Josphat Kyalo Mulwa

Chapter 19 - Seismic Hazard

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 and 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 11ᵒS to its intersection with the Kenyan coast at 2ᵒS, 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 EARS, 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 EARS. The earthquake focal mechanism indicates that the Davie Ridge is characterized by predominantly normal faulting with occasional oblique faulting. Consequently, Kenya and generally the East African coast are prone to both seismic hazards on land and tsunami-generating earthquakes. This chapter begins with general overview of the seismicity in Kenya from the 1900s to the present. Seismicity in Kenya up to 1963 is mainly based on macroseismic data while that from 1963 to the 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.