to tectonic movements along theŽ .fault. A nearly parallel trend of epicenters II, Fig. 5

extends from the center of the area SSE to the




Fig. 5. Map of epicenter locations of 123 well-located events and the six recording stations within the Olkaria Geothermal area. Recording

stations are denoted by triangles. Arrows and Roman numerals indicate the interpreted linear alignments of epicenters. Circles represent

geothermal wells where high producers are shown as open circles while the poor producers are shown as crossed circles.

vicinity of the southern terminus of Hell’s gorge.

This trend does not fall along any mapped geologic

feature, but we suspect that a buried fault correlates

with these epicenters.

Two other prominent alignments occur alongŽ .NW–SE trends III and IV, Fig. 5 . One crosses

north of the Ololbutot lava field and one is located

south of this lava field. These alignments appear to

correlate with faults that have recently been identi-

fied by geologic mapping and analysis of gravityŽ .anomalies Mungania, 1995 . Consequently, these

events could also be associated with fluid movement

in the geothermal field.

ŽTwo shorter E–W-trending alignments V and VI,.Fig. 5 occur north of the EPF. These alignments do

not extend west of the Ololbutot fault zone. These

events have some of the shallowest hypocenters ob-Žserved, about 2.3 km. A final linear alignment VII,

.Fig. 5 is composed of events that form a NNE trend

starting north of the EPF and extending to the vicin-

ity of station B. These events occurred at estimated

depths of 2.5–3.5 km.

Earthquake hypocenters located between grid linesŽ .9903 and 9905 Figs. 2 and 5 were projected to

Ž .produce an E –W profile Fig. 6 . This profile shows

that hypocentral depths change rapidly near fault




Fig. 6. Earthquake hypocentres within UTM grid 9903–9905 north projected onto an EW vertical plane. Arrows represent locations of

selected wells.

systems. The best example is the intersection of the

profile and the Ololbutot fault zone between wells

OW-201 and OW-709. At the Olkaria west field near

well OW-301, the events also change depth at the

intersection of the profile and a prominent N–S-Ž .trending fault zone Fig. 2 .

These alignment of epicenters and the depth dis-

tribution suggest that the seismicity is controlled by

faults rather than by some diffuse percolation of

geothermal fluids. It further shows that the seismicity

should be an aid in locating fault zones, which iscritical in efforts to find high-permeability zones

with more-than-average fluid movement and, hence,

heat transfer.

4. Conclusion and recommendations

Seismic monitoring of the Olkaria Geothermal

area has revealed interesting patterns in epicenters,

which are mainly located along linear trends. Several

of these trends follow known fault zones, in particu-

lar the Ololbutot fault zone and two sub-parallel

NW–SE faults that bound the Ololbutot lava field onŽ .the north and south Fig. 5 , and we suggest that the

linear alignments are probably associated with fault-

ing. Events have hypocenters which do not exceed 6

km in depth. Microseismic activity at the intersection

of the different epicentral trends shows a higher

concentration of smaller and shallow events that are

interpreted to be caused by fluid movement along

fault zones.

Three classes of seismic signals were observed at

Olkaria based on their waveforms and spectral con-

tent. The two endmember classes are interpreted asŽ .representing: 1 volcano-tectonic events with well-

Ž .defined P- and S-phases; and 2 events due to

possible fluid movement within the reservoir that are

characterized by lack of clear phases after the first

arrival and spectra with a well-defined dominantfrequency.

The possible relationship between seismicity and

fluid movement within the geothermal systems is

important and needs further study. This will require

more instruments closely spaced within the area. A

longer period of monitoring with a larger and denser

network will also be necessary to obtain reliable

focal mechanisms. One of the objectives of future

studies will be to constrain and separate seismicity

due to tectonicsrvolcanism and those due to fluid

movement within the reservoir. It will also be impor-

tant to continue monitoring in a way designed to

observe the relationship of re-injected fluid move-

ment within the reservoir to seismicity. If fluid injec-

tion and production seismicity can be analyzed, the

induced events can also be interpreted.

The significance of fault zones as conduits for

heat and fluid movement within the reservoir is




clearly shown by the presence of shallow type 3

events. Wells drilled in these zones have the largest

mass output. Future studies should aid in the delin-

eation of such zones as targets for drilling high fluid

producing wells.

Acknowledgements

We wish to thank Mr. Hudson I. Viele, Elvis O.

Oduong and Tom K. Mboya of KenGen for working

tirelessly to collect data both before and during the

period of this study. Special thanks are due to the

Geothermal Development Manager and the Chief

Geothermal Scientist for their support. Many thanks

to Drs. G.R. Raquemore and A.W. Hurst for their

constructive reviews of the original manuscript. Our

grateful thanks are extended to Dr. Jeff Karson forhis informal review that led to the improvement of

this paper. The Department of Geological Sciences,

University of Texas at El Paso provided the RefTek

seismic instruments as part of its participation in the

Kenya Rift International Seismic Project which was

funded by the Continental Dynamics Program of the

U.S. National Science Foundation.

References

Albores, A., Reyes, A., Brune, J.N., Gonzalez, J., Garcilazo, F.,

Suarez, F., 1980. Seismicity studies in the region of the Cerro

Prieto geothermal field. Geothermics 9, 65–77.

Chen, W.P., Molnar, P., 1983. Focal depths of intracontinental

and intraplate earthquakes and their implications for the ther-

mal and mechanical properties of the lithosphere. J. Geophys.

Res. 88, 4183–4214.

Clarke, M.C.G., Woodhall, D.G., 1987. Geological and geother-

mal mapping. British Geological Survey, Exploration for

Geothermal Energy Project, Progress report BGSKENr2, 135

pp.

Combs, J., 1975. Geophysical techniques in geothermal explo-

ration. Proceedings of the 2nd Symposium Development and

Use Geothermal Resources 1, 95–99.

Fairhead, J.D., Stuart, G.W., 1982. The seismicity of the East

African rift system and comparison with other continentalŽ .rifts. In: Palmason, G. Ed. , Continental Rifts. Geodynamic

Series 8 American Geophysical Union, Washington, DC, pp.

41–61.

Fehler, M., 1983. Observation of volcanic tremor on Mount St.

Helen volcano. J. Geophys. Res. 88, 3476–3484.

Foulger, G.R., Long, R.E., Einarsson, P., 1989. Implosive earth-

quakes at the active plate boundaries in Iceland. Nature 337,

640–642.

Foulger, G.R., Grant, C.C., Ross, A., Julian, B.R., 1997. Industri-

ally induced changes in Earth structure at the Geysers geother-Ž .mal area, California. Geophys. Res. Lett. 24 2 , 135–137.

Hamilton, R.M., Smith, B.E., Knapp, F., 1973. Earthquakes in

geothermal areas near Lake Naivasha and Hannington. United

States Geological Survey, Unpublished report UNDPrEAPL,231 pp.

Hay, D.E., Wendlandt, R.F., Keller, G.R., 1995. The origin of

Kenya Rift plateau-type flood phonolite: integrated petrologic

and geophysical constraints on the evolution of the crust and

upper mantle beneath the Kenya Rift. J. Geophys Res. 100,

10549–10557.

Henry, W.J., Mechie, J., Maguire, P.K.H., Khan, M.A., Prodehl,

C., Keller, G.R., Patel, J.P., 1990. A seismic investigation of

the Kenya Rift valley. Geophys J. Int. 100, 107–130.

Keller, G.R., Prodehl, C., Mechie, J., Fuchs, K., Khan, M.A.,

Maguire, P.K.H., Mooney, W.D., Achauer, U., Davis, P.M.,

Meyer, R.P., Braile, L.W., Nyambok, I.O., Thompson, G.A.,

1994. The East African rift system in the light KRISP 90. In:Ž .C., Keller, G.R., Khan, M.A. Eds. , Crustal and Upper Man-

tle Structure of the Kenya Rift. Prodehl, Tectonophysics 236,

465–483.Ž .Lee, W.H.K., Lahr, J.C., 1975. HYPO 71 revised : a computer

program for determining hypocenter, magnitude and first mo-

tion pattern of local earthquakes. U.S. Geol. Surv., Open File

Rep. 75-311, 16 pp.

Lees, J.M., Crosson, R.S., 1989. Tomographic inversion for

three-dimensional velocity structure at Mt. St. Helen using

earthquake data. J. Geophys. Res. 94, 5716–5728.

Lister, R.G.B., 1974. On the penetration of water into hot rock.

Geophys. J.R. Astron. Soc. 39, 465– 501.

Maguire, P.K.H., Swain, C.J., Masotti, R., Khan, M.A., 1994. A

crustal and uppermost mantle cross-sectional model of theKenya Rift derived from seismic and gravity data. In: C.,

Ž .Keller, G.R., Khan, M.A. Eds. , Crustal and Upper Mantle

Structure of the Kenya Rift. Prodehl, Tectonophysics 236,

217–249.

Mariita, O.N., Otieno, C.O., Shako, J.W., 1996. Microseismic

monitoring at the Olkaria Geothermal field, Kenya. Kenya

Power, Unpublished Report, 15 pp.

McEvilly, T.V., Schechter, B., Majer, E.L., 1978. East Mesa

seismic study. Annual Report, Earth Sciences Division,

Lawrence Berkeley Laboratory, University of California, pp.

26–28.

Mechie, J., Keller, G.R., Prodehl, C., Gaciri, S., Braile, L.W.,

Mooney, W.D., Gajewski, D., Sandmeier, K.J., 1994. Crustal

structure beneath the Kenya Rift from axial profile data. In:Ž .C., Keller, G.R., Khan, M.A. Eds. , Crustal and Upper Man-

tle Structure of the Kenya Rift. Prodehl, Tectonophysics 236,

179–199.

Muchemi, G.G., 1994. Geological interpretation of the Olkaria

North East field. KenGen Internal Report, 25 pp.

Mungania, J., 1995. Tephra deposits in Olkaria and the surround-

ing areas. Kenya Power, Unpublished Report, 7 pp.




Naylor, J., 1972. The geology of the Olkaria and Eburru prospects,

UNDPrKEN-GOVT Report No. UNDPrKEN-125r1640r72.

Ntepe, N., Dorel, J., 1990. Observations of seismic volcanicŽ .signals at Stromboli volcano Italy . J. Volcanol. Geotherm.

Res. 43, 235–251.

Palmason, G., 1975. Geophysical methods in geothermal explo-

ration. In: Proceedings of the 2nd Symposium on the Develop-

ment and Use of Geothermal Resources, San Francisco. pp.

1175–1184.Sherburn, S., Clotworthy, A., Hunt, T., 1993. No induced seismic

activity at Ohaaki geothermal field. In: Proceedings of the

15th NZ Geothermal Workshop. pp. 201–2054.

Simiyu, S.M., 1996. Integrated geophysical study of the crustal

structure of the southern Kenya Rift. PhD Dissertation, Uni-

versity Texas El Paso, 240 pp.

Simiyu, S.M., Keller, R.G., in review. Seismic and gravity inter-

pretation of the shallow crustal structure in the Southern

Kenya Rift Valley., Geophys J. Int.

Smith, M., 1994. Stratigraphic and structural constraints on mech-

anisms of active rifting in the Gregory Rift, Kenya. In: C.,Ž .Keller, G.R., Khan, M.A. Eds. , Crustal and Upper Mantle

Structure of the Kenya Rift. Prodehl, Tectonophysics 236,

3–22.

Smith, M., Mosley, P.N., 1993. Crustal heterogeneity and base-

ment influence on the development of the Kenya Rift, East

Africa. Tectonics 12, 591–606.

Strecker, M., Bosworth, W., 1991. Quaternary stress-field changes

in Gregory rift, Kenya. EOS Trans., AGU 72, 3.

Stroujkova, A.F., Malin, P.E., in review. A magma mass beneath

Casa Diablo? Further evidence from seismic reflected waves.

Bull. Seismol. Soc. Am., in review.

Swain, C.J., Maguire, P.K.H., Khan, M.A., 1994. Geophysical

experiments and models of the Kenya Rift before 1989. In: C.,Ž .Keller, G.R., Khan, M.A. Eds. , Crustal and Upper Mantle

Structure of the Kenya Rift. Prodehl, Tectonophysics 236

23–32.

Tongue, J.A., Maguire, P.K.H., Young, P.A.V., 1992. Seismicity

distribution from temporary earthquake recording networks in

Kenya. Tectonophysics 204, 71–79.

Ward, P.L., 1972. Microearthquakes: prospecting tool and possi-

ble hazard in the development of geothermal resources.

Geothermics 1, 3–4.

Ward, P.L., Bjornson, S., 1971. Microearthquakes, swarms, and

the geothermal areas of Iceland. J. Geophys. Res. 76, 3953–

3982.

Zollweg, J.E., 1990. Seismicity following the 1985 eruption of

Nevado del Ruiz Colombia. J. Volcanol. Geotherm. Res. 41,

355–367.

Documents

Seismic Monitoring of the Olkaria Geothermal Area, Kenya Rift