Investigation of crustal thickness and uppermost mantle velocity beneath Gujarat, western India, utilizing Moho reflected P phases

Abstract

Crustal thickness and its variation can provide important insights into the tectonics of a region. The most popular methodology to determine crustal thickness – receiver function analysis utilizes converted phases from the Moho discontinuity. Although the reflection co-efficient is comparable to the transmission co-efficient, reflected phases are rarely used for determining crustal thickness. We tested and utilized such a rare methodology to determine the crustal thickness and uppermost mantle P-velocity beneath Gujarat, a seismically active region of the northwestern Deccan Volcanic Province. We used waveforms of ~2900 local and regional earthquakes (ML≥2.0) recorded by a network of broadband seismometers spanning the region. After manually marking the P arrivals from all the seismograms, computing and applying several signal-to-noise ratio criteria and aligning each seismogram to their P-onsets, a total of 9134 seismograms were stacked together. This stack enabled identification of the Moho reflected phases (PmP), based on the amplitude. Forward modelling of the observed PmP data yielded estimates of crustal thickness (H) and uppermost mantle velocities (v), given a crustal velocity structure or equivalent average velocity of the region. We divided the region into 0.5° × 0.5° grids and estimated the values of H and v at each grid point, using stacks of seismograms corresponding to Moho reflection points (within 50 km) of the grid. These values provided the lateral variation of the crustal thickness and uppermost mantle velocities across Gujarat. We found that the average uppermost mantle velocity is higher in the Kachchh region compared to that in Saurashtra, probably indicating past high-temperature events that transported and accumulated high velocity materials from greater depths beneath Kachchh. The crustal thickness also follows the same trend and maximum thickness of 43 km was observed within the center of the Kachchh rift with values gradually decreasing outwards from it. It is very likely that a thick crust beneath Kachchh includes a root. The average thickness and uppermost mantle velocity in Saurashtra were found to be 38.5 km and 7.9 km/s, respectively.