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Mulwa JK, Kimata F, Duong NA. "Seismic hazards in Kenya.". In: Developments in Earth Surface Processes. Amsterdam: Elsevier B.V.; 2013. Abstract

The East African Rift System (EARS), and by extension the Davie Ridge, which is considered as the seaward extension of eastern branch (Kenya Rift Valley) of the East African Rift Valley (Mougenot et al., 1986), are characterized by divergence whose maximum rate is estimated to be about 7 mm/year (Chase, 1978). This rate of divergence is somewhat much slower than that found at most active mid-ocean ridges, or even the convergence of India-Burma plates or that between the Australian-Sunda plates (Stein and Okal, 2006). Despite this slow rate of divergence, the East African Rift Valley and the Davie Ridge are characterized by frequent seismicity with large and shallow earthquakes occurring occasionally.

Seismic reflection, gravity and magnetic data from offshore East Africa allow the Davie Fracture Zone to be traced from 11°S to its intersection with the Kenyan coast at 2°S, constraining the relative motion of Madagascar and Africa (Coffin and Rabinowitz, 1987). Further, numerous faults and fractures probably associated with the Davie fracture have been mapped using recent gravity and magnetic data between latitudes 2o21'S and 3o03'S and longitudes 40o08'E and 40o45'E by Gippsland Offshore Petroleum Limited (2009). Seasat-derived free air gravity anomalies and slope/rise positive magnetic anomalies observed in shipboard data help to locate the continent-ocean boundaries (COB) off the shore of East Africa and Madagascar.
Furthermore, the East African Rift System, and precisely the Kenya Rift Valley is characterized by ~3 km thick sediments and normal faulting mechanism. Deformation has been active along the Kenya Rift valley as evidenced by high seismic activity. Surface deformation studies from SAR Interferometry in the southern sector of the Kenya rift valley in Magadi show that it is characterized by 14 cm of deformation over 10 km long stretches (Kuria et al., in press). If the Davie ridge is an extension of the East African Rift Valley, we cannot rule out the occurrence of tsunami generating earthquakes, which are bound to have devastating consequences on the eastern coast of Africa.

Earthquakes as deep as 40 km have been recorded below Davie Ridge (Grimison and Chen, 1988). However, evaluation of recent seismic data shows that magnitude 6.0 – 7.2 earthquakes at relatively shallow depths of 10 - 30 km are a common occurrence along the Kenya Rift Valley and the Davie Ridge in the Mozambique channel. The focal mechanism of these earthquakes supports what has previously been proposed that the Davie Ridge is a southward extension of the eastern arm of the East African Rift System. The earthquake focal mechanism indicates that the Davie ridge is characterized by predominantly normal faulting with occasional obligue faulting. Consequently, Kenya and generally the East African coast are prone to both seismic hazards on land as well as tsunami generating earthquakes.
Chapter 19 begins with general overview of the seismicity in Kenya from 1900s’ to present. Seismcity in Kenya up to 1963 is mainly based on macroseismic data while that from 1963 to present is based on data from instrumental recordings. In the past, a number of microseismic and seismicity studies in Kenya have previously been undertaken and the results from these studies are rather disjointed. In this chapter, we have made an attempt to merge all the existing results into one database from which the general seismicity, and subsequently seismic hazard in Kenya has been evaluated. The main goal of this chapter is to bring into focus the area(s) in Kenya more prone to seismic hazards either due to ground shaking occasioned by an earthquake or due to tsunami as a result of earthquakes occurring along the Davie ridge.

Mulwa JK, Kimata F, Suzuki S, Kuria ZN. "The seismicity in Kenya (East Africa) for the period 1906 – 2010: A review." Journal of African Earth Sciences. 2014;89(1):72-78. AbstractWebsite

Kenya has had a seismic station since 1963 as part of the World Wide Standardized Seismograph Network (WWSSN). In 1990, the University of Nairobi in collaboration with GeoForschungsZentrum (GFZ) started to build up a local seismological network, the Kenya National Seismic Network (KNSN), which operated for about ten years between 1993-2002. This, however, experienced a myriad of problems ranging from equipment breakdown, vandalism and lack of spares. Kenya is seismically active since the Kenya rift valley traverses through the country from north to south bisecting the country into eastern and western regions. In the central part, the Kenya rift branches to form the NE-SW trending Kavirondo (Nyanza) rift. The Kenya rift valley and the Kavirondo (Nyanza) rift are the most seismically active where earthquakes of local magnitude (Ml) in the order of 2.0 – 5.0 occur. Furthermore, historical records show that earthquakes of magnitudes of the order of Ml  6.0 have occurred in Kenya. Such large magnitude earthquakes include the January 6, 1928 Subukia earthquake (Ml 7.1) and an aftershock (Ml 6.2) four days later, as well as the 1913 Turkana region earthquake (Ml 6.2). Since early 1970’s, numerous seismic investigations have been undertaken in Kenya in order to understand the formation and structure of the Kenyan part of the East African rift valley. Earthquake data from these studies is, however, rather disorganized and individual datasets, including that acquired during the period 1993-2002, cannot furnish us with comprehensive information on the seismicity of Kenya for the past ~100 years. The purpose of this paper is, therefore, to review the seismicity in Kenya for the period 1906-2010 by utilizing data and results from different sources. The general seismicity of Kenya has been evaluated using historical data, data recorded by local seismic networks, the United States Geological Survey catalogue as well as earthquake data from the numerous seismic investigations by different individuals and research groups. On the basis of earthquake data from these sources, the entire N-S trending Kenya rift valley and the NE-SW trending Nyanza (Kavirondo) rift are characterized by a high rate of seismicity, and the USGS network has been effective in detecting local M > 3.0 earthquakes. A peculiar trend is exhibited by earthquakes of Ml  5.1 in that these occur along the N-S and NE-SW trending Kenya rift valley and the Kavirondo (Nyanza) rift zone respectively. Earthquake data from the various sources for the period 1906-2010 is complete for Ml  4.4 earthquakes with a b-value of 0.79 which is characteristic of tectonic active regions like rifts. There is need to revive and extend the KNSN for a greater coverage and effective seismic monitoring in Kenya.

Wamalwa RN, Nyamai CM, Ambusso WJ, Mulwa JK, WASWA AARONK. "Structural controls on the Geochemistry and output of the Wells in the Olkaria Geothermal Field of the Kenyan Rift Valley." International Journal of Geoscience. 2016;7:1299-1309. Abstract

The Olkaria geothermal field is located in the Kenya Rift valley, about 120 km from Nairobi. Geothermal activity is widespread in this rift with 14 major geothermal prospects being identified. Structures in the Greater Olkaria volcanic complex in- clude: the ring structure, the Ol’Njorowa gorge, the ENE-WSW Olkaria fault and N-S, NNE-SSW, NW-SE and WNW-ESE trending faults. The faults are more prom- inent in the East, Northeast and West Olkaria fields but are scarce in the Olkaria Domes area, possibly due to the thick pyroclastics cover. The NW-SE and WNW- ESE faults are thought to be the oldest and are associated with the development of the rift. The most prominent of these faults is the Gorge Farm fault, which bounds the geothermal fields in the northeastern part and extends to the Olkaria Domes area. The most recent structures are the N-S and the NNE-SSW faults. The geoche- mistry and output of the wells cut by these faults have a distinct characteristic that is the N-S, NW-SE and WNW-ESE faults are characterized by wells that have high Cl contents, temperatures and are good producers whereas the NE-SW faults, the Ring Structure and the Ol’Njorowa gorge appear to carry cool dilute waters with less chlo- ride concentration and thus low performing wells. Though the impacts of these faults are apparent, there exists a gap in knowledge on how wide is the impact of these faults on the chemistry and performance of the wells. This paper therefore seeks to bridge this gap by analysis of the chemical trends of both old wells and newly drilled ones to evaluate the impacts of individual faults and then using buffering technique of ArcGis estimate how far and wide the influence of the faults is. The data was ob- tained after the sampling and analysis of discharge fluids of wells located on six pro- files along the structures cutting through the field. Steam samples were collected with a stainless steel Webre separator connected between the wellhead and an atmospher- ic silencer on the discharging wells whereas the analysis was done in house in the KenGen geochemistry laboratory. The results indicates that Olkaria field has three categories of faults that control fluid flow that is the NW-SE trending faults that bring in high temperature and Cl rich waters, and the NE-SW trending Olkaria frac- ture tend to carry cool temperature waters that have led to decline in enthalpies of the wells it cuts through. The faults within the Ol Njorowa gorge act to carry cool, less mineralized water. Though initially, these effects were thought to be in shallow depths, an indication in OW-901 which is a deeper at 2200 m compared to 1600 m of OW-23 well that proves otherwise. This is, however, to be proved later as much deeper wells have been sited.

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