Seismogenesis of earthquakes occurring in the ancient rift basin of Kachchh, Western India - Intraplate Earthquakes - page 137

lherzolite (Sen et al ., 2009 ) . Eclogitization or serpentinization of these may release carbon
dioxide. The availability of such sources of melts is assured by the thin crust and lithosphere.

6.2.5 Low-velocity fluid-filled zones in and around magmatic
bodies in the lower crust

Mandal et al . (2004a) detected a low-velocity high Poisson's ratio zone within the mafic
body at the hypocentral depth of the mainshock (

18-25 km), which is inferred to be a fluid-
filled fractured rock mass and which might have acted as an asperity for generation of the
2001 Bhuj earthquake ( Figure 6.5 ) . From further detailed work on Vp and Vs tomography,
Mandal and Chadha ( 2008 ) inferred several low-velocity patches ( Figure 6.7 ) . From 3D
mapping of b-values (Mandal and Rastogi, 2005 ; Singh et al ., 2011 ; Nagabhushan Rao,
2012 ) , a high b-value ( 1) zone is found to be sandwiched within the maximum rupture

zone at depths of 15-25 km with a low b-value (0.6 to 0.8) above and below. In the
same zone Mishra and Zhao ( 2003 ) and Singh et al .( 2012 ) found low Vp and Vs, high
crack density, porosity, saturation rate and Poisson's ratio, and suggest the presence of a
fluid-filled and fractured rock matrix.
Such a low-velocity fluid zone was inferred by Kato et al .( 2009 ) from high-resolution
3D tomography around solidified intrusive bodies in the intraplate eastern margin of the
Japan Sea back-arc basin, and they attributed the large earthquakes of the area to the
presence of an intrusive body and fluids around it. It is speculated that the trapped aqueous
fluids resulted from metamorphism or were released from degassing of mantle magmatic
material or volatiles such as carbon dioxide (Zhao et al ., 1996 ; Miller et al ., 2004 ; Wang
and Zhao, 2006 ; Mandal and Pandey, 2010 ) .

6.2.6 Paleoseismological investigations

Based on Baker's (1846) leveling survey, Bilham ( 1999 ) inferred 11 m slip (uplift and
subsidence) along the Allahbund fault due to the 1819 earthquake. The inferred fault is
north-dipping with possibly listric fault geometry, with a steep dip at shallow depth but a
gentle dip at larger depth. Rajendran ( 2000 ) estimated lesser uplift of 4.3 m and suggested
a growing fold at shallow depths along a north-dipping thrust (Rajendran and Rajendran,
2001 ) . From radiocarbon age data of liquefaction features they suggested occurrence of a
previous earthquake 800-1000 years ago and suggested the ABF to have been previously
active, as a historic event was possible in AD 893 (Rajendran and Rajendran, 2002 , 2003 ) .
At Dholavira in northern Kachchh the archeological evidence indicates an earthquake
during the closing decades of stage III of the settlement (2500-2200 BC) (Singh, 1996 ;
Rajendran and Rajendran, 2003 ; Kovach et al ., 2010 ) . Bisht ( 2011 ) suggests that this event
occurred during 2100-2000 BC and identifies two earlier earthquakes also during stage II
(2900 BC) and stage III (2700 BC).