Probabilistic Seismic Hazard Assessment of Southern Kenya

Abstract

Kenya’s Indian Ocean coastline is a passive continental margin that resulted from the breakup of the East and West Gondwanaland supercontinent during the Jurassic-Tertiary age and is consequently expected to have minimal seismic activity. However, earthquake data from 1938 to 2019 contains several shallow moderate earthquakes that occur both onshore and offshore. Previous works done in quantifying the seismic hazard in the region of Southern Kenya propose a quasi-uniform low seismic hazard or assume a uniform bedrock foundation with seismic hazard curves limited to Mombasa city. The main aim of the study was to carry out a detailed probabilistic seismic hazard assessment of the study area. The specific objectives included demarcation of the seismic source zones, computation of the peak horizontal earthquake ground accelerations and drafting an updated seismic hazard map of the area. The outputs are presented in terms of peak ground acceleration (PGA) in cm/s/s (gals) for a 10% probability of exceedance in 50 years with a return period of 475 years. Results indicate that there are three distinct seismic source zones covering the area, which align with the trends of major faults. PGA values range from 59.0 cm/s/s at Voi town in Taita-Taveta County, 68.4 cm/s/s at Malindi town in northern Kilifi County to 88 cm/s/s at Vanga town in southern Kwale County. Seismic hazard spectra indicate that PGA values are highest at short periods (<0.5seconds). Thus, the seismic hazard in the study area increases southwards. The PGA maps are an aid in seismic risk analyses for the preparation of County Seismic Building Codes. The seismic hazard curves and spectra are an aid in anti-seismic design of future critical civil engineering projects in the area of study.