Seismogenesis of earthquakes occurring in the ancient rift basin of Kachchh, Western India - Intraplate Earthquakes - page 154

We speculate that the lower crust has a mafic intrusive with high seismic wave velocities,
which acts as stress concentrator. The low-velocity patches at 14-34 km depths indicate
the presence of fluids or volatile carbon dioxide, which may act as asperity zones and along
which large earthquakes are associated. The presence of patches of partial melt below the
lithosphere-asthenosphere boundary (LAB) suggested from receiver function analysis in
the central Kachchh Rift zone may be imprints of the Deccan mantle plume (65Ma) beneath
the region (Sen et al ., 2009 ; Mandal, 2010 ) . These may provide compressed carbon dioxide
or favorable circumstances for eclogitization and consequent release of fluid, either or both
of which facilitate the occurrence of earthquakes. Thin crust and lithosphere provide easy
access to the magma source.
Large spatial variations in Vp (from -10 to 13%), Vs (from -11 to 13%), and Vp/Vs ratio
up to 1.6 to 1.8 are seen in the main rupture zone of the 2001 Bhuj earthquake at 6-34 km
depth, where the tomography model is fairly sampled by the aftershock data (Mandal and
Chadha, 2008 ) . At shallower depth (0-10 km) the low Vp, low Vs, and high Vp/Vs could
indicate the presence of soft sediments with higher water content.
The aqueous fluid or volatiles containing carbon dioxide are released from the eclog-
itization of olivine-rich lower crustal rocks. The presence of 2 weight percent carbon
dioxide fluid components is suggested in the crystalline basement rocks of the 1993 Latur
earthquake (Pandey et al ., 2009 ) . Interestingly, John and Schenk (2006) suggest, based on
petrologic analysis of eclogites, that, once rupture begins in gabbroic crustal rocks, fric-
tional melting can promote intermediate-to-lower crustal depth earthquakes under eclogite
facies conditions and this seismic event can produce permeabilities for external fluids. Thus,
the eclogitization of lower crustal olivine-rich gabbroic rocks can provide aqueous fluids
or gaseous fluids (such as carbon dioxide and nitrogen) in the intermediate to lower crust.
Besides, strong attenuation of seismic waves (low coda Qc) and shear-wave splitting were
reported in the Bhuj aftershock zone, which could be explained in terms of the presence of
fluids, cracks, or both in the fault zone (Mandal et al ., 2004b ; Mandal, 2009 ; Padhy and
Crampin, 2006 ) .
Mandal (2012a) finds the crust (35 km) and lithosphere (62 to 63 km) to be thin in the
central part of the KR, as compared to 40-42 km thick crust and 65-77 km thick lithosphere
in the outer parts of the KR, due to updoming of the lithosphere and asthenosphere (

6-

12 km). Mandal finds a decrease of Vs at 62-77 km depth over an area of 130 km ×

90 km, which may indicate the presence of patches of partial melt below the LAB in
the central Kachchh rift zone as a remnant of the Deccan mantle plume (65 Ma) beneath
the region. Further support for this model is given by the rift axis parallel azimuthal
anisotropy, with a delay of 1.6 s evaluated from the SKS splitting study, which is attributed
to anisotropy induced by the rift-parallel flows within the 76 6 km thick lithosphere and

anisotropy associated with the rift-parallel pockets of partial melts in the asthenosphere,
inherited from the plume-lithosphere interaction during the Deccan/Reunion plume episode
(

±

65 Ma) (Mandal, 2011 ) . Thus, there is a possibility that these patches of partial melts
could provide a high input of volatiles containing carbon dioxide into the lower crust. If so,
this would contribute significantly to seismo-genesis and continued aftershock activity in
the intracontinental Kachchh rift zone.