Seismogenesis of earthquakes occurring in the ancient rift basin of Kachchh, Western India - Intraplate Earthquakes - page 135

69.6 70 70.4 70.8

10

20

2001 (Mw 7.7)

30

40

≥ 8.0 7.5 6.5 ≤ 6.5

Vp (km/s)

Figure 6.5 Cartoon displaying Vp distribution with depth along an E-W profile through the 2001
Bhuj earthquake epicenter. The hypocenter of the 2001 mainshock projected on this line lies in a
low-velocity patch. The outer portion of the intrusive body also has low-velocity fluid-filled zones.
For color version, see Plates section.
this zone and the concentration of aftershock hypocenters around it further validate this
conclusion.
Aftershock data suggest a reverse slip on a fault plane (NWF) dipping 40-60 to the

south as the causative fault for repeated earthquake nucleation (Mandal and Horton, 2007 ) .
Continued continental compression and repeated thrusting generate fractures related to
the main fault-forming highly stressed fault zone, which seems to be the zone of repeated
earthquake nucleation (Biswas, 2005 ) . This earthquake nucleation zone is defined by several
subsidiary faults branching off the main KMF. The causative fault, the NWF (North Wagad
Fault), of the Bhuj 2001 earthquake seems to be one such fault that lies north of it. The
occurrence of the 2001 Bhuj earthquake has been explained in terms of large local stress
perturbations associated with the pre-existing fault intersection (with respect to favorably
optimally oriented maximum horizontal stress direction) and mafic crustal intrusive bodies
below the KR zone (Gangopadhyay and Talwani, 2003 ; Mandal and Pandey, 2010 ) . The
presence of faults transverse to the NWF may lead to large stress concentrations in the
lower-crustal magmatic layer (at 14-34 km depth), thereby, the favored zone of earthquake
nucleation (Mandal and Pandey, 2010 ) . Presumably, the fluid released by the eclogitization
of olivine-rich lower-crustal rocks aids the slippage along this causative fault and hence
the occurrence of the 2001 Bhuj earthquake sequence (Mandal and Pandey, 2010 ; Mandal,
2011 ; Mandal, 2012a, b).

°

6.2.4 Identification of magmatic intrusive bodies

The P and S tomography results (Mandal et al ., 2004a) suggest the presence of a regional
high-velocity, low Poisson's ratio body around the 2001 mainshock epicenter with a head
extending 60 km in the N-S direction and 40 km the in E-W direction at 10-25 km depth
( Figure 6.5 ) , which may be a mafic intrusive of the rifting stage. At deeper depths it may
be larger. It has been speculated that this body is causing stress build-up. Its velocity is
inferred to be high (Vp: 7.15-8.11 km/s) at 24-42 km depth and density 3.06-3.37 g/cm 3