Seismic evidence of magmatic underplating in the epicentral zone of 2001 Bhuj earthquake, Kachchh region, Gujarat, Western India: P-receiver function perspective

Kachchh, a region of high seismicity on the western India, is an intraplate rift basin and has experienced several earthquakes of magnitude > 5.5 over the past 1000 years. In this paper, we studied the crustal structure beneath the epicentral region of the 2001 Bhuj earthquake in Kachchh using data from broadband seismic array consisting of 53 stations at spacing of about 5 km between each and 3 profiles at a distance of 15 km. We used receiver function analysis for over 80,000 waveforms of the 526 teleseismic earthquakes. Using Hk and CCP stacking methods, we determined the crustal thickness to range from 33.8 km to 45.2 km, with the deepest Moho located at the junction of the Kachchh Mainland Fault (KMF) and the South Wagad Fault (SWF). A thinner crust is observed in the Samakhiyali graben, while a thicker crust is found in the surrounding areas. The Vp /Vs ratio values range from 1.62 to 2.02, with about 38 stations showing values above 1.8, which is high for the continental crust. This implies that the Bhuj epicentral zone has been affected by magmatic underplating, a process that significantly influences crustal dynamics through crustal thickening and thinning. At 17 stations, the RF stack shows a significantly positive phase at 2–3 s after the direct P phase, which could be related to high-velocity layer at 16–24 km or an upper crustal interface. A distinct low-velocity layer has been identified at depths between 24 and 32 km throughout the study area, indicating the possible presence of fluids within the fractured rock matrix. Moreover, high regional heat flow, elevated electrical conductivity, and a pronounced decrease in intracrustal S-wave velocity align with partial melting in the lower crust. The results observed in this study and evidence from previous studies suggest interaction of the Reunion mantle plume with the lithosphere beneath the epicentral region of the 2001 Bhuj earthquake, thus supporting the plume tectonics theory.