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Implications of the Distribution of Seismicity Near Lake Bogoria in the Kenya Rift

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Implications of the Distribution of Seismicity Near Lake Bogoria in the Kenya Rift Geophys. J. Int. (1991) 105, 665-674 Implications of the distribution of seismicity near Lake Bogoria in the Kenya 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 Nakuru 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 Lake Magadi 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 Lake Baringo 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.
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