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6.7 Conclusions
The Kachchh rift consists of a network of conjugate faults, some of which extend to large
depths. Activity on these faults has produced a series of horsts and grabens, some of which
have been leveled by erosion or filled by deposition. The 2001 mainshock that occurred
along a hidden North Wagad fault triggered seismicity on adjoining faults by a stress pulse.
New seismological and geophysical data have revealed the subsurface orientations of
some active faults. Frequent large earthquakes up to M
w
7.8 have occurred along different
faults in the past 200 years. The 2001 Bhuj earthquake has been associated with over a
decade of aftershock activity of M
w
up to
6. In addition to that, about 20 earthquakes
of M
w
4-5.6 were triggered along different large and small faults up to 75 km away
in Kachchh. The large vertical displacements of up to 13 mm/yr detected from GPS and
10-27 mm/yr detected from InSAR studies are contributing more to strain build-up than
the 2-5 mm/yr small horizontal displacements measured from GPS network and campaign
stations. There are some earthquakes that may be triggered in the zones of positive Coulomb
stress but some earthquakes are in the negative Coulomb stress zone. Earthquakes 8-10
years after the mainshock, away from the rupture zone and also in the negative Coulomb
stress zones indicate that the stress due to a viscoelastic process and/or rheologic process
subsequent to the 2001mainshock adds to the stress due to plate tectonics to cause triggering
of earthquakes.
High-velocity mafic intrusives inferred in the lower crust may act as stress con-
centrators. The postulated low-velocity patches within the mafic bodies may represent
fluid-filled zones that act as asperity zones and along which large earthquakes might
nucleate. A velocity decrease at 62-77 km, covering an area of 120 km
×
80 km, as
revealed by receiver function analysis was attributed to the updoming of the asthenosphere
or/and the presence of patches of partial melts beneath the central Kachchh rift zone.
These may provide favorable circumstances for eclogitization and release of compressed
carbon dioxide or fluids that accumulate in some pockets and facilitate occurrence of
earthquakes.
We propose that large crustal stresses associated with ongoing uplift, the presence
of intrusive mafic bodies, fault intersection, marked lateral variation in crustal thick-
ness, and related sub-crustal thermal anomalies are accumulating in the heterogeneous
causative fault of the 2001 mainshock. These stresses are mainly concentrated in the
denser and stronger lower crust (at 14-34 km depths) beneath the Kachchh rift zone,
within which most of the earthquakes nucleate along the fluid-filled fractured low-velocity
zones.
Acknowledgements
A thorough review and numerous suggestions by Pradeep Talwani have tremendously
improved the manuscript. Prantik Mandal is grateful to the Director, NGRI, for support
and permission to publish this work. The Government of Gujarat and Ministry of Earth-
Sciences, New Delhi, supported this work. Girish Kothyari drafted some of the figures.
