The Bhuj earthquake and the
western boundary of the Indian plate
Giovanni Sella, Emile Okal, and I have been looking into
the tectonic setting of the January, 26, 2001,
Bhuj, India, M = 7.7 earthquake.
Although it has been suggested that this
earthquake was a continental intraplate
earthquake with analogies to the New Madrid
seismic zone in the central U.S.,
it seems more plausible that
the earthquake occurred within the Indian
plate's diffuse western boundary. In general,
continental plate boundary zones are broad. The
Indian plate's northern boundary is a zone of
faulting and earthquakes extending thousands of
km north from the Himalayas. Its southern
boundary is also a diffuse zone of seismicity
and deformation, first recognized from the fact
that models of a single plate containing both
India and Australia were unable to account for
these presumed "intraplate" earthquakes.
Diffuse seismicity and faulting similarly shows that India has a
broad western boundary in the India - Arabia - Eurasia triple
junction region.
![[Image]](http://www.earth.northwestern.edu/people/seth/research/bhujfig1.jpg)
Thus the Bhuj earthquake's
location lies in a previously recognized band of
seismicity and active faulting,
implying that the boundary zone extends
further east than previously drawn.
Although present data are
inadequate to determine the geometry of the boundary
zone and its relation to the earthquake,
a possible tectonic model consistent with the faulting,
seismicity, and major plate motions would be
that a Sind block or microplate has broken,
or is breaking, off from the Indian plate
near the triple junction,
as occurs at other plate boundaries. A few
mm/yr of motion relative to India along
its south boundary, causing N-S compression,
would yield the observed zone of seismicity and
active thrust faulting. Its north and west boundaries
with Eurasia would have the observed thrust and
strike-slip motion. Its eastern boundary would
have thrust and strike-slip motion relative
to India, and may be evolving with time.
![[Image]](http://www.earth.northwestern.edu/people/seth/research/bhujfig2.jpg)
This
model has some similarities to aspects of the
Sierra Nevada microplate and eastern California
shear zone, shown below to the same scale.
![[Image]](http://www.earth.northwestern.edu/people/seth/research/bhujfig4.jpg)
The scales involved are comparable:
Bhuj is about 400 km from the nominal boundary,
a distance which in U.S. terms is about halfway
across the boundary zone between the Pacific and
North American plates, in the central Nevada
seismic belt where magnitude 7 earthquakes
occur. In contrast, New Madrid is about 2400 km
from the San Andreas, the nominal plate boundary.
![[Image]](http://www.earth.northwestern.edu/people/seth/research/bhujfig3.jpg)
Thus the earthquake seems
to give insight into the geometry and kinematics
of the Indian plate's western boundary rather than into
intraplate tectonics.
These results are published as Stein, S., G. Sella, and E. Okal,
The January 26, 2001 Bhuj earthquake and the
diffuse western boundary of the Indian plate,
in Plate Boundary Zones,
edited by S. Stein and J. Freymueller,
Geodynamics Series 30,
AGU, Washington, D. C., 2002.
For pdf click here
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