Pop-up tectonics of the Shillong Plateau in the great 1897 earthquake (Ms 8.7):: Insights from the gravity in conjunction with the recent seismological results

It was reported that a hidden 110-km-long south-dipping fault, named Oldham Fault, was responsible for the 1897 great earthquake (M-s 8.7) in the Shillong Plateau by reverse faulting due to 'pop-up' tectonics of the plateau in northeast India. Here we report the results of our geophysical investigations, where we critically examine the crustal structure of the plateau on the basis of gravity modeling and attempt to shed light on the hitherto debated hidden Oldham Fault at the northern boundary of the Shillong Plateau. Our gravity model, constrained by broadband seismological data, suggests that the Moho beneath the Shillong Plateau is at a shallower depth of about 35 km when compared to the Bengal basin to the south and the Assam valley to the north, which is about 42 km. Thinning of the crust under the plateau may be a consequence of the 'pop-up' mechanism. To examine the possible 'pop-up' of the plateau during the great 1897 earthquake, we have estimated the energy released by this earthquake and compared it with the energy required for the 'pop-up' of the plateau, using a simplistic rigid model. It is found that the Shillong Plateau between the Oldham Fault and Dauki-Dapsi Thrust would require an energy of 4.5 x 10(19) J, which does match well with the energy released by the great earthquake of M-s = 8.7.