Geoscience Reference
In-Depth Information
thrusting is along the E-W-trending Gedi Fault and strike-slip is along a northeast-trending
conjugate transverse fault. The M
w
5.1 of June 19, 2012 along the Khadir transverse fault
indicates left-lateral strike-slip faulting.
Moment tensor focal mechanism solutions of M
>
4 earthquakes in Kachchh dur-
ing 2001-10 for a few earthquakes along the Gora Dungar, Kachchh Mainland, and
South Wagad faults, surrounding the 2001 rupture zone and believed to have been trig-
gered since 2006, indicate strike-slip. The focal depths are around 3 km for the Gedi
and South Wagad earthquakes, 8 km for the Gora Dungar earthquake, and 12 km for
the Kachchh Mainland fault earthquake. The focal depths of earthquakes showing thrust
mechanisms in the 2001 rupture zone are usually deeper than 10 km. It confirms the view
that there could be strike-slip at shallow near-surface depths and thrusting at deeper than
10 km.
Stress inversion using focal mechanism data from five areas across the main rupture
zone reveal a N-S orientation of maximum compressive stress with 7-32
rotation, which
is attributed to the sizable local horizontal stress component associated with the crustal
°
6.6 Results and discussions
The normal faults of the Mesozoic Kachchh Rift were reactivated after the continent-
continent collision (40 Ma) in the form of transpressional faults (with thrusting at depth)
geophysical data reveal the southward dip of the Kachchh Mainland fault and northward
dip of the South Wagad fault, along which large uplifted areas have been formed. Until
2000, seismicity in the region was considered low but with high hazard potential because
of historical records of the destruction of Dholavira in 2700 BC due to an earthquake of
estimated M
w
6.5, the 1819 Allahbund earthquake of M
w
7.8, the 1845 Lakhpat earthquake
of M
w
6.3, and the 1956 Anjar earthquake of M
w
6. These earthquakes occurred along
different faults. The picture of low seismicity changed after the 2001 Bhuj earthquake.
The KR has been associated with over a decade of aftershock activity of M
w
5 and
triggering of
20 earthquakes of M
w
4-5.7 along different large and small faults at up
to 75 km distance in a decade. GPS measurements, which started soon after the January
2001 earthquake, revealed 12 mm horizontal displacement for the first 6-month period.
It reduced to 6, 3, and 4 mm in the next three 6-month periods. Initially large horizontal
displacements favor a shear deformation mode of aftershocks. However, occurrence of
M
w
4-5.6 earthquakes, despite small horizontal displacements of 2-5mm/yr across different
faults, was suggested to be due to a stress pulse. The stress pulse is transferred, causing
earthquakes along the pre-existing critically stressed faults. Large vertical deformation
of up to 13 mm/yr was estimated by GPS for 2006-9 and 10-27 mm/yr from InSAR
studies for 2008-9 north of the KMF, and during 2004-7 as well as 2007-10 south of the
KMF.
