Sediment Thicknesses and Q(S)-Q(P) Relations in the Kachchh Rift Basin, Gujarat, India, Using Converted Phases

摘要

In the seismic-hazard assessment of the seismically active Mesozoic Kachchh rift zone (KRZ), the sediment thickness and Q(S) versus Q(P) relations were estimated using the S-to-P converted phase (S-P). These estimations were conducted at 12 three-component broadband stations in Kachchh, Gujarat, operated by the National Geophysical Research Institute (NGRI) in Hyderabad, during the period of 2001-2013. In general, the S-P phase is clear on the vertical component and has been generated by a large impedance contrast between the sediment sequence and the basement. The difference in the travel times between S and S-P phases and velocities of P and S waves were used to calculate the sediment thicknesses at 12 broadband seismic stations. The estimated sedimentary thicknesses varied from (0.99 +/- 0.18) km at the BCH and JMN seismic stations to (1.46 +/- 0.29) km at the TPR seismic station. The spatial distribution of estimated sediment thicknesses (including estimates from earlier studies) reveals an east-west-trending basin structure (extending 44 km from north to south and 120 km from east to west), with the larger sediment thicknesses varying from 1.75 to 2.5 km, underlying the KRZ. Also, it depicts another zone of larger sediment thicknesses (1.5-1.8 km) below the Gedi fault zone. The calculated Q(S) versus Q(P) relations at 12 broadband seismic stations range from Q(S) - 0.467Q(P) at TPR with a high correlation coefficient (0.93) to Q(S) = 0.779Q(P) at MND and JMN with a low correlation coefficient (0.51). Our estimates for Q(P) versus Q(S) relations suggest the presence of wet and attenuative sediments in the Kachchh rift basin, indicating a high earthquake hazard for the region.