Seismic Anisotropy Beneath Lodwar and Kibwezi Broadband Stations, Kenya, Revealed by Shear Wave Splitting

Abstract

Seismic anisotropy beneath Lodwar (LODK) and Kibwezi (KIBK) broadband seismic stations is investigated in this dissertation. This is achieved by use of shear wave splitting analysis of SKS teleseismic phases. Shear wave splitting parameters; polarization of fast direction, Φ, and delay time, dt, are determined using both minimum energy and rotation- correlation methods. From the shear wave splitting analysis, the fast directions, Φ, are 19° and 4° for LODK and KIBK respectively. This shows a NNE to SSW trend and are aligned sub-parallel to the earth’s surface geological structures around the stations and to the rift axis. The delay time increases in magnitude moving northwards in Kenya. KIBK and LODK have a dt of 1.0 and 1.2 respectively. Since LODK occurs to the north of KIBK, this shows a northward thickening anisotropic layer. The cause of anisotropy in the stations is melt filled pockets aligned parallel to the rift and rift parallel asthenospheric flow. The splitting measurement in KIBK and LODK are consistent and do not vary with backazimuth which suggest a single simple anisotropic layer. The depth of anisotropy beneath the seismic stations could not be determined, however, it can be inferred to be of lithospheric origin. This study has managed to determine seismic anisotropy beneath LODK and KIBK which has not been done before