Seismogenesis of earthquakes occurring in the ancient rift basin of Kachchh, Western India - Intraplate Earthquakes - page 150

6.4.3 Geodetic observations in Kachchh

Starting in 2006, ISR operates up to 22 permanent differential GPS stations across geo-
logical faults in Gujarat. Some 11 campaign stations in Kachchh are occupied bi-annually.
Processing is done with 1 mm/yr accuracy (Choudhury et al ., 2013a). Local deformation
has been estimated with respect to Gandhinagar station, operating more than 200 km east
of the epicenter of the main earthquake of 2001. The Indian Institute of Geomagnetism
operated two GPS close to two of the ISR stations during 2001-5. Combining the two
datasets, the composite plot for the period 2001-9 indicates that near the epicenter the
postseismic relaxation was initially large, being 12, 6, 3, and 4 mm for four consecutive
6-month periods of 2001-2, but subsequently reduced ( Figure 6.16 ) . Since 2007 all the
ISR stations in Kachchh indicate the horizontal deformation to be low, i.e., of the order
of 2-5 ± 1 mm/yr. Nevertheless, the horizontal deformation gives a significant strain of

0.05 s/yr, even as late as 2008-11 (Dumka and Rastogi, 2013 ; Choudhury et al ., 2013b ),

triggering earthquakes along the SWF ( Figure 6.16 inset). However, vertical deformation
is found to be quite large, i.e., 2-13

µ

3 mm/yr, as observed by GPS measurements
near the epicenter of the 2001 mainshock and up to 75km north and northeast, being
13 mm/yr at Dholavira and 10 mm/yr at Dudhai during 2006-11 (Rastogi et al ., 2012 )
( Table 6.4 ) .
The interferogram generated with Differential Interferometric Synthetic Aperture Radar
(DInSAR) using the ENVISAT ASAR datasets of June 22, 2008 and October 25,
2009, with a baseline separation of 125 m in the area up to 75 km north of the main-
shock epicenter ( Figure 6.17 ; Rastogi et al ., 2012 ) indicates vertical deformation rates of
7-27 mm/yr.
Uplift of 16-27 mm/yr was detected along two faults (KMF and KHF) up to 50 km
south of the epicenter, measurements of which were done during 2004-7 using ENVISAT
ASAR datasets and during 2007-10 using ALOS PALSAR data (Choudhury et al ., 2012 ;
Sreejith and Rastogi, 2013 ) . Each interferogram-generated ring indicated 2.8 cm of vertical
uplift for the ENVISAT data and 11 cm for ALOS data.

±

6.4.4 Cause of triggered earthquakes

We propose, based on the seismological data, that the viscoelastic process and rheologic
changes appear to be the plausible causes of the long-distance and delayed triggering of
earthquakes with diffusion rates of 5-30 km/yr, whichmight have also been facilitated by the
migration of a stress pulse of 20 MPa stress drop caused by the 2001 M w 7.7 earthquake.
The rate of transmission of the stress pulse ( Figure 6.11 ) , considering the seismic area
involved, matches a seismogenic permeability of 0.5 to 5 m 2 /s (Talwani et al ., 2007 )
suggesting fluid pressure diffusion. Vertical deformation is observed to be high: measured
as up to 13 mm/yr by 6 years of GPS observations and 10-27 mm/yr by 10 years of InSAR
observations. We suggest that the transmission of the stress pulse into the upper crust could
have resulted in vertical deformation as observed in GPS and InSAR measurements.