Modelling of source parameters of local earthquakes and seismotectonic implications in Siang Valley, NE India

A moderate size earthquake (ML 5.9) occurred near Mechuka in Siang Valley on 23rd April 2019 at 20:15:50.00 UTC in the rupture zone of the great Assam earthquake of 15th August 1950. This shallow-focused earthquake occurred close to NE trending Main Central Thrust has a thrust fault mechanism striking parallel to this major tectonics of the Himalaya. Including this event, a dataset of 46 local, micro and low magnitude earthquakes (1.5 ≤ ML ≤ 5.9) of the Siang Valley of Arunachal Pradesh, NE India, are evaluated for the characteristics of the earthquake source, seismicity, stress drop and tectonic structures. The earthquake events were recorded by a close digital network of eight broadband seismic stations installed in December 2018 in the Siang Valley. The stress drop (Δσ) obtained for the Mechuka earthquake in the Siang Valley is 114.71 bars, the seismic moment (Mo) of 6.86E + 16 Nm and the source radius (r) of 1392.41 m. The seismic moments obtained from the microearthquakes vary between 2.44E + 11 and 6.86E + 16 Nm, and the source radius varies from 172.90 to 1392.41 m based on Brune’s circular source model 1970. The corner frequencies (fc) of the events range from 1.03 to 8.04 Hz. Seismically intense clustering of microearthquakes has a low-stress drop below 10 bar except for the Mechuka earthquake, which varies from 0.05 to 114.72. This high-stress drop of moderate earthquake compared to its low values of lower magnitude earthquakes indicates that the earthquake of 23rd April 2019 is associated with high-strength material accumulating high strain during the earthquake-building process. Moment magnitude of whole data set ranges from 1.52 to 5.16. The scaling relationship between Mo and fc is obtained as 9.089E + 16fc–9.52, and for Mo and Δσ are modelled to be related as Mo = 8.96E + 10Δσ2.86. Evaluated low-stress drop for the smaller size of earthquakes shows brittleness of the upper crustal region.