P-SV conversions of teleseismic waves beneath the Chyulu Hills volcanic field, Kenya

Abstract

Observations of P-SV converted waves provide new insights on the structure of the lithosphere beneath the Chyulu Hills, a Quaternary volcanic field on the SE shoulder of the Kenya rift (East Africa). A 3D teleseismic delay-time tomography and a seismic refraction model revealed a zone of reduced P-wave velocity (-3.5%) extending from the lower crust into the uppermost mantle (limited to about 70 km depth). In this paper, waveform and particle-motion analyses are used to identify P-SV conversions in teleseismic recordings and to search for lateral variations of seismic discontinuities. Data from 9 teleseisms contain clear SV-phases within the first 8 seconds of the P-coda. The depth of the generating interfaces is determined with a local velocity-depth function based on the seismic refraction model. Our results show that there are no lateral depth variations of the Conrad (19 - 21 km) and Moho (43 - 45 km) discontinuities, indicating no large-scale modification of the crustal structure through the magmatic processes. In the time window between the SV-arrivals from the Conrad and Moho discontinuities additional SV-energy is observed at some stations, but its origin cannot be located.