The Seismogenic Structure of the Seismically Active Samakhiali Basin and Its Surrounding Region of Kachchh Rift Basin, Gujarat

Abstract

Receiver function data from 50 closely spaced seismic stations of the Kachchh array network, along with data from 17 nearby permanent stations, were analyzed to determine the 3D crustal structure of the seismic source zone in the Kachchh rift basin, Gujarat. The stations were located within the seismically active source zone of the catastrophic Mw 7.7 Bhuj earthquake (2001) and its surrounding tectonic features, including the Wagad uplift (WU), Samakhiali basin (SB), and Kachchh mainland uplift (KMU). We performed velocity–depth inversions for all sites using a Bayesian inversion technique, which provides estimates of crustal thickness, individual crustal layer thicknesses, corresponding S-wave velocities, and VP/VS ratio. To image the seismogenic layer, we generated common conversion point stacked sections both along and across the study area. Our results reveal a relatively thick crust (∼42 to 45 km) beneath the highly active SB, which is uplifted by ∼5 km toward the north (WU) and south (KMU). We infer the process of underplating for the thick crust beneath this area. In and around the seismically active SB, we identified a sedimentary layer 1.5–2.5 km thick, consistent with previous results from gravity, resistivity, and seismic studies indicating 1–3 km sedimentary thickness. This sedimentary layer is overlying a prominent high-velocity zone around 4.0 km in thickness, which appears consistently throughout the study region and is interpreted as a gneiss–granitic basement. A notable feature of the crustal structure is the presence of an undulating intracrustal low-velocity zone (LVZ) with a thickness of 5–8 km, occurring at depths between ∼18 and 26 km. Despite the existence of this LVZ, seismicity is distributed from the mid-crust to the lower crust, suggesting complex seismotectonic interactions and possible fluid or compositional heterogeneities within the crust beneath the SB and its associated seismicity.