Intraplate or Interplate?

The Mw=7.7 Republic Day (01/26/2001) earthquake near Bhuj in western In-dia killed nearly 20,000 people and destroyed tens of thousands of homes. The cause and the tectonic implication of this earthquake have been the subject of in-tensive debate. Located ~400 km from the plate boundary, the Bhuj earthquake bears significant similarities with some intraplate earthquakes such as those in the New Madrid seismic zone in central United States. On the other hand, the plate boundary in western India is known to be diffuse, thus the Bhuj earthquake may reflect broad plate boundary deformation. To investigate the cause of the Bhuj earthquake and numerous other historic earthquakes concentrated in this part of the Indian plate, we have developed a viscoelastic finite element model to simulate the stress state within the lithosphere of western India. Our results indi-cate that the intracontinental thrusting and shearing along the northwestern Indian plate boundary may have caused deviatoric stresses to broadly diffuse into the Indian continent. When the internal rheologic heterogeneities of the Indian plate, such as the inherited mechanical weakening in the Kutch rift basin, is included, the model predicts a broad earthquake-prone zone extending hundreds of kilome-ters into the interior of the Indian plate that includes the Bhuj earthquake and most historic earthquakes in western India. 1