Geophys. J. Int. (1991) 105, 665-674

Implications of the distribution of seismicity near in the Rift

P. A. V. Young,'" P. K. H. Maguire,' N. d'A. Laffoley' and J. R. Evans2 'Department of Geology, Leicester University, University Road, Leicester LE1 7RH, UK 'British Geological Survey, Murchbon House, West Mains Road, Edinburgh EH9 3LA, UK

Accepted 1991 January 2. Received 1990 July 19; in original form 1988 May 5

SUMMARY Previous seismicity studies in the Kenya Rift have not been able to determine accurately depths of earthquakes, nor have most of them determined epicentres precisely enough to allow correlation of the seismicity with particular surface features. We operated a small dense seismic array for 3 months near Lake Bogoria in the Kenya Rift with the aim of locating microearthquakes in 3-D. 572 earthquakes, 81 per cent with ML< 1.0, have been located. The majority of the events are associated with the larger older faults on the Rift shoulder rather than the young 'grid' faults in the centre of the Rift. Seismic activity in the central trough cannot be related directly to the surface faulting; we infer that it indicates the presence of deep buried faults. This possibility has important implications for extension estimates and models of the Rift. Most of the activity occurs at depths less than 12 km, and no normal activity is deeper than 16 km. There is a peak in seismic activity at a depth of 9-10 km and the cut-off depth for brittle failure is taken at 12 km. The depth distribution of these earthquakes is similar to that found in other intracontinental areas with similar heat flow which suggests that the crust beneath the Kenya Rift is of normal rheology. Key words: crustal rheology, Kenya Rift, microearthquakes.

INTRODUCTION (Hamilton, Smith & Knapp 1973; Pointing et al. 1985). This project, part of the Kenya Rift International Seismic Project Despite earth scientists' considerable interest in the Kenya of 1985 [KRISPSS (Khan et al. 1986)], had two main Rift, we know comparatively little about its seismicity, objectives: to locate the local earthquakes accurately, and to Large magnitude earthquakes (M> 5.5) are rare, but zones use this data in a shear-wave splitting study. This paper of microseismic activity (M< 3.5) have been identified comprises a description of the network and a discussion on (Fairhead & Stuart 1982; Shah 1986; Pointing et al. 1985). the distribution of the recorded earthquakes, the shear-wave Small-scale seismicity can only be monitored by local seismic splitting and other studies will be the subjects of further networks; the closer the network to the earthquakes, the papers. The stations covered a 20 x 30 km' area including more accurate the epicentre locations. If the recording the southern part of Lake Bogoria, and this paper describes network is almost on top of the earthquakes, accurate the seismicity within an 80~50 km2 area centred on the depths as well as epicentres can be determined. Only if the array. An analysis of the seismic activity within this small seismicity distribution has been defined in 3-D can the area provides insight into the crustal structure and stress correlation of seismicity with particular surface features, regime of the Rift as a whole. such as faults or geothermal sites, be attempted with any confidence. Geology and structure We operated a 15 station short-period seismic array for 3 months near Lake Bogoria in the Kenya Rift (Fig. l), a site The network was situated in the central part of the Kenya shown to be seismically active by previous workers Rift, just north of the equator (Fig. 1). In this region the structure changes in direction, from generally NNW to *Formerly P. A. V. Cooke, now at N.E.R.C. Unit for Thematic Rift Information Systems, Department of Geography, University of NNE, and a variety of fault trends are apparent (King Reading, Whiteknights, Reading, RG6 2AB. 1978). This leads to complex surface fault patterns,

665 666 P. A. V. Young et al. particularly on the eastern flank of the Rift (Fig. 2). big enough to be recorded at teleseismic or even regional Although exposure of fault dips is rare, all the available distances are rare in the Kenyan section of this Rift System geological evidence indicates normal faulting (McCall 1967; (Shah 1986; Maguire et al. 1988; Fairhead & Stuart 1982). Griffiths 1977). The geological succession is almost entirely Therefore, most Kenyan seismicity can only be recorded by volcanic and lavas cover most of the flanks as well as infilling seismic stations within Kenya. the central Rift. The 1985 network was situated in the The largest instrumentally recorded earthquake within the central trough, where the most recent eruptives (younger Kenya Rift occurred within 25 km of Lake Bogoria in 1928. than 2Ma) hide all earlier formations and structure, with This was a magnitude 6.0 event [magnitude unified to the the exception of the older Emsos-Bogoria fault (McCall International Seismological Centre scale by Shah (1986)]. 1967; Griffiths 1977). Formations visible in the eastern flank The fact that surface cracking and the greatest damage were increase in age from 6 to 10, 11 and then 12 Ma as distance reported along the base of the Marmanet-Laikipia from the centre of the Rift increases (Griffiths 1977). escarpment (Willis 1936; Richter 1958; McCall 1967) Similarly, faulting also becomes older towards the Rift strongly suggests that this event was caused by movement on boundary. The major older faults which define the eastern the Laikipia-Marmanet fault (see Fig. 1). Rift shoulder have proven throws of up to 1OOOm (McCall Shah (1986) compiled macroseismic reports for 1880-1979 1967; Griffiths 1977); sometimes this movement has been and found that earthquakes were reported throughout the achieved in several phases of tectonic activity millions of southern Kenya Rift and the Kavirondo Rift. She points out years apart (McCall 1967; Griffiths 1977). The small-scale that northern Kenya is very thinly populated and that this faults in the central trough are less than 1Ma old and may account for the paucity of earthquake reports there. generally have throws of less than 70 m (Griffiths 1977). This McCall (1967) notes that in the 1950s earthquakes were young, dense, subparallel faulting, termed ‘grid’ faulting, is reported fairly frequently in the Lake Bogoria region, and particularly prominent in the 20 km wide strip to the west of we were given similar accounts by the local inhabitants Lake Bogoria (Fig. 2), disappearing northwards and during the fieldwork in 1985. On the basis of the number southwards under sedimentary and pyroclastic cover. Whilst ahd size of the earthquakes here, both Shah (1986) and the major faults can be visualized to extend deep into the McCall (1967) consider the Lake Bogoria region to be at crust, the small horizontal scale of the grid faults suggests a ‘considerable’ earthquake risk. relatively small vertical scale; they are unlikely to reach depths greater than approximately 3 km (e.g. Bosworth, Microseismic studies Lambiase & Keisler 1986). The structure seen here, with the young grid faulting in the centre and the older major faults Microearthquakes were first identified within the Kenya Rift defining the Rift shoulders, is typical of the Rift structure as by reconnaissance surveys, in which single seismographs a whole (Baker, Mohr & Williams 1972; Baker, Mitchell & were deployed at a series of sites, usually remaining at each Williams 1988). Tectonic activity appears to have become site for 1 or 2 days only. Events with small S-minus-P arrival concentrated along a progressively narrower central zone times recorded at a particular site are known to have throughout the Rift’s history (McCall 1967). occurred within a short distance of that site but the direction Bosworth et af. (1986) visualize the Lake Bogoria region of approach and the depth of that event remain unknown. as an ‘accommodation’ zone between two large detachment Tobin, Ward & Drake (1969) conducted such a survey and systems of opposite polarity: to the north a Baringo concluded that microseismic activity in the Rift was detachment moves eastwards from the Elgeyo fault and to associated mainly with the young grid faulting. However, the south a detachment moves westwards from the Molnar & Aggarwal (1971), on the basis of a similar but Sattima fault. Such accommodation zones are expected to more extensive project, asserted that all parts of the Rift be dominated by wrench style tectonics, but there is little were equally likely to be tectonically active. They set up a surface expression of east-west trending features in this small array of seismographs near and were area. able to constrain focal depths of earthquakes to be shallower Lake Bogoria is one of the major geothermal fields in than 15km. Neither of these surveys identified Lake Kenya, and was considered worthy of geophysical and Bogoria as a particularly active area. geochemical exploration including a microseismicity study in Even a small array of recording stations can be used to the 1970s (Noble & Ojiambo 1975; Hamilton et al. 1973). estimate epicentres of earthquakes at some distance away, There are hot or warm springs and fumaroles at various sites and two such small arrays within Kenya have been able to in the locality (McCall 1967), activity being most obvious at locate activity at distances of up to 200 km. One operated the southern end of Lake Bogoria (Fig. 1). for 2 years at Kaptagat (0.S”, 35.5”E) and located activity mainly in the Kavirondo Rift but also in the and Lake Bogoria area (Maguire & Long 1976). The other, PREVIOUS SEISMICITY STUDIES IN THE which was situated at Ngurunit (1.7”N, 37.3”E) and KENYA RIFT, WITH EMPHASIS ON THE monitored seismicity for 8 months, located events in the LAKE BOGORIA REGION northern Kenya Rift, 100km east of the Rift and in the Teleseismic and macroseismic studies Lake Bogoria and Baringo region (Pointing et af. 1985). As the earthquakes were distant from the networks, neither of Large earthquakes occur throughout much of the East these experiments was able to determine focal depths. African Rift System and data from the global network are The only previous local earthquake studies conducted in available to determine their focal mechanisms and depths the Kenya Rift were operated as part of a geothermal (Fairhead & Stuart 1982; Shudofsky 1985). However, events exploration initiative (Hamilton et al. 1973; Noble & Seismicity in the Kenya Rijt 667

Ojiambo 1975). Networks were set up around geothermal and the remainder at 16/160i.p.s., leading to passbands of fields at , Eburru and Lake Bogoria and operated for 16 and 32Hz respectively. To take advantage of the a few weeks at each site (Hamilton et al. 1973). Depth high-quality digital transmission, incoming signals were also determination was generally poor for all three networks, monitored by a PDP 11/23 mini computer which triggered particularly as many earthquakes were located using arrivals to record digitally any coherent disturbance. This digital at only three recording stations. Seismicity at Olkaria and triggering system, developed by the British Geological Eburru was shallow and apparently associated with the Survey on the Turkish Dilatancy Projects (Evans et al. geothermal activity and Eburru volcano. Activity was 1987), worked well but problems with the power supply greatest at Lake Bogoria (104 events in 7 weeks) and some meant that its coverage was intermittent. This purely digital appeared to be deep, down to 19 km. Unfortunately, many subset of the data has significantly lower noise and greater events were outside the Lake Bogoria network and only the bandwidth than the analogue tape recordings, and these P arrivals were used for the locations, hence the depth advantages would be useful in frequency analysis, for determinations were even worse than for the Olkaria and instance. The work presented here uses the continuous Eburru networks; a large proportion of the activity was analogue (tape) data set. estimated to occur 3 or 4 km deep, close to the trial depth used in the location program. However, epicentre locations Data analysis close to the array should have been reasonably accurate. The strongest concentration of epicentres found in the Lake Earthquakes recorded on the analogue tape were identified Bogoria region was partially within their array, and formed by audio monitoring and paper records were produced for a 10 km long linear feature trending north-south immedi- all types of event (teleseismic, regional and local). Events ately beneath the Legisianana-Emsos escarpment (Fig. 1). with S-minus-P arrival times less than 3-4s, that is, those which occurred within approximately 30 km of the array, THE 1985 LAKE BOGORIA EARTHQUAKE were digitized from tape at a rate of 100 samples per second. PROJECT The seismograms were then displayed on a high resolution graphics terminal in order to pick the arrival times. Events Seismicity studies prior to 1985 had thus identified some were located by the program HYPOINVERSE,using the regions of seismic activity, and detailed studies in several velocity model for the centre of the network (Fig. 5) small areas had been attempted. However, those detailed determined from the KRISP85 refraction line (Henry et al. studies had been directed towards geothermal exploration 1990). This line crossed the Lake Bogoria area and the shot rather than general tectonic information. A number of the distribution ensured good control over the velocity structure aims of the Lake Bogoria project required that the network within the network (Henry 1987; Henry et al. 1990). The be situated in the immediate vicinity of the seismicity. V,/V, ratio was determined by constructing individual Firstly, it was intended to map active faults at depth and Wadati plots for 67 events which had clear arrivals and were therefore, it was necessary to record a large number of as evenly distributed as possible; the mean Vp/Vsratio was earthquakes for which not only the epicentres but also the found to be 1.74, with a standard deviation of f0.07 (Young depths could be well determined. Secondly, the project was 1989). Station corrections were calculated using station to study shear-wave splitting, which requires three- elevation above the refence level and the velocity of the component recordings of shear waves incident within the second layer (Fig. 5). The reference level used throughout shear-wave window. Thirdly, focal mechanisms of local this paper is 1430 m above sea level, being halfway between events can only be constrained if they occur within the the maximum and minimum station elevations. network. The Lake Bogoria region, as an area of known seismicity, was chosen as the network site in order to record Location errors as many local events as possible in the 3 months available for field operations. HYPOINVERSE uses Geiger’s method to determine an The Lake Bogoria network was the second earthquake earthquake’s hypocentre, origin time and their errors. network, after those of the Turkish Dilatancy Projects Although, it has been recognized that this method has (Evans et al. 1987), to be designed specifically for a limitations (e.g. Thurber 1986) HYPonwERsE remains in shear-wave splitting study. The aim was to cover the common use and is readily available. The program theoretical shear-wave window, that is, the circle at the calculates the size and orientation of the principal axes of ground surface within which earthquake arrivals have the standard error elipsoid for each earthquake location. incident angles of less than 35” (Evans 1984; Booth & Under the assumptions implicit in this method, there is a 95 Crampin 1985). Therefore, the array was small and dense per cent probability that the ‘true’ location is contained with a station spacing of approximately 6 km (Fig. 1) and within an error ellipsoid with major axes 2.4 times those of was centred on the concentration of epicentres found by the standard error ellipsoid (Klein 1978). Hamilton et al. (1973). Also, all but one of the stations (the An independent determination of likely location errors northernmost) were three-component sets. was carried out (Young 1989). A set of synthetic arrival We used Willmore Mark IIIA seismometers from which times for a hypocentre was constructed by ‘fixing’ an original the data were amplified and telemetered digitally, via the hypocentre, and then allowing HYPOINVERSE to calculate the Earth Data 9690 system, to the base camp, where they were expected arrival times at each station. These calculated recorded with a common time base. Five Geostore tape arrival times (with some realistic errors added) were then recorders provided the basic data set of continuous analogue used as input for HYPOINVERSE and a final location records. About half of the data was recorded at 161320 i.p.s. produced. The difference between the original point and the L'Gb yo D W- :?

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r. / major fault E x../ + caldera 0 10 krn uf fracture - a D earthquake 0.2'5 Figure 4. East-west cross-sections of the best located seismicity. Earthquake hypocentres within two strips 11 km wide have been projected Figure 3. Epicentres of 435 well-located earthquakes, each showing the principal axes of onto the vertical planes defined by A-A' and B-B' (Fig. 4). As depth its error ellipsoid. North of 0.05"N, the events plotted are those with epicentral errors constraint falls off outside the network, only events which occurred within within f1.5 km, depth errors within f2.5 km and location residuals less than 0.1 s. In 10 km of the nearest recording station have been plotted. The locations are order to retain a significant proportion of the epicentres south of 0.05"N,locations have further restricted to have horizontal and vertical errors within flkm and not been so restricted and events with epicentral errors within f2 km, no depth error residuals less than 0.1 s. The location error ellipsoids are indicated. restrictions and residuals of less than 0.5s are plotted. The letters A, A', B, B' etc. indicate the ends of cross-sections (Figs 5 and 6). 670 P. A. V. Young et al.

final location is an indicator not only of the magnitude of likely hypocentral error, but also of the direction of any error. The results show that for events deeper than 4 km, ______epicentre constraint is good (within k2 km) up to 30km sea level outside the network, and that the standard error calculated within HYPOINVERSE is of a similar magnitude and orientation to the difference between the original point and 10 - the final location. HYPOINVERSE is unable to determine the depths of shallow (less than 4 km deep) earthquakes outside -l the network and unusually large vertical errors are produced h when calculated arrival times for events in this region are input. Depth constraint falls off rapidly outside the network, :U and beyond 10 km from the network edge and deeper than Q aJ 4 km, the standard vertical error may underestimate the TJ I difference between the original and final locations. Note that this method takes no account of errors in the velocity model. Another indication of location accuracy is provided by groups or 'families' of similar earthquakes, each member of which produces very similar seismograms at each station. This similarity may be accounted for if all the members have a similar source and originate within a comparatively small Figure 5. Velocity-depth model for the Lake Bogoria region, from volume-at most a few 100 m across (Tsujira 1983; Love11 et Henry (1987; Henry et af. 1990). The reference level is 1430 m. of. 1987). A group of 15 such events (all of which occurred

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Figure 6. North-south cross-sections of the best located seismicity along lines C-C' and D-D' (Fig. 4). The same strip widths and error constraints as for Fig. 5 apply. Seismicity in the Kenya Rift 671 within 1 hr) is found 8 km outside the network. The depth (e.g. King 1978; Bosworth et of. 1986). Studies of calculated epicentres lie within 1 km in the N-S direction large earthquakes and their aftershocks in various tectonic and within 5 km in the E-W direction, but the depths are regimes indicate that large faults have dips between 30" and smeared out over a 10 km range. The calculated standard 60" (e.g. Jackson 1987). The bulk of the aftershock error ellipsoids do, at this distance from the network, reflect epicentres occur on the downthrown side of the surface the spread in locations for this family. Another family of fracture caused by the main earthquake (e.g. Stein & events near the centre of the network, where locations Barrientos 1985). Therefore, seismicity associated with a would be expected to be good, are indeed located in a tight large normal fault downthrown to the west might be cluster, with small standard errors in all directions. expected to lie to the west of the surface fault and its Within the real data set, locations for events to the south distance from the fault equal, very approximately, to its of the network have very large vertical standard errors. depth. There are several possible explanations for this. One is that On this basis, it can be seen that most of the activity the velocity structure in this area deviates significantly from located by the 1985 Lake Bogoria network appears to be the model used. Subsurface structure varies considerably associated with the larger, older faults on the Rift flank, along the axis of the Rift (Henry et af. 1990), and Henry rather than the younger grid faults (Figs 2 and 3). This is (1987) introduces a large high-velocity block into the unexpected, given that the episodes of major tectonic KRISP85 refraction model on a similar latitude to activity are thought to have occurred within progressively caldera. However, errors in the velocity model would not narrower and more centralized sectors throughout geologi- generally be expected to lead to sudden changes in cal time (McCall 1967). We conclude that at present minor the calculated errors. A more likely possibility is that the tectonic adjustments are occurring across the width of the southernmost earthquakes are much shallower than the Rift, not in the central grid-faulted zone alone. others, in which case calculated depth errors would be Most of the recorded seismicity is found to the east of the expected to be large (see the previous discussion). The base network, contained by and following the line of the of activity does tend to become shallower towards the south Marmanet-Laikipia and Solai faults (Fig. 3). As mentioned (cross-section D-D', Fig. 6), supporting this explanation. previously, it was probably the Marmanet-Laikipia fault which moved in 1928, therefore, its continued activity is not surprising, Hamilton et al. (1973) also located activity to the The earthquakes recorded northwest of Lake Bogoria, but that activity represented a lower proportion of the total compared with the present 572 earthquakes within 30 km of the network were large project. It is difficult to tell whether this is a true indication enough to be located. Their local magnitude determinations of a lateral shift of stress release, as their equipment may range from 0 to 2.7, with 81 per cent being less than 1.0 have been less sensitive and the sample sizes are small. (Young 1989). None of these earthquakes was large enough Baker et al. (1972, 1988) infer a major fault joining the to be felt at base camp (Fig. 1). The activity was generally Marmanet fault with the Solai fault via the E-W trending evenly dispersed in time and space throughout the recording small faults (Fig. 2), and this inference is supported by the period, although there were occasional bursts of activity clusters of activity in the curve of the Marmanet-Chui within a small area lasting perhaps an hour. Although the escarpment (Fig. 3). Also, after the 1928 earthquake, bulk of the activity is found outside the array, according to surface cracking along the base of the Marmanet fault was the above error analysis the locations are well enough found to bend slightly to follow this E-W fault zone (Willis determined to interpret their distribution (Figs 3, 4 and 6). 1936; Richter 1958). McCall (1967) notes that the seismic activity felt in the 1950s was mainly reported in Solai and Subukia, and seemed to emanate from the Laikipia Earthquake locations and surface faulting escarpment. The present concentration of activity within the There are several reasons for analysing the seismic activity Marmanet and Solai escarpments, therefore, extends back in relation to the surface faulting. An association of for at least 60 years. microseismicity with the young grid faulting has been There is some seismicity in the central trough. The most claimed, and used td support the hypothesis of a large axial obvious feature is a N-S trending narrow strip of intrusion beneath the Rift (e.g. Fairhead 1976; Mohr 1987). well-located events within the network. The grouping is This association is based on Tobin et al.'s (1969) 12 km long, 2 km wide and 8-9 km deep (see Figs 3, 4 and recconaissance survey. Does this claim hold up under a 6, cross-sections A-A' and C-C'). This linear feature more thorough investigation? Conversely, if seismicity does follows the trend of the grid faulting, but does not correlate not correlate with surface features, this may have important with any prominent surface fault. The activity is deeper than implications. Models of the deep structure of the Kenya Rift the small-scale grid faults would be expected to extend and tend to assume that there are only small-scale, shallow, forms a strip longer than most of the grid faults, which tend surface faults in the central trough (e.g. Mohr 1987; to branch or fade out within short distances. This linear Bosworth et af. 1986). However, there may be deeper faults group of events suggest the presence of a deep active fault in buried by the lavas and pyroclastics which have flooded the the central zone, a buried fault that is parallel with but on a central Rift. larger scale than the surface grid faults. Baker et al. (1988) When attempting to relate seismic activity to a surface also propose that older faults lie buried beneath the Rift fault, the possible dip of the fault at depth must be floor, in order to explain individual faults' extension rates considered. Although these normal faults appear to be near when compared with the average regional spreading rate. vertical at the surface, they are likely to become less steep at The dimensions and orientation of the linear group of 672 P. A. V. Young et al. events found by this project are remarkably similar to those percentage of total number of the concentration of activity found 13 years earlier by Hamilton et al. (1973). The 1985 strip of epicentres is 5 km to the NW of the earlier one, which cut beneath the Legisianana-Emsos escarpment (Fig. 1). It is unlikely that these two groups of activity, although so similar, are actually one and the same. The 1985 group is well within our network, its position is therefore well determined. As Hamilton er af.’s (1973) linear group was partially within ~~ 1 their network, its position was also fairly well determined; 7I activity 5 km to the NW would have led to very different sets of arrival times. There does not appear to be a constant 5 km offset between the two data sets, as the remainder of the epicentres show similar distributions. Like most of Hamilton et al.’s (1973) data set many of the events in their linear concentration locate at depths of 3-4 km, about 4 km shallower than the 1985 concentration. Although this is shallow enough to allow an association with the surface grid faulting, their strip of activity, like the 1985 grouping, cannot be related to any obvious surface feature. Thus we believe that the concentration of activity found by the previous workers also indicated the presence of a different 30 buried fault, and that the seismic activity has therefore Figure 7. Depth distribution for the 284 best located events which moved from one fault to another in the intervening years. have location‘I errors within fl km, residuals of less than 0.1 s and In general, the seismicity parallels the surface fault trends which occurred within 10 km of the nearest station. and there is no strong evidence for a seismic trend cross-cutting the Rift, which might have indicated the earthquakes. These events are provisionally interpreted as accommodation zone proposed by Bosworth et al. (1986). the movement of magma (Young 1989), as have similar Note that some dominant surface features, such as the events at this crustal level (Ukawa & Ohtake 1987; Walter Legisianana-Emsos-Bogoria fault, do not appear to be 1986). These deep events are thus not regarded as examples seismically active. The depth and distribution of the seismic of brittle failure. activity preclude a direct connection with geothermal The Kenya Rift is an area of high heat flow, which is activity, a fact recognized by Noble & Ojiambo (1975) from indicated by the geothermal activity as well as measure- Hamilton et d’s (1973) data. ments averaging 98 (f48) wMm-2 (Morgan 1982), and apparently has a crustal thickness of about 37 km, near the continental average (Henry 1987; Henry et al. 1990). The Earthquake depths and crustal rheology earthquake depth distribution (Fig. 7) shows that over 95 The earthquake depth distribution indicates the distribution per cent of the activity occurs above a depth of 12 km, and of brittle failure throughout the crust, and comparisons of that no ‘normal’ activity occurs below 16 krn. The depth of depth distributions from different tectonic regimes may be 12 km is taken here to be the typical cut-off depth, given used to highlight differences or similarities in crustal that most of the deeper events occur towards the limits of rheology. The depth variation of crustal strength depends the well-constrained locations. (A choice of a 16 km cut-off on, among other factors, the crustal lithology, geothermal depth would assume no location errors.) This cut-off depth gradient, crustal thickness and strain rate, whatever the of 12km is similar to that of 12-15km which has been exact modelling procedure used (Meissner & Strehlau 1982; reported for other young intracontinental areas (Chen & Sibson 1982; Kusznir & Park 1987). It can be an important Molnar 1983) with medium to high heat flow, 70- feature in Rift models (e.g. Bosworth et al. 1986; Bott & 105 wMm-2 (Sibson 1982; Meissner & Strehlau 1982). The Mithen 1983). The earthquake depth distribution of this cut-off depth appears flat on E-W sections (Fig. 6), but data set is thus discussed here from two viewpoints. Firstly, appears to shallow from north to south (Fig. 7), perhaps due is it normal or abnormal compared with other seismic to an increase of heat flow towards Menengai caldera, site of regions? Secondly, what (if any) inferences can be made the youngest volcanic activity in the area (Young 1989). about the rheology of this area? The Lake Bogoria activity has a sharp peak at 9-10 km The earthquakes were located implementing a variety of depth (Fig. 7). An increase in seismic activity towards the velocity models, including one with no velocity contrasts base of the seismogenic zone is observed in many places below 3.5 km, and using a range of VJV, ratios. In all cases (Meissner & Strehlau 1982), thus the Lake Bogoria activity the depth distribution and the number of ‘best’ located appears normal in this respect. Increased stress drops have events (those with standard errors within flkm) were very also been noted near the peak in background activity and similar (Young 1989). It can be seen from the earthquake large earthquakes (M,> 5.5) tend to originate at similar depth distribution (Fig. 7) and cross-sections (Figs 4 and 6) depths (Sibson 1982). A major earthquake in the Lake that a small group of earthquakes are unusually deep. All Bogoria region would therefore be expected to originate at a these events were of abnormally low frequency, 2-3Hz depth of 9-10 km. compared to approximately 8Hz for the majority of the The nature of the ‘transitional’ band between the brittle Seismicity in the Kenya Rift 673

(seismogenic) upper crust and the ductile (non-seismogenic) cooperation with this project. Many thanks to P. Bowry, M. lower crust is poorly understood, and its exact relationship Brooks, J. Davies, W. Henry, Kimani, R. Kiprop, D. to the seismicity depth distribution is debatable. For Mutea, and J. Patel for their assistance in the operation of instance, Sibson (1982) and Meissner & Strehlau (1982) the array. The UK Overseas Development Administration consider the peak of earthquake activity to occur within this funded the fieldwork, principally through a grant to transitional layer, whilst Jackson (1987) and Sholz (1987) Leicester University (R4149) and also through the Overseas argue that the transitional zone is usually ductile and breaks Development Administration & British Geological Survey only rarely in a large earthquake. It seems likely that the (BGS) Earthquake Prediction Research Programme. The earthquake cut-off depth actually represents the transition Natural Environment Research Council (NERC) provided a from semi-brittle or plastic behaviour to purely ductile flow, research studentship (GT4/85/GS/llO) and fieldwork but such a cut-off appears to be used to define the expenses for P. Young (previously P. Cooke) and supported ‘brittle-ductile transition’ (e.g. Fuchs el al. 1987; Chen & other aspects of the work at BGS. This work is published Molnar 1983). The brittle-ductile transition may therefore with the approval of the Director of the British Geological be defined at the cut-off depth of 12 km for crude two-layer Survey (NERC). modelling of crustal extension, for example. A number of workers model the variation of stress/strain/strength with depth in the crust (Meissner & Strehlau 1982; Sibson 1982; Kusznir & Park 1987; Tse & REFERENCES Rice 1987). Despite the fact that little is known of the Baker, B. H., Mohr, P. A. & Williams, L. A. J., 1972. Geology of conditions in the region of a deep fault and that large the eastern rift system of Africa, Spec. Pap. geol. SOC. Am., extrapolations from laboratory measurements are involved, l36. these models do predict a peak and cut-off in seismicity at Baker, B. H., Mitchell, J. G. & Williams, L. A. J., 1988. roughly the depth these phenomena are observed. As these Stratigraphy, geochronology and volcano-tectonic evolution of models usually assume a quartz-based rheology, at least in the Kedong--Kinangop region, Gregory Rift Valley, the upper crust, it appears that our observations at Lake Kenya, 1. geol. SOC. Lond., 145, 107-116. Booth, D. C. & Crampin, S., 1985. Shear-wave polarizations on a Bogoria are in keeping with a normal (i.e. quartz-based) curved wavefront at an isotropic free surface, Geophys 1. R. rheology. This is in contrast with the large proportion of astr. 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