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Fault Geometry of the Mw7.7 Western India Intraplate Earthquake Constrained from Double Difference Tomography and Fault-Plane solutions

Globally, one of the largest intraplate earthquakes of Mw 7.7 occurred on 26 January 2001 in the Kachchh rift basin (KRB), western India. The continuing long aftershock sequence over decades has generated much debate on the seismogenic fault(s). We have analyzed more than 10,000 aftershocks (Mw >1:0) recorded by a 50-station broadband network in the region during 2006–2014. A total of 834 aftershocks (Mw >2:4), each recorded by at least eight broadband seismic stations with a minimum of eight P and six S phases, are relocated in this study by double-difference tomography (tomoDD) method. The relocated aftershocks and velocity images reveal a near-vertical or steeply south-dipping deeper structure as the source zone of the mainshock and aftershocks; the structure correlates well with the geologically mapped South Wagad fault (SWF). Among the other geologically known faults, the Kachchh Mainland fault (KMF) and the Gedi fault (GF) are also well identified in the seismic sections. Further, fault-plane solutions of 109 aftershocks having Mw ≥3:5 corroborate well with the known faults. The geological model and seismological observations suggest that the SWF overstepped the KMF and intersects it at depth. The intersecting fault zone is the source area for the deeper (10–35 km) reverse faulting earthquakes in the KRB. At the fault end of the SWF, a heterogeneous velocity structure is imaged, which is attributed to a fluid-filled rock matrix that triggered the mainshock. On the other hand, the GF is reported to be a later-activated fault to the north of SWF; it generated some shallower aftershocks (0–20 km) mostly by strike-slip mechanisms.

Fault Geometry of the Mw7.7 Western India Intraplate Earthquake Constrained from Double Difference Tomography and Fault-Plane solutions

Absolute VP and VS along eight north–south cross sections. The major geological faults are marked. The area bounded by the curvilinear line indicates the area of high resolution. The velocity scales are shown.

Further details:

Singh, A. P., C. Dorbath, M. Ravi Kumar, S. Kumar, I. Chaoudhry, J. R. Kayal (2016), Bulletin of the Seismological Society of America, 661 (4), 1446-1460

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