Probabilistic Seismic Hazard Assessment for Kenya and Adjoining Areas With Emphasis on Populated Areas and Sites

Abstract

Probabilistic seismic hazard assessment (PSHA) modeling aims to forecast earthquake occurrence and the resultant ground shaking. We present probabilistic seismic hazard maps at 10% and 2% probability of exceedance (PoE) in 50 years for a return period of 275 and 2475, resulting to 0.01g to 0.30g and 0.01g to 0.46g, respectively. The results at selected urban centers of Nairobi, Nakuru, Mombasa, and Kisumu show an exceedance probability from 0.0038g to 0.22g. The hazard level in Kenya is moderate, ranging from medium to high level of seismic hazard. A new seismicity map comprising 19 catalogs, updated and unified; and structural and tectonic datasets spanning 128 years is presented. The seismicity patterns delineate the Kenya Rift Valley, the Nyanza Rift Valley, the east southeast Kenya, and the Mt Kenya region as the most active zones. The seismicity along the Nyanza Rift Valley confirms that this rift section extends in a westsouthwest direction. In contrast, the seismicity east of the southern Kenya Rift Valley is diffuse and occasionally clusters in a west-northwest direction. Eight active faults are identified; The Subukia fault, the Nandi-Aswa shear zone, Nyando fault, Lambwe-Samanga fault, Sirisia-Vitumbara fault, Legisiana Emsos fault, Nguruman fault, and little Magadi active fault. A new seismogenic model for Kenya with twenty-two seismogenic zones is used to model the hazard. The seismic hazard maps can be used to assess risk and be part of the national building codes as input to standard buildings design for significant buildings and infrastructure projects, land use planning, and determining insurance rates.