The Bhuj Earthquake and the Western Boundary of the Indian Plate

Giovanni Sella, Emile Okal, and I looked into the tectonic setting of the January, 26, 2001, Bhuj, India, M = 7.7 earthquake. Although it had been suggested that this earthquake was a continental intraplate earthquake with analogies to the New Madrid seismic zone in the central U.S., it is more useful to recognize 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.

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.

This model has some similarities to aspects of the Sierra Nevada microplate and eastern California shear zone, shown below to the same scale.

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.

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.