May 13, 2016

Featuring

Satish Singh
Institut de Physique du Globe de Paris

The equatorial Indian Ocean is deforming actively with
deformation taking place along E
-
W trending thrust faults in the
Central Indian Basin, west of the Ninety
-
East Ridge, and along N
-
S
trending re
-
activated fracture zones in the Wharton Basin. The
existence of active deformation in the Wharton Basin was con
firmed
by the 2012 twin Mw=8.6 and Mw=8.2 earthquakes, the largest intra-
plate earthquakes ever observed on Earth. Seismological and
geodetic data for the Mw=8.6 earthquake suggest the development of
new lithospheric faults. Using a combination of newly acquired
bathymetry and seismic reflection data, we report the existence of
pervasive 294º
-
striking shear zones, oblique to the plate fabric, and
reactivated N9ºE trending oceanic fracture zones, forming a
conjugate set of faults that might have ruptured dur
ing the Mw=8.6
event in echelon form.
We have imaged faults down to 45 km depth,
suggesting that the 2012 earthquakes might have ruptured the whole
lithosphere. We find that the deformation in the Wharton Basin can be
divided into two layers; a highly frac
tured fluid
-
filled serpentinized
upper layer and a pristine brittle lithospheric mantle where great
earthquakes initiate and large stress drops occur. The intense
serpentinization of the upper layer could release hydrogen, and
subsequently forming methane,
that may explain the widespread
presence of high
-
amplitude negative polarity reflection below Bengal
-
Nicobar fan sediments. This weak layer may act a decollement
surface during the subduction. On the other hand, the dehydration of
serpentinite at the boundary of these two layers may create a
reaction zone leading to seismicity that may explain the presence of
the second Benioff zone along the subduction zones worldwide