A rupture model for the great earthquake of 1897, northeast India

We assume that the unusually deep, extensive and long-lasting floods of 1897 along the section of the Brahmaputra River north of the western Shillong plateau were due to local ground subsidence associated with the great earthquake which occurred on June, 12 of that year in the western part of northeast India. Numerical simulations of ground-level changes due to slip on a buried low-angle thrust fault, dipping due north, then show that the northern limit of the rupture zone of this earthquake should have been along the E-W-flowing Brahmaputra River, about 40 km north of the northern edge of the Shillong plateau and about 70 km south of the Himalayan mountain front. A similar interpretation of a ground tilt observation suggests that the western limit of the 1897 rupture zone was along the western margin of the Shillong plateau. The E-W and N-S dimensions of the rupture zone are estimated to be 170 km and 100 km respectively, so that it enclosed the western half of the Shillong plateau and areas north of it up to the Brahmaputra River. The rupture depth could not be estimated from the available data on ground-level changes, and was constrained at 15 km beneath the southern margin of the Shillong plateau, on other evidence. The above thrust fault should be of the nature of a detachment at midcrustal depth, which arose because the continental crust associated with the Indian Shield terrains of the Shillong plateau and Mikir Hills immediately to the east could not subduct under the continental crust of the Eurasian plate to the north and east. It is tentatively suggested that, although this detachment may extend under the Himalaya, it may not be the detachment on which the great earthquakes of 1905, 1934 and 1950 have occurred in the northwestern, central and eastern Himalaya, respectively. It is also suggested that a distinction should be made between the seismicity of the Himalaya and the seismicity of the Himalayan convergent plate margin (HCPM). An earthquake of the Himalayan seismic belt is also an earthquake of the HCPM, but the converse need not hold true. Since the inferred northern limit of the 1897 rupture zone is about 70 km south of the Himalayan mountain front, it is suggested that this earthquake belongs to the HCPM but not to the Himalayan seismic belt. Thus, conservatively, a seismic gap of about 700 km may exist along the Himalayan seismic belt between the eastern and western limits of the ruptures zones of the great 1934 and 1950 earthquakes respectively