September 5, 2916
The Chedrang Valley Fault, the epicentre of the June 12, 1897 Great Assam Earthquake of 8.7 magnitude, is still seismically active. Moreover, on its northern fringe, the Brahmaputra Fault has also been found to be a major potential earthquake source zone in Northeast India.A sudden change in the stress regime in the Chedrang Valley Fault in the Shillong Plateau from its prevailing north northeast (NNE) to north northwest (NNW) orientation may have the potential of leading to an earthquake of significant magnitude.
These are the findings of a team of seismologists comprising Saurabh Baruah, Devajit Hazarika, Santanu Baruah, Rajib Biswas and Aditya Kalita of the Geoscience Division of the North-East Institute of Science and Technology (NEIST), Jorhat in their research paper titled – Seismotectonics and the current state of stress in Chedrang Valley and its vicinity – The rupture area of Great Assam Earthquake of June 12, 1897 (M= 8.7).
Talking to this correspondent, Saurabh Baruah, who led the team of seismologists in this study, said their paper was published in journals like Geophysical Journal International, Current Science and The Tectonics, among others.
These seismologists have identified the Brahmaputra Fault, on which the Brahmaputra river is flowing, as one of the major potential earthquake source zones in the northeastern region of India.
These scientists have prepared an earthquake catalogue of the Chedrang Valley Fault area based on the relocated events since 1982, when the inception of real time seismic monitoring network took place in the northeastern region of India, till 2006.
“The microtremor activity is relatively more intense in case of Chedrang fault in comparison to Samin and Dudhnoi faults having the earthquake magnitude ranged between 2.1 and 5.0. Notably, intense microseismic studies are found to occur within the depth range of 30 km,” the seismologists said in their paper.
“…The focal depth of the earthquake that we have studied along Chedrang fault deepens north of northeast beneath the western part of Shillong plateau with the orientation of the thrust plane towards Brahmaputra valley.”
The Chedrang Valley Fault (latitude 24.8 – 26.6 degree N; longitude 89.8 – 91.6 degree E) ran for a distance of about 20 km in the NNW direction. This fault and its vicinity are very active with sizeable energy release daily, Barua said.
Chedrang fault holds an importance. Here, noted geologist and the then Director General of the Geological Survey of India RD Oldham (1899) observed an 11-metre co-seismic slip down to the west of location of the fault. Recent palaeoseismic and seismotectonic studies (Rajendran et al 2004; Kayal et al, 2006) identify its northern boundary fault as a major seismic source, now termed as the Brahmaputra fault, said Barua.
It needs mention here that the Chedrang fault dips towards north of northeast. The stress tensor inversion from focal mechanisms suggests that the region is characterised by NNW-south southeast (SSE) compression in Chedrang.
