Geoscience Reference
In-Depth Information
6
Seismogenesis of earthquakes occurring in the ancient
rift basin of Kachchh, Western India
BAL KRISHNA RASTOGI, PRANTIK MANDAL,
AND SANJIB KUMAR BISWAS
Abstract
This chapter describes the mode of formation and seismogenesis of the very
active Mesozoic-age intraplate 200 km
300 km Kachchh Rift, which contin-
ues to be seismically active. As well as aftershocks of the 2001 M
w
7.7 Bhuj
earthquake, seismicity has continued to occur to M
w
5.6 levels along other
newly activated faults up to a distance of 240 km over the past dozen years.
This ongoing activity has provided a natural laboratory for studying seismoge-
nesis of intraplate rifts. Over the past decade, detailed investigations included
intense seismicity monitoring with up to 75 broadband seismographs, ground
motion detection with a GPS network of 22 stations and InSAR, active fault
investigation and subsurface mapping with various types of geophysical sur-
veys. The results of these studies led to the detection and mapping of subsurface
faults. Though the low GPS-derived horizontal deformation (2-5 mm/yr) may
be adequate to trigger earthquakes along pre-existing faults away from the rup-
ture zone, pockets of high vertical deformation (1-27 mm/yr) were detected by
InSAR, indicating greater vertical deformation. The uplift was possibly aided
by migration of the stress pulse due to a 20 MPa stress drop associated with
the mainshock. It is inferred that tectonic inversion of the rift is causing uplift
of the region. The seismicity and receiver function analysis suggest a thin crust
overlying a thinned lithosphere. Tomographic studies reveal the presence of a
high-velocity
×
100 km wide solid mafic intrusive with embedded low-velocity
zones, which suggest the presence of metamorphic fluids or volatile carbon
dioxide. The thin crust and lithosphere facilitated placement of mafic intrusive
and metamorphic fluids and/or volatile carbon dioxide from the underlying
magma chambers. It is inferred that the high-density mafic body acts as a local
stress concentrator and the low-velocity fluid-filled patches act as asperities.
