Abstract

In order to resolve an enigmatic issue relating to the existence of fluid‐related or temperature‐related anomalies at the mainshock hypocenter of the 2001 Bhuj, India, earthquake (Mw 7.6), an estimate of the 3D bulk‐sound velocity structure is made from the inverted high‐quality P‐ and S‐arrival times from a total of 368 aftershocks recorded by 12 temporary seismic stations installed following the 2001 Bhuj earthquake. Results reveal strong lateral and vertical heterogeneity in bulk velocity (forumla) beneath the source zone. The 2001 Bhuj mainshock and its aftershock source zones are associated with anomalously high bulk‐sound velocity (high forumla), indicating high bulk elastic strength of the source rocks at the mainshock hypocenter due to high pore pressures of the fully saturated cracked rocks associated with solute precipitation through the processes of acoustic fluidization and cementation. The interpretation of bulk velocity tomograms suggests that the processes of mineral dehydration and permeation of sea/surface water through several active Quaternary faults down to the deep crust might have contributed to in situ fluid‐related material heterogeneity in bulk velocity within the fluid‐filled fractured rock matrix in the paleorift zone at the 2001 Bhuj mainshock hypocenter, which in turn increased pore pressure, lowered the effective stress, and brought the system into a brittle failure. High forumla in the intersecting fault geometry in the fractured rock matrix at the mainshock hypocenter can be taken as evidence for the strong role of fluids in association with the intraplate earthquake of the Indian peninsula.

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