Seismic-reflection images of the crust beneath the 2001 M = 7.7 Kutch (Bhuj) epicentral region, western India

Three short ( approximately 35 km) seismic-reflection profiles are presented from the region of the 2001 Mw = 7.7 Bhuj (western India) earthquake. These profiles image a 35-45-km-thick crust with strong, near-horizontal reflections at all depths. The thickness of the crust increases by 10 km over a distance of approximately 50 km from the northern margin of the Gulf of Kutch to the earthquake epicenter. Aftershocks of the Bhuj earthquake extend to a depth of 37 km, indicating a cold, brittle crust to that depth. Our results show that all of these aftershocks are contained within the crust. Furthermore, there is no evidence for offsets in the crust-mantle boundary associated with deep (mantle) faulting. The existence of a thick ( approximately 45 km) and highly reflective crust at the epicentral zone may be indicative of crustal thickening due to the compressive regime of the past 55 m.y. Alternatively, this crustal thickening could be attributable to magmatic intrusions that date back to Mesozoic rifting associated with the breakup of Gondwanaland.