BHUJ: A DIFFUSE PLATE BOUNDARY ZONE EARTHQUAKE?

Although it has been suggested that the Bhuj earthquake of January 2001, is a continental intraplate earthquake with analogies to the New Madrid seismic zone, it seems more plausible that the earthquake occurred instead 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 with 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. Thus a common map of plate boundary zones (Gordon and Stein, Science, 256, 333-342, 1992) assumes from the diffuse seismicity that India also has a broad western boundary.

The earthquake's thrust mechanism and its location in a clear band of seismicity and previously recognized active faulting imply that the boundary zone may extend somewhat farther east than previously thought, which would not represent a major change in the boundary geometry. A possible tectonic model consistent with the faulting, seismicity, and major plate motions would be that a Bhuj block or microplate has broken, or is breaking off from the Indian plate. 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 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, perhaps analogous to the eastern California shear zone. The scales involved are plausible: in terms of a different tectonic environment, but familiar geometry, Bhuj would be about halfway across the boundary zone between the Pacific and North American plates, in the central Nevada seismic belt where events as large as M 7.75 (Pleasant Valley, 1915) have occurred. Thus the earthquake seems more likely to give insight into the geometry of the Indian plate's boundaries than into the intraplate tectonics of the central U.S.