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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2094
Tmographic Imaging of the Kachchh seismic zone, western India : A
mechanism for stress concentration in the lower crust
Dr. Prantik Mandal
Seismology NGRI, HYDERABAD, INDIA IASPEI
Dr. R.K. Chadha
Aiming at obtaining a model for the causative mechanism of the continued occurrence of earthquakes in
Kachchh, Gujarat for about last six years, we estimated high-resolution three-dimensional Vp, Vs and
Vp/Vs structures in the aftershock zones of the 2001 Mw7.7 Bhuj and 2006 Mw5.6 Gedi earthquakes.
We used 13,862 P and 13,736 S wave high-quality arrival times from 2303 events recorded by the 5-18
three-component seismograph stations during 2001-06. Seismic images revealed a marked spatial
variation in the velocities (up to 10% in Vp, up to 15% in Vs, and up to 4% increase in Vp/Vs) in the 10
42 km depth range beneath the Bhuj aftershock zone. Relatively more in crease in Vp than Vs, and an
increase in Vp/Vs in the crust beneath the seismically active causative fault (North Wagad Fault, NWF)
zone in Kachchh, Gujarat suggests a rigid, mafic crust beneath the region. They also delineate an
increase of 8% in Vp and 14% in Vs, and a decrease of 4% in Vp/Vs in the almost vertical rupture zone
of the 2006 Gedi earthquake extending up to 12 km depth. This high velocity body associated with the
Gedi mainshock is inferred to be a gabbroic intrusive. The Banni region and the Wagad uplift are found
to be associated with high velocity intrusive bodies (inferred to be mafic) extending from 5 to 35 km
depth, which might have intruded during the rifting in early Jurassic (~160 Ma). Aftershock activity
mainly confines to the zones characterized by high Vp, high Vs and low Vp/Vs ratio, which might be
representing the strong, competent and brittle parts of the fault zone / intrusive bodies that could
accumulate large strain energy for generating aftershocks for more than five years. A few patches of
slow (Vp and Vs) and high Vp/Vs between 10 to 30 km depth have also been detected on the causative
40o south dipping north Wagad fault (NWF) for the 2001 mainshock, which may be attributed to the
fluid filled fractured rock matrix. Interestingly, the 2001 Bhuj mainshock hypocenter is found to be
associated with such a low velocity patch.
Keywords: intraplate earthquake, local earthquake velocity tom, crustal mafic intrusive