DR. MULWA JOSPHAT KYALO

The Earth Processes course unit is one of the three core courses in Geology. The other two core courses are SGL 101 – Materials of the Earth and SGL 103 - Introduction to Paleontology. Geology is a science of the study of the earth with reference to its evolution, composition and processes that have prevailed from the time of its evolution to the present time.

The earth is a dynamic body that has undergone various changes. These changes are both of internal and external origin. The internal processes are referred to as diastrophism and they tend to elevate the earth’s surface. They are counterbalanced by the external processes that wear down the land surface. The constant interaction between these two processes determines the configuration of the earth’s surface. The external processes are as a result of solar energy and gravitational forces whereas the internal processes are as a result of the earth’s internal heat.

Weather pattern, for example, is to a large extent due to the solar energy on the one hand. Along the equator there is a substantial amount of heating because the sun is always overhead and therefore this results in the rising up of hot air. The rising hot air is replaced by cold air from the colder regions. This cyclic process is closely related to ocean waves and currents generated by solar heating. Waves are effective forces for determining the shape of the landscape along shorelines of oceans and seas.

The force of gravity on the other hand is due to mutual attraction between bodies. The greater the attracting bodies the greater the gravitational force. Because the mass of the earth is greater than any other body on its surface, materials are attracted towards the earth’s center. Rain and snow precipitate due to gravitational attraction of the earth. Water moves towards the oceans because of gravitational force. Glaciers on higher mountains are normally pulled down by the force of gravity.

Internal earth processes are due to heat energy which keeps rocks in the mantle below the earth’s crust in a molten state. This break forth as a volcanic flow during volcanic eruptions. Heat energy is also responsible for large-scale processes such as earthquakes and mountain building, and small scale processes such as geysers, hot springs, steaming ground and hydrothermal processes.

We can therefore conclude that all earth processes are manifestations of energy and these processes are responsible for sculpturing the land surface.

The Earth Processes course unit begins with an overview of the types of energy which contribute to earth processes. These are discussed in Lecture 1. The unit is thereafter subdivided into two parts. Lectures in Part I of the course unit discusses the External Earth processes where else lectures in Part II of the unit discusses the Internal Earth Processes.

The general objective of the Earth processes course unit is to introduce you to the basic concepts of geosciences. More specifically, at the end of this course unit you should be able to:

 describe the internal and external processes which shape the earth;
 explain the present configuration of the earth and attempt to reconstruct its original form;
 explain the natural processes of the earth;
 categorize hazardous and non-hazardous processes of the earth;
 outline the contribution of the earths natural fields in exploration of natural resources;
• propose measures of minimizing hazards due to earth processes.

You are required to have writing materials e.g. books or foolscaps, pens, lead pencils, coloured pencils or crayons, a scientific calculator, a ruler and a mathematical set. Although every effort has been made to provide you with an up-to-date lecture notes, you are expected to do further reading for a better understanding of Geology, Geological concepts and Processes.

Practicals are compulsory in this course unit and a separate practical manual will be availed to you.

Arus-Bogoria geothermal prospect, located in the central Kenya rift valley (KRV), encompasses several features of geological significance that are indicators of possible geothermal potential. These include surface manifestations, such as fumaroles, steam jets, mud pools, hot springs, spouting geysers, and high rate of micro-seismic activity of about 500 earthquakes recorded within a period of three months in comparison to other geothermal fields and prospects along the Kenya rift valley (KRV).

A comparison of the results of gravity survey, undertaken between 2005 and 2006 for geothermal resource evaluation of Arus and Lake Bogoria geothermal prospects, to results of micro-seismic monitoring undertaken in 1985 during the Kenya Rift International Seismic Project (KRISP 85) was undertaken to map the existence of heat source(s), presumably due to dyking, and define the brittle-ductile transition zone. The results indicate that the heat source is due to a series of north-south trending dyke injections occurring at depths of ~3 – 6 km in the vicinity of the Arus steam jets. The geothermal prospect is seismically active and approximately 95% of the seismic activity is probably associated with tectonic activity due to reactivation of north-south trending faults.

Further, only ~5% of micro-earthquakes can be correlated with the geothermal activity such as dyking, as mapped using gravity data, and hydrothermal processes. The change in seismic activity at Arus-Bogoria geothermal prospect occurs at a depth of 8 – 15 km with a peak in micro-seismic activity at 12.5 km depth. We therefore conclude that 8-15 km represents the brittle-ductile transition zone in Arus-Bogoria geothermal prospect.

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.

The May 20, 1990 earthquake which occurred in southern Sudan is so far the strongest earthquake to occur in the eastern part of African continent within the past 21 years. It caused damage in southern Sudan as well as severe shaking in parts of Uganda and Kenya, and was accompanied by aftershocks on May 24, 1990 of moment magnitudes Mw = 6.5 and 7.1.

Inversion of teleseismic body-waves has been undertaken for the purpose of this study in an attempt to reassess the seismo-tectonics of northern and central Kenya as well as southern Sudan. The results show that the best solution for the inversion of teleseismic body waves for the May 20, 1990 earthquake consists of only one event with a source mechanism of 315o/84o/-3o (strike/dip/rake) and the fault plane is characterized by left-lateral strike-slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment Mo = 7.65 x 1019 Nm and moment magnitude, Mw = 7.19 (7.2). The fault rupture started 15 seconds earlier and lasted for a duration of 17 seconds along a fault plane having dimensions of length  60 km and width  40 km. The average dislocation along the fault is 1.1 m and the stress drop,  is 1.63 Mpa.

The distribution of historical earthquakes from southern Sudan through central Kenya shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW-SE alignment of epicenters is characterized by earthquakes of local magnitude Ml  4.0. This NW-SE alignment of epicenters confirms the existence of an active fault zone, the Aswa-Nyangia fault zone, from southern Sudan through central Kenya and further into the Indian Ocean. However, owing to lack of waveform data for these historical earthquakes, it is not possible to determine the source mechanism of the fault. Further work on inversion of short period waveform data is required so as to precisely determine the fault mechanism of this NW-SE trending fault zone in the central and southeastern parts of Kenya.

A systematic approach has been applied in the selection of suitable sites for borehole drilling in a quest to provide adequate water supply to a rural pastoral community in Lolmolok area. The study area lies in samburu district in Kenya and is bound by latitudes 0°56’21”N and 0°57’58”N and longitudes 36°34’42”E and 36°36’35”E. The geology of this area is comprised of tertiary volcanics. Basalts, which have weathered into residual black cotton soil, are underlain by phonolitic lavas and tuffs. The systemat-ic approach for the exploration of groundwater was followed to enable selection of an optimum drill site(s) within a quadrant with three-kilometer radius identified by the pas-toral community. The approach consisted of the following multi-steps:-
(i) Hydrogeological reconnaissance of the whole area, mapping different groundwa-ter potential areas on the basis of aerial photo interpretation;
(ii) Geophysical field surveys involving very low frequency electromagnetic (VLF-EM) and Vertical Electrical Sounding (VES).
(iii) Processing and interpretation of the data acquired in the field, which led to selection of suitable drill sites, indication of potential yield and depth of aquifers.
This paper describes the success of combined geophysical survey techniques in siting boreholes whose yield ranges between 5 m3/hr and 10 m3/hr.

Groundwater is a natural resource of the earth that sustains and supports domestic, agricultural and industrial activities. It is distributed fairly and evenly throughout the world and over half of the world’s population depends on groundwater for drinking water supplies. Its usage is increasing due to rapid population growth, high rate of urbanization, industrial growth and agricultural utilizations. This has resulted to rapid depletion of groundwater which leads to water stress and degradation of these resources. The situation is further worsened by inadequate information on groundwater resource which has been and is still a big obstacle to the proper management of these resources. Remote sensing and GIS techniques have emerged as very effective and reliable tools in the assessment, monitoring and conservation of groundwater resources. This paper has made an attempt to identify and delineate groundwater potential zones in Lake Chala Basin in Kenya using Remote sensing and GIS. In the process of groundwater delineation in the area, different thematic maps on lithology, land use/land cover, drainage density, slope and rainfall were prepared, assigned with different weighting values as per their importance on groundwater occurrence and overlaid using spatial analyst tool in ArcGis 10 to generate groundwater potential map. The generated groundwater potential zone map was classified into four groundwater potential zones namely, very high, high, moderate and low. The study revealed that the area has very high groundwater potential. The generated groundwater potential map will be used for further groundwater exploration, proper planning, sustainable utilization and management of groundwater resources in the Lake Chala Watershed.

Over the last 5-10 years, there has been accelerated geothermal resources development for geothermal power generation in order to meet the country’s electricity demands and hence make Vision 2030 feasible. However, owing to the tectonic setting of Kenya, earthquakes are likely to pose a threat and subsequent challenge to attaining Vision 2030 despite the numerous efforts made by the power sector players.

Earthquakes in Kenya are common along the Kenya Rift Valley due to the slow divergent movement of the rift and hydrothermal processes within the geothermal fields. This implies slow but continuous radiation of seismic energy which relieves stress in the subsurface rocks. It is therefore unlikely that the Kenya rift poses significant earthquake hazard in Kenya.

On the contrary, the NW-SE trending rift/shear zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift zones have been the sites of a number of strong earthquakes in the past such as the Mw = 7.2 southern Sudan earthquake of May 20, 1990 and aftershocks of Mw = 6.5 and 7.1 on May 24, 1990; the 1937 Ms = 6.1 earthquake north of Lake Turkana close to Kenya-Ethiopian border, and the 1913 Ms = 6.0 Turkana earthquake among others.

We have attempted to determine the source parameters of the May 20, 1990 southern Sudan earthquake by inversion of teleseismic body-waves, and the implications of this earthquake on the seismotectonics of southern Sudan, as well as the central and southern parts of Kenya. The results of teleseismic body-waves inversion show that the best solution for the May 20, 1990 southern Sudan earthquake consists of only one event on a fault having strike, dip and rake of 315o/84o/-3o. The fault plane is characterized by left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment Mo = 7.65 x 1019 Nm and moment magnitude, Mw = 7.12 (7.2). The fault rupture started 15 seconds earlier and lasted for a duration of 17 seconds along a fault plane having dimensions of length  60 km and width  40 km. The average dislocation along the fault is 1.1 m and the stress drop, , due to this earthquake is 1.63 Mpa.

The distribution of historical earthquakes from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW-SE alignment of epicenters is characterized by earthquakes of local magnitude Ml  4.0, except the 1928 Subukia earthquake (Ms = 6.9) in central Kenya. This NW-SE alignment of epicenters is consistent with the trend of Aswa-Nyangia fault zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean.

We therefore conclude that, the NW-SE trending rift/shear zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.

Arus-Bogoria geothermal prospect encompasses several features of geological significance that are indicators of possible geothermal potential. These include surface manifestations, such as fumaroles, steam jets, mud pools, hot springs, spouting geysers, and high rate of micro-seismic activity of about 500 earthquakes recorded within a period of three months in comparison to other geothermal fields and prospects along the Kenya Rift Valley (KRV).

A comparison of the results of gravity surveys undertaken between 2005 and 2006 for geothermal resource evaluation of Arus and Lake Bogoria geothermal prospects, to results of micro-seismic monitoring undertaken in 1985 during the Kenya Rift International Seismic (KRISP 85) Project was undertaken to map the existence of heat source(s). The results indicate that the heat source may be due to a series of NS trending dyke injections occurring at depths of ~3 – 6 km in the vicinity of the Arus steam jets. Some of the seismic activity is also probably associated with tectonic activity due to reactivation of NS trending faults.

The Lake Bogoria basin, here in referred to as the study area, is located in the greater Baringo-Bogoria basin (BBB), about 100 km to the north of Menengai geothermal prospect on the floor of Kenya Rift Valley (KRV). It is bound by latitudes 0o 00’ and 0o 30’N and longitudes 35o45’E and 36o15’E within the rift graben. The study area is characterized by geothermal surface manifestations which include hot springs, spouting geysers, fumaroles/steam jets and mud pools. The area is overlain by Miocene lavas lavas, mainly basalts and phonolites, and Pliocene to recent sediments and pyroclastics such as tuffs, tuffaceous sediments, superficial deposits, volcanic soils, alluvium and lacustrine silts. The terrain is characterized by extensive faulting forming numerous N-S ridges and fault scarps.
Gravity and magnetotelluric (MT) surveys were undertaken in the area to determine the heat source, characterize the geothermal reservoir, and evaluate the geothermal resource potential of the basin.
Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north-South direction and interpreted to be due to a series of dyke injections and hence the heat source in the basin. The interpretation of Bouguer anomaly has been constrained by using previous seismic results. The MT survey results show three distinct layers in the basin. The first layer, overlain by high resistivity thin layers, is ~3 km thick and has resistivities ranging between 4-30 -m. This layer is interpreted to be due to a combination of saline sediments and circulation of high temperature geothermal fluids. The second layer is ~10 km thick and resistivity values range between 85-2500 -m. This layer is interpreted to be fractured basement metamorphic rocks hosting a steam reservoir where circulating fluids are heated by underlying dyke injections. The substratum is characterized by resistivities ranging between 0.5-47 -m and is interpreted as hot dyke injections which are the heat sources for this geothermal prospect. The magnetotelluric results in this study are consistent with results of previous microseismic study in Lake Bogoria basin by Young et al. (1991).
On the basis of gravity and MT results, the heat source in Lake Bogoria basin is due to cooling dyke injections occurring at depths of ~6 – 12 km in the subsurface. Gravity method however favours depths of ~3 – 6 km. The geothermal reservoir is probably two-phase and is attributed to condensation of high temperature steam from the underlying fractured basement metamorphic rocks.

The Rotoma-Tikorangi geothermal prospect is situated about 26 km northeast of Rotorua in New Zealand. It is associated with Okataina Volcanic Centre in the Taupo Volcanic Zone. A low level airborne magnetic survey (760 m a.s.l) was undertaken in 1984 for the purpose of delineating the geothermal prospect. Detailed analysis of the aeromagnetic data shows that some residual total force magnetic anomalies are independent of terrain effects. Positive magnetic anomalies are probably due to young rhyolites whereas negative magnetic anomalies are due to hydrothermal alteration. A three dimensional (3-D) magnetic interpretation indicated that hydrothermally demagnetized rocks associated with Rotoma-Tikorangi geothermal prospect have variable thickness ranging between 800 and 1250 m and dip towards the north. The geothermal prospect is characterized by concealed outflow. The results of this study also suggest that the Rotoma-Tikorangi geothermal prospect extends further to the east of a resistivity boundary delineated from previous studies.

Kenya is the first and so far the only country in the African continent to generate electricity from geothermal resources. Currently the power output from geothermal resources stands at about 57 MWe. This output is expected to rise with the planned commissioning of other power plants in Olkaria and elsewhere. Geothermal energy is reliable, environmentally sustainable and the least cost source of base load power for Kenya. The least cost power development plan (KPLC, 2001) has proposed that the geothermal sources provide approximately an additional 500 MWe of base load electric power over the next 20 years (Omenda, 2001; Mwangi, 2001). It should be noted that the Rift System in Kenya has a potential of producing 2000 MWe of geothermal energy that can be generated using conventional steam condensing turbines. This power generation can even exceed 3000 MWe when combined cycle and binary systems are used (Omenda, 2001). For these reasons, exploration for geothermal resources is quite active within the Kenya Rift System and the potential to be exploited is quite substantial.

Geothermal resource exploration has largely been undertaken by the Kenya Electricity Generating Company (KenGen) and the Ministry of Energy. There has also been an input from international organizations and consulting companies. Nevertheless, there has been low level contributions by the local universities in geothermal resource studies and research works. In other countries such as U.S.A, New Zealand, Iceland, Japan, Philippines, Indonesia, just to mention a few, geothermal resources have been collaboratively studied and researched on by both the university and the industry and there is no satisfactory reason why this cannot be emulated in our continent. This paper therefore outlines some of the areas where collaborative work can be undertaken by both the industry and the local universities. Some of the areas include feasibility studies, exploration, construction and installation, production, research and development. This is essential for better knowledge dissemination, improvement and training for posterity.

Rotoma-Tikorangi geothermal prospect is situated about 26 km northeast of Rotorua in New Zealand. It is associated with Okataina Volcanic Centre in the Taupo Volcanic Zone. A low level airborne magnetic survey (760 m a.s.l) covering the Rotoma-Tikorangi prospect was undertaken by staff of the Geothermal Institute in 1984. Detailed analysis of the aeromagnetic data carried out as part of the present study showed that some residual total force magnetic anomalies are independent of terrain effects. Positive magnetic anomalies are probably due to young rhyolites whereas negative magnetic anomalies are due to hydrothermal alteration. A three dimensional (3-D) magnetic interpretation indicated that hydrothermally demagnetized rocks associated with Rotoma-Tikorangi geothermal prospect have variable thickness ranging between 800 and 1250 m and dip towards the north. The geothermal prospect is characterized by concealed outflow. The results of this study also suggest that the Rotoma-Tikorangi geothermal prospect extends further to the east of a resistivity boundary delineated from previous studies. Detailed modeling of the demagnetized anomalies along respective flight lines is recommended as a follow-up to this study, to determine more accurately the extent of the hydrothermally demagnetized rocks.

The Olkaria geothermal field is located in the Kenyan Rift valley, about 120 km from Nairobi. Development of geothermal resources in the Olkaria area, a high temperature field, started in the early 1950s. In the subsequent years numerous expansions have been carried out with additional power plants being installed in Olkaria. These include a binary plant at Olkaria South West (Olkaria III) in 2000, a condensing plant at Olkaria North East (Olkaria II) in 2003, another binary plant at Olkaria North West (Oserian) in 2004 and finally condensing plants in the year 2014 within East production field (EPF) and Olkaria Domes (OD) areas. The total generation from this field is about 730 Mw. The study considered samples from 4 producing wells from 3 fields of the Olkaria geothermal area (OW-44 from the Olkaria East, OW-724A from the Olkaria North East, and OW-914 and OW-915 from the Olkaria Domes field). The chemical data were first analyzed using SOLVEQ. This helped in the determination of the equilibrium state of the system, the reservoir temperatures and the total moles to be run through CHILLER. The run CHILLER considered the processes that have been proven to be occurring in the Olkaria field i.e., boiling and condensing processes, fluid-fluid mixing rocks and titration resulting from water-rock interaction. The effects on gas evolution were evaluated based on the resulting recalculated gas pressures. The results indicate that the gas species are not in equilibrium with the mineral assemblages. The CHILLER evaluation shows boiling as the major process leading to the evolution of gases. OW-44 had the least gas concentrations, arising from the considered reservoir processes due to degassing, and near surface boiling, besides the removal of NH3 , H2 and H2S are through the reaction with steam condensate. The gas breakout is most likely in OW-914 and least in OW-44. The study proposes different reservoir management strategies for the different parts of the Olkaria geothermal field. That is by increasing hot reinjection in the eastern sector around well OW-44. The reservoir around OW-914 is to be managed by operating the wells at a minimum flow rate (or even to close them) or the use of chemical inhibitors to prevent calcite scaling.

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.

A geo-electrical investigation was carried out in Lake Chala Watershed in Kenya to determine the roundwater potential of the area. The Vertical Electrical Sounding using schlumberger configuration with a maximum current electrode spread varying from 250 320m and potential electrode spread of 25m was utilized to provide information of the aquifers and subsurface lithology. A total number of 50 VES were carried out. The data obtained were interpreted by computer iteration process. Interpreted results revealed four to six distinct subsurface layers which comprised of top soil (clay, sandy clay soil intercalated with silt, sand and gravel)
, highly weathered ryholite, and moderately weathered basalt volcanic ash, highly weathered fractured basalt, weathered basalt, slightly fractured dry and fresh basalt and basalt basement rock layers. The results showed that the auriferous layer was composed of highly weathered fractured basalt, moderately weathered basalt & volcanic ash and weathered basalt geological material. The layer had a resistivity range of 40 to 200 and a thickness range of 1.38 to 91m. The results showed that lake chala watershed have high groundwater potential which can be exploited as an alternative source of water in the area.

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.

Earthquakes in Kenya are common along the Kenya Rift Valley due to the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the Mw = 7.2 southern Sudan earthquake of May 20, 1990 and aftershocks of Mw = 6.5 and 7.1 on May 24, 1990; the 1937 Ms = 6.1 earthquake north of Lake Turkana close to Kenya-Ethiopian border, and the 1913 Ms = 6.0 Turkana earthquake among others.

Source parameters of the May 20, 1990 southern Sudan earthquake shows that, this earthquake consists of only one event on a fault having strike, dip and rake of 315o/84o/-3o. The fault plane is characterized by left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment Mo = 7.65 x 1019 Nm and moment magnitude, Mw = 7.19 (7.2). The fault rupture started 15 seconds earlier and lasted for 17seconds along a fault plane having dimensions of  60 km x 40 km. The average fault dislocation is 1.1 m and the stress drop, , is 1.63 MPa.

The distribution of historical earthquakes (Mw ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW-SE alignment of epicenters is characterized by earthquakes of local magnitude Ml  4.0, except the 1928 Subukia earthquake (Ms = 6.9) in central Kenya. This NW-SE alignment of epicenters is consistent with the trend of Aswa-Nyangia fault zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean.

We therefore conclude that, the NW-SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.

Arus-Bogoria geothermal prospect encompasses several features of geological significance that are indicators of possible geothermal potential. These include surface manifestations, such as fumaroles, steam jets, mud pools, hot springs, spouting geysers, and high rate of micro-seismic activity of about 500 earthquakes recorded within a period of three months in comparison to other geothermal fields and prospects along the Kenya Rift Valley (KRV). A comparison of the results of gravity surveys undertaken between 2005 and 2006 for geothermal resource evaluation of Arus and Lake Bogoria geothermal prospects, to results of micro-seismic monitoring undertaken in 1985 during the Kenya Rift International Seismic (KRISP 85) Project was undertaken to map the existence of heat source(s). The results indicate that the heat source is due to a series of north-south trending dyke injections occurring at depths of ~3 – 6 km in the vicinity of the Arus steam jets. Most of the seismic activity is probably associated with tectonic activity due to reactivation of north-south trending faults.

Lake Chala is a transboundary fresh water resource with no surface water inflow or outflow and is located in the southwestern part of Kenya on the Kenya-Tanzania border. The lake catchment area is bound by longitudes 370 41’ E and 370 43’ E and latitudes 30 18’ S and 30 20’ S. The Lake has a surface area of 4.2 km2 and lies within a surface catchment area of about 16.23 km2, which falls within a semiarid region frequently facing severe water scarcity especially during periods of prolonged drought. The major economic activities in this area are agriculture, horticulture and animal husbandry which account for about 75-80% of household income. Due reliance on rain fed agriculture, water scarcity has often had negative impact on the people and there is need to tap the lake water for irrigation purposes. As such, water samples were collected on the Kenya and Tanzania sides from eleven (11) sites in March 2011 and subjected to analysis for chemical characteristics. Ten of the water samples show that the type of water that predominates in the study area is Ca-Mg-HCO3 type, while one water sample from a shallow well is a Ca-Mg-chloride type based on hydro-chemical facies. The suitability of water for irrigation has been evaluated based on sodium percent, residual sodium carbonate, sodium adsorption ratio and salinity hazard and is therefore suitable for irrigation purposes.

The Lake Bogoria basin, here in referred to as the study area, is located in the greater Baringo-Bogoria basin (BBB), about 100 km to the north of Menengai geothermal prospect on the floor of Kenya Rift Valley (KRV). It is bound by latitudes 0o 00’ and 0o 30’N and longitudes 35o45’E and 36o15’E within the rift graben. The study area is characterised by geothermal surface manifestations which include hot springs, spouting geysers, fumaroles/steam jets and mud pools. The area is overlain by Miocene lavas mainly basalts and phonolites, and Pliocene to recent sediments and pyroclastics such as tuffs, tuffaceous sediments, superficial deposits, volcanic soils, alluvium and lacustrine silts. The terrain is characterized by extensive faulting forming numerous N-S ridges and fault scarps.

Gravity and magnetotelluric (MT) surveys were undertaken in the basin in an attempt to determine the heat source, characterize the geothermal reservoir, and evaluate the geothermal resource potential of the basin.

Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north-South direction and interpreted to be due to a series of dyke injections and hence the heat source in the basin. The interpretation of Bouguer anomaly has been constrained by using previous seismic results. Seismic velocities were converted to densities using the expression derived from Gardner et al. (1974). The MT survey results show that the geothermal prospect in Lake Bogoria basin is overlain by high resistivity (50-1000 -m) thin (100-500 m) layer which forms a cap rock for the geothermal reservoir, which is subsequently underlain by three distinct relatively thick layers within the geothermal prospect. The first of these thick layers is ~3 km thick and has resistivities ranging between 4-30 -m. This layer is interpreted as the geothermal reservoir and the low resistivities are due to circulating hot mineralized geothermal fluids. The underlying layer is ~10 km thick and resistivity values range between 85-2500 -m and is interpreted to be a fractured and hydrothermally altered basement metamorphic rocks. The relatively high degree of fracturing allows deep circulation of water where it gets heated up by the underlying dyke injections, and convective heat transport to the geothermal reservoir. The substratum is characterized by resistivities ranging between 0.5-47 -m and is interpreted as hot dyke injections which are the heat sources for this geothermal prospect.

On the basis of gravity and MT results, the heat source in Lake Bogoria basin is due to cooling dyke injections occurring at depths of ~6 – 12 km in the subsurface. Gravity method however favours depths of ~3 – 6 km for the heat source. The geothermal reservoir is probably two-phase and the temperature ranges between 150-400oC (Karingithi, 2006). Previous microseismic studies by Young et al. (1991), Tongue (1992) and Tongue et al., (1992, 1994) show that Lake Bogoria basin geothermal prospect is characterised by high frequency of low magnitude (< 3) seismic events which are correlated with surface faulting and multiple episodes of dyke injections. The gravity and MT results in this study are therefore fairly consistent with results from previous microseismic studies undertaken in the basin.

The Lake Bogoria basin, here in referred to as the study area, is located in the greater Baringo-Bogoria basin (BBB), about 250 km to the north of the city of Nairobi and about 100 km to the north of Menengai geothermal prospect on the floor of Kenya Rift Valley (KRV). It is bound by latitudes 0o 00’ and 0o 30’N and longitudes 35o45’E and 36o15’E within the rift graben. The study area is characterised by geothermal surface manifestations which include hot springs, spouting geysers, fumaroles/steam jets and mud pools. The area is overlain by Miocene lavas lavas, mainly basalts and phonolites, and Pliocene to recent sediments and pyroclastics such as tuffs, tuffaceous sediments, superficial deposits, volcanic soils, alluvium and lacustrine silts. The terrain is characterized by extensive faulting forming numerous N-S ridges and fault scarps.

Gravity survey was undertaken in the study area to map the possible heating source and evaluate the geothermal resource potential of the basin. Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north-South direction and interpreted to be due to a series of dyke injections and hence the heat source in the basin. The interpretation of Bouguer anomaly has been constrained by using results from previous seismic surveys undertaken in the Kenya rift valley by Simiyu and Keller (2001), Keller et al., (1994a) and Braile et al., (1994). P-wave velocities have been converted to densities using the equation derived from Gardner et al. (1974).

Microseismic studies in Lake Bogoria basin by Young et al. (1991), Tongue (1992) and Tongue et al., (1992, 1994) show that the basin is characterised by high frequency of low magnitude (< 3) seismic events which are correlated with surface faulting and multiple episodes of dyke injections. The gravity results in this study are therefore fairly consistent with results of previous microseismic studies undertaken in the basin.

On the basis of the gravity results, the heat source in Lake Bogoria basin is due to cooling dyke injections occurring at depths of ~3 – 6 km. Since a heat source, in addition to fluid circulation, is an integral component of a geothermal system, it is evident that a geothermal reservoir exists in Lake Bogoria basin and the fluids can be tapped for generation of geothermal power.

Groundwater is increasingly becoming an important resource within and around the Ngong area, of Kenya. This is mainly as a result of increase in human population due to rural-urban migration, industrial expansion and farming activities. Ngong area forms a water catchment zone to the west of the city of Nairobi in Kenya. The geology of the area is comprised of volcanic layers of basalts, trachytes, phonolites and tuffs all overlain by thick layers of clay soil. The soil retains water for an extended period of time after the rains and this partly recharges aquifers in the area. Basalts and trachytes form good aquifers whereas tuffs are aquifers only when fractured. Faults delineated from a study of aerial photographs trend in a North-South direction in conformity with the structural pattern of the eastern branch of the Great Rift Valley. The influence of the faults on groundwater is two fold. They act as drainage channels to the flow of groundwater and also as aquifers in the area.

The Lake Bogoria basin, herein referred to as ‘the study area’, is located in the greater Baringo-Bogoria basin (BBB), about 250 km from the city of Nairobi on the floor of Kenya Rift Valley (KRV). It is bound by latitudes 0o 00’ and 0o 30’N and longitudes 35o45’E and 36o15’E within the rift graben. The study area is characterised by geothermal surface manifestations which include hot springs, spouting geysers, fumaroles/steam jets and mud pools. The area is overlain by Miocene lavas mainly basalts and phonolites, and Pliocene to Recent sediments and pyroclastics such as tuffs, tuffaceous sediments, superficial deposits, volcanic soils, alluvium and lacustrine silts. The terrain is characterized by extensive faulting which forms numerous N-S ridges and fault scarps.

Gravity and magnetotelluric (MT) surveys were undertaken in the area in order to determine the heat source and evaluate the geothermal resource potential of the basin for generation of geothermal power. The gravity data used was from the University of Texas at El Paso and Leicester University gravity data bases. New gravity measurements’ comprising 260 data points was undertaken for the purpose of this study. In addition, magnetotelluric data comprising about fourty sites was also acquired in the study area.

Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north-south direction and this has been interpreted to be due to a series of dyke injections and hence the heat source in the basin. The dyke injections occur at depths of 3-6 km on average, but at 1 km depth at the shallowest. The gravity models show a north-south gradual variation in thickness of the uppermost low density layer comprising rift-fill volcanics from 1-4 km on average. The variation in thickness of this layer from south-north suggests that these volcanic deposits are as a result of volcanic eruption(s) outside Lake Bogoria basin such as Menengai to the south.

The MT survey results show three distinct relatively thick layers in the basin. The first of these layers, which is overlain by high resistivity (50-1000 m) thin (100-500 m) layers, is ~3 km thick and has resistivities ranging between 4-30 -m. This layer is interpreted as the geothermal reservoir and the low resistivities are due to a combination of circulating hot mineralized geothermal fluids, hydrothermal alteration and saline sediments. The second layer is ~10 km thick and resistivity values range between 85-2500 -m and is interpreted to be a fractured and hydrothermally altered basement metamorphic rock. The relatively high degree of fracturing has considerably enhanced circulation of water which gets heated by the underlying dyke injections and thus inducing convective heat transport to the geothermal reservoir. The substratum is characterized by resistivities ranging between 0.5-47 -m and is interpreted as hot dyke injections at depths of about 6-12 km, which are the heat sources for the geothermal system.

Consequently, a heat source and a geothermal reservoir exist in Lake Bogoria basin. The heat source(s) is/are due to cooling dyke injections occurring at depths of 3-6 km on average, but 1 km depth at the shallowest near Arus where steam jets and fumaroles occur. The magnetotelluric method, however, favours depths of 6-12 km for the heat source and this may be attributed to lack of significant resistivity contrast between the dyke injections and the basement rocks where the former have intruded the latter rocks.
More gravity data is warranted so as to precisely define the geometry and areal extent of the heat source in Lake Bogoria basin. However, based on the results of this study, it is recommended that:- 1) exploratory drilling be undertaken in the area near Arus steam jets, 2) even though the study area is not prone to any pre-historic eruptions, microgravity and seismic monitoring be undertaken so as to help in tracking possible magma migration and variations in magma input. Such data could, in turn, play an important role in predicting future eruptive events in Lake Bogoria basin.