Seismogenesis of earthquakes occurring in the ancient rift basin of Kachchh, Western India - Intraplate Earthquakes - page 127

6.1 Introduction

Large earthquakes are continuously occurring along different faults in the Kachchh
intraplate rift basin of Western India. In this chapter we describe geological features and
details of the lower crust and upper mantle derived from tomography, receiver function, and
shear-wave splitting; we speculate on the cause of stress accumulation; and give details of
continuing seismicity triggered after the M w 7.7 earthquake along nearby faults, possibly
caused by a stress pulse, and the unusually high uplift detected by InSAR and GPS.
During the continental breakup of India from Africa in the late Triassic (210 Ma),
Western India experienced crustal stretching (thinning) and the formation of three failed
rifts of Kachchh (also spelled as Kutch), Cambay and Narmada. In the Kachchh Rift (KR),
magmatic intrusives were emplaced in the lower crust (possibly starting in the late Jurassic,
175 Ma). During the process, several granitic intrusive rocks have come up to the surface all
over Kachchh (Biswas, 2005 ) . The xenoliths found in these intrusives may have originated
in the lower lithosphere and are dated 75 Ma (Sen et al ., 2009 ) . The Deccan/Reunion
mantle plume during the end of the Cretaceous caused lithosphere thinning and emplaced
the Deccan volcanic flood basalt. The KR and the intraplate region in or close to the Deccan
Traps has been known to be active for centuries, and many earthquakes have occurred in
recent times (inset of Figure 6.1 ) : Koyna (reservoir induced seismicity along with M w 6.3
in 1967 to M w 5.6 in 2011) along the Kurdwadi Rift; Latur (1993, M w 6.2), Jabalpur (1997,
M w 5.8), and Bharuch (1970, M w 5.4) in the Narmada Rift; and Bhuj (2001, M w 7.7) in the
Kachchh Rift. The 2001 Bhuj earthquake caused 14,000 deaths and destruction in a heavily
populated and industrialized region (Rastogi, 2001 ; Rastogi et al ., 2001 ) .
Worldwide intraplate earthquakes, which mainly occur along rifts, are not well under-
stood due to the low level of seismicity. Measuring about 200 km 300 km, the Kachchh

Rift of Gujarat is seismically one of the most active intraplate regions of the world and has
experienced large earthquakes for many centuries and intense seismicity for over a decade.
Its six major E-W-trending faults are being reactivated by thrusting. This detailed study of
the seismogenesis of the KR has provided insights into understanding intraplate seismicity
associated with rift zones.
Large earthquakes and faults in the KR are shown in Figure 6.1 . These are: the
1819 M w 7.8 earthquake along the Allahbund Fault (ABF), the 1845 M w 6.5 Lakhpat earth-
quake along the western part of the Kachchh Mainland Fault (KMF), the 1956M w 6.0 Anjar
earthquake along the Katrol Hill Fault (KHF), the 2001 Bhuj M w 7.7 earthquake along the
eastern extension of the Kachchh Mainland Fault, and the 2006 M w 5.6 Gedi earthquake
along the Gedi Fault (GF).
A seismic reflection survey detected a fault 10 km north of Anjar as part of the KHF
along which the 1956 earthquake may have occurred (Sarkar et al ., 2007 ) .
Aftershocks of the 2001 Bhuj earthquake of M w up to 5 have continued for over a
decade. Also, several triggered earthquakes of M w

×

5.6 occurred along different faults in
Kachchh up to about 75 km from the mainshock epicenter and also up to 240 km south in
Saurashtra, which is a different tectonic province.