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

Seismic monitoring of the Olkaria Geothermal area, Kenya Riftvalley

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

a Kenya Electricity-Generating Company, Olkaria Geothermal Project, PO Box 785, NaiÕasha, Kenyab 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 outsince 1985. The initial purpose of this effort was to determine the background level of seismicity before full exploitation ofthe geothermal resource was started. This monitoring began with one seismic station. However, since May 1996, a seismicnetwork comprising six stations was operated and focused mainly on the East Production Field. During the 5 months ofnetwork 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 5s 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 notexceed 3.0. Preliminary spectral analysis shows three kinds of seismic signals, with only the first type displayingwell-defined P- and S-phases. The seismicity is mainly concentrated in the central area of the recording network, and thelinear 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 otherprominent alignments occur along NW–SE trends that coincide with fault zones which have been detected by geological andgravity studies. Consequently, they are interpreted to be associated with fluid movement in the geothermal field. Thesepreliminary results suggest that seismic monitoring will be useful to both monitor the field during production and to help siteadditional wells. q 2000 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 ofan active continental rift. It is part of the eastern armof 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 Valleyjust south of Lake Naivasha and covers an area of 35

2 Ž .km Fig. 1 . It is located in one of the largeQuaternary axial volcanic centers that occur along

Ž .the Kenya Rift Valley Fig. 1 . To date, it is the onlycenter to produce geothermal energy, but explorationin the region is active. Geothermal exploitation atOlkaria started in the early 1980s. Since then, more

0377-0273r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0377-0273 99 00124-9

( )S.M. Simiyu, G.R. KellerrJournal 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 theRift Valley of Kenya. Also shown are the rift valley lakes.

than 90 wells have been drilled, and a power planthas been constructed that produces 45 MW, which is6% of the national electric consumption.

The Olkaria region, an area dominated by youngvolcanic rocks and where igneous activity occurred

Žin 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 oldcaldera complex that has been subsequently cut byN–S normal faulting. The faults provided the loci forlater eruptions of rhyolitic and pumice domes. Areasof altered and warm ground are extensive throughout

Ž .the Olkaria area Glover, 1972 . Surface manifesta-tions show a close association with dominant near

( )S.M. Simiyu, G.R. KellerrJournal 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 afew areas of the Kenya Rift Valley region anddocument the presence of local centers of seismic

Ž .activity Tongue et al., 1992 . In the first seismicmonitoring at Olkaria, both passive and active seis-mic experiments were conducted by the U.S. Geo-

Ž .logical Survey Hamilton et al., 1973 using aneight-station network. Explosions were detonated inthe area to determine a velocity model and stationtime corrections. Only earthquakes with body wavemagnitudes less than 2 were recorded, and the epi-centers were mainly located along the N–S-trendingOlolbutot fault zone. Time distance plots were alsoconstructed and indicated that crystalline basementrocks with a P-wave velocity of 6.38 kmrs underliethe Olkaria area.

Large-scale seismic refraction profiles wererecorded 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 overall

Žcrustal 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 generallyŽ .what one would have expected Maguire et al., 1994 .

However, variations in crustal structure along the riftare surprisingly large, generally correlating with the

Želevation 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 isonly 22 km thick. The main source of the differencein 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 floodphonolite which covered much of the central portion

Ž .of the rift Hay et al., 1995 . Another result of theKRISP 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 many

Žearly models of crustal structure in the rift Swain et

Table 1P-wave velocity model developed from the KRISP 85–90 seismic

Ž .refraction data by Simiyu and Keller in review : This velocitymodel was used in the location of earthquake hypocentres. Alsogiven are the layers thicknesses and lithology estimated from drillhole data, geology and seismic velocity.

Layer Thickness P-wave LithologyŽ .number km velocity

1 0.2 2.0 Pyroclastics2 0.8 3.7 Trachytesrtuffs3 1.5 4.2 Lavarintrusives

Ž .4 4.0 5.0 Fractured granite ?5 )6.5 6.0 Crystalline basement

.al., 1994 . For the Olkaria region, the picture ofcrustal structure provided by the KRISP effort hasbeen 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 structurewith velocities higher than the average upper crustal

Žvelocities 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 KenyaElectricity-Generating Company started in early 1985with one analog MEQ 800 seismic system equippedwith a vertical component sensor. Data collected bythis station up to early 1996 have been processed and

Ž .discussed by Mariita et al. 1996 . These data showthat most of the events at Olkaria have body wave

Ž .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 thelocations and nature of the earthquakes in order to:

Ž .1 Delineate zones of high fracture permeabilitythat may channel hot fluids from a deep heat sourceto shallow levels;

Ž .2 Get a better definition of the reservoir, itsrecharge and the associated structures;


Ž .3 Evaluate the main characteristics of the localseismic activity to define the initial state of seismic-ity before the whole geothermal area is under fullexploitation; and

Ž .4 Delineate zones of high microseismic activityand use this as a basis for future seismic networkplanning.

The network was designed to cover the geother-mal production area with instrument locations closeenough to detect events of low magnitude. Thispaper presents a qualitative description of the seis-

micity recorded from May to September of 1996 andof its nature and relationship to specific geologicfeatures. It is expected that with more data andrefinement of the velocity model used in the loca-tions, the patterns of seismic activity will becomeclearer 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 linesshow major faults and some of the production wells are shown as circles. Field divisions are shown as shaded areas; EPF, East ProductionField; OWF, Olkaria West Field; NEP, Northeast Production Field.


ment rocks and the inner stations would record directwaves traveling through the shallow layers. Giventhe logistic constraints, we felt that this was anoptimal station configuration for epicentral and focaldepth determinations for the area. However, we rec-ognize the need to have a closer-spaced network in

order to allow determination of hypocenters forsmaller earthquakes in localized areas.

The seismograph network comprised of six sta-tions including four MEQ-800 drum recorders andtwo 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 ofs pŽ .local events T yT -5 s recorded between May and September 1996.s p


equipped with GURALP three-component broadbandsensors and digital GPS timing systems. Using theGPS, output through a laptop computer provided thetiming on the MEQ-800s. Station locations weredetermined using a hand-held GPS system whichalso provided grid coordinates that were converted togeographical coordinates for use in the hypocenterdetermination computer program.

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

were scanned to identify earthquakes and to notetheir time of occurrence. The events identified werephotographically enlarged in order to provide accu-rate picks of the arrival times. The P- and S-wave

Ž .arrival times T and T , respectively read from thep s

sections were entered manually into the PCSUDSprogram that included the HYPO71 package for

Žepicenter and hypocenter location Lee and Lahr,.1975 . The RefTek data were downloaded directly

from the instruments onto a PC and converted fromthe RefTek format to the PCSUDS format using thepackage 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 pŽ . Ž .2 Regional events 5 s-T yT -40 s ; ands pŽ . 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 with

Ž .small peaks greater than five events over periods ofabout 4–7 days, corresponding to swarms of eventswith linear alignments in space and time.

In the absence of any specific magnitude formuladetermined for the southern Rift Valley of Kenya,

Ž .we used the formula given by Lee and Lahr 1975to derive duration magnitudes:

Ms0.97q2 log 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 arealso shown in Fig. 3. The continuous character ofseismicity is clear. There were no duration magni-tudes greater than 3.0, but magnitudes around 2.0occurred regularly. Negative magnitudes were also

detected in areas with very low noise levels. Epicen-ters and hypocenters for events with clear P- andS-wave onset were determined with the computer

Ž .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 ofŽ .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 showedthat the P-wave residual errors were rather high for

Ž .the four MEQ-800 stations A, B, C and D becausethe picks were less accurate from single-component,analog, vertical records. The S-wave picking errorswere also very large on the single-component verti-cal records. The residual errors are low on the RefTekinstruments since the P- and S-wave arrival timeswere more accurately picked on the broadband,

Ž .three-component data Table 2 . The mean locationerrors that were estimated by HYPO71 for the 123local events are as follows:

Horizontal error: 0.32 km;Vertical error: 0.67 km; andRMS error in arrival times: 19.35 ms.

Based on waveforms and spectral content, threegeneral types of seismic signals were observed dur-

Ž .ing this study Fig. 4 . For most of the events, theamplitudes are higher for the horizontal componentsthan for the vertical components. For all compo-nents, the amplitudes are higher for station B thanany other station.

Table 2Mean residual errors calculated by HYPO71 for each stationshowing the number of observed events used and the residual inmilliseconds

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

A 2.3 84 12.5 76B 4.7 99 45.3 87C 57.2 110 62.4 100D 53.4 74 58.5 67E 1.24 88 1.11 81F 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 andspectra 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 Hzand smaller frequency peak is noticeable at 4 Hz.These types of signals are from deep events and are

Žprobably related to volcano-tectonic activity Ward,.1972; Ward and Bjornson 1971 .


2.2. Type 2

These signals with characteristics between Types1 and 3 have a more complicated shape with twophases. The first phase is a low-frequency signalfollowed by a second phase, which is more enrichedin high frequencies. The dominant frequency is 2 Hzfor the first phase and secondary frequency peaksoccur at about 5 and 8 Hz for the second phase. Weare 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 thefirst arrival and have spectra characterized by onedominant frequency. These events have higher fre-quencies than Types 1 and 2 events and the onset ofthe 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 withinthe reservoir.

3. Interpretation

Seismic studies in active tectonic and volcanicareas show that some high temperature geothermalfields are characterized by a relatively high level ofmicroearthquake activity than the surrounding areasŽe.g., Combs, 1975; Palmason, 1975; Albores et al.,

.1980; Foulger et al., 1989, 1997 . There are alsoother geothermal fields that are characterized by low

Žearthquake activity McEvilly et al., 1978; Sherburn.et al., 1993 . However, numerous studies show that

recent intrusions are associated with high levels ofŽearthquake 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 andDorel, 1990; Casa Diablo: Stroujkova and Malin, in

.review . Spectral analysis of individual events fromthese studies shows that they are characterized byunique, low-frequency source mechanisms. Theevents are often emergent, lack clear phases, andcontain several characteristic frequencies. Theseevents gave information on the dimensions of their

associated magmatic and hydrothermal systems. Inpractical terms, these are the features that control thepotential energy present in a given geothermal field.It has also been shown that the depth distribution ofevents in a geothermal field is mostly controlled bythe 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 ofmicroseismic activities in geothermal areas but by afew large earthquake sequences along the rift bound-ary faults. In many cases, the location of a geother-mal system coincides with an area where regionalstress is being released at a different rate to the

Ž .surrounding areas Foulger et al., 1989, 1997 . Thedifferences in the seismicity of different geothermalareas reflect differences in their regional tectonicset-ups. In seismically active geothermal areas suchas Olkaria, the location of seismic events can providedata necessary to determine the location of activefault zones that function as subsurface conduits forgeothermal fluids. The focal depths are used topredict the depth of circulation in a geothermalsystem as well as the depth to the brittle–ductiletransition 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 largepart, on their correlation with geologic structuresidentified during the numerous geological and geo-physical studies that have been carried out in thearea by the Kenya Electricity-Generating Company’sgeothermal 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 from1.2 to 2.5. The spatial correlation with the Ololbutotfault suggests that the activity within this zone is dueeither to the movement of geothermal fluids withinthe fault zone or to tectonic movements along the

Ž .fault. A nearly parallel trend of epicenters II, Fig. 5extends 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. Recordingstations are denoted by triangles. Arrows and Roman numerals indicate the interpreted linear alignments of epicenters. Circles representgeothermal 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 geologicfeature, but we suspect that a buried fault correlateswith 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 locatedsouth of this lava field. These alignments appear tocorrelate with faults that have recently been identi-fied by geologic mapping and analysis of gravity

Ž .anomalies Mungania, 1995 . Consequently, theseevents could also be associated with fluid movementin 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. Theseevents 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 estimateddepths 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 showsthat 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 ofselected wells.

systems. The best example is the intersection of theprofile and the Ololbutot fault zone between wellsOW-201 and OW-709. At the Olkaria west field nearwell OW-301, the events also change depth at theintersection 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 byfaults rather than by some diffuse percolation ofgeothermal fluids. It further shows that the seismicityshould be an aid in locating fault zones, which iscritical in efforts to find high-permeability zoneswith more-than-average fluid movement and, hence,heat transfer.

4. Conclusion and recommendations

Seismic monitoring of the Olkaria Geothermalarea has revealed interesting patterns in epicenters,which are mainly located along linear trends. Severalof these trends follow known fault zones, in particu-lar the Ololbutot fault zone and two sub-parallelNW–SE faults that bound the Ololbutot lava field on

Ž .the north and south Fig. 5 , and we suggest that thelinear alignments are probably associated with fault-ing. Events have hypocenters which do not exceed 6km in depth. Microseismic activity at the intersectionof the different epicentral trends shows a higher

concentration of smaller and shallow events that areinterpreted to be caused by fluid movement alongfault zones.

Three classes of seismic signals were observed atOlkaria based on their waveforms and spectral con-tent. The two endmember classes are interpreted as

Ž .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 arecharacterized by lack of clear phases after the firstarrival and spectra with a well-defined dominantfrequency.

The possible relationship between seismicity andfluid movement within the geothermal systems isimportant and needs further study. This will requiremore instruments closely spaced within the area. Alonger period of monitoring with a larger and densernetwork will also be necessary to obtain reliablefocal mechanisms. One of the objectives of futurestudies will be to constrain and separate seismicitydue to tectonicsrvolcanism and those due to fluidmovement within the reservoir. It will also be impor-tant to continue monitoring in a way designed toobserve the relationship of re-injected fluid move-ment within the reservoir to seismicity. If fluid injec-tion and production seismicity can be analyzed, theinduced events can also be interpreted.

The significance of fault zones as conduits forheat and fluid movement within the reservoir is


clearly shown by the presence of shallow type 3events. Wells drilled in these zones have the largestmass output. Future studies should aid in the delin-eation of such zones as targets for drilling high fluidproducing wells.

Acknowledgements

We wish to thank Mr. Hudson I. Viele, Elvis O.Oduong and Tom K. Mboya of KenGen for workingtirelessly to collect data both before and during theperiod of this study. Special thanks are due to theGeothermal Development Manager and the ChiefGeothermal Scientist for their support. Many thanksto Drs. G.R. Raquemore and A.W. Hurst for theirconstructive reviews of the original manuscript. Ourgrateful thanks are extended to Dr. Jeff Karson forhis informal review that led to the improvement ofthis paper. The Department of Geological Sciences,University of Texas at El Paso provided the RefTekseismic instruments as part of its participation in theKenya Rift International Seismic Project which wasfunded by the Continental Dynamics Program of theU.S. National Science Foundation.

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