Crustal velocity model beneath Lodwar station, Kenya using teleseismic P-wave receiver function

Abstract

Receiver functions are an important tool to study the structure of the crust and upper mantle including the discontinuities within them. Due to the regional complex geologic history and ongoing tectonism, Turkana area has been a subject of many geophysical studies. These included the KRISP (Kenya Rift International Seismic Project) in the 1980’s and 1990’s and other studies done by the oil companies. The method used in this research is the receiver function. A receiver function is a seismological technique that exploits the phenomenon of wave conversion. The upcoming P-wave interacts with seismic velocity impedance contrasts below the receiving station to produce polarized P–to- S or PS converted phases. The time delay between the first arriving P-wave and the PS converted phase is interpreted to infer the depth of interfaces and the velocity structure directly below the receiver, allowing estimates to be made of the physical properties of the layers. Passive seismic data recorded at the Lodwar seismic station were used in this study. The data were recorded for a period of one year from 1st January, 2012 to 31st December, 2012. A total of 18 teleseismic events were picked for receiver function analysis. The iterative time domain deconvolution was applied to those teleseismic events. Only the resulting receiver functions with a signal recovery of over 85% were used for subsequent analysis. Reconstructing the local velocity structure by comparing the observed receiver functions with synthetically generated receiver functions is usually regarded as a highly non-linear inverse problem. Forward and inverse methods were used in modeling the receiver functions waveform to obtain the P-wave velocity structure beneath the station. Two major layers were obtained, namely the first layer with a thickness of 10 km and increasing P-wave velocity of 5.75 to 7.25km/s and the second layer with a thickness of 10 km and an increasing P-wave velocity from 6.6 to 8.25km/s whose lower interface coincides with a sharp Moho discontinuity. These results agree quite well with previous geophysical results obtained in the area.