Seismic b-value reduction prior to the 28th April 2021, Mw 6.5 Sonitpur earthquake of Assam Valley

• Published in: Journal of Earth System Science 2022-09, Vol.131 (4), p.194, Article 194
• Main Authors: Sarma, Banashree, Borah, Kajaljyoti, Bora, Dipok K, Anand, Aakash
• Format: Article
• Language: English
• Subjects: Crustal structure; Datasets; Depth; Earth and Environmental Science; Earth Sciences; Earthquake prediction; Earthquakes; Fault lines; Group velocity; Lithosphere; Modelling; Moho; Moho discontinuity; Rayleigh waves; Reduction; Seismic activity; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Upper mantle; Velocity; Wave dispersion; Wave groups; Wave velocity
• ISSN: 0973-774X; 0253-4126; 0973-774X
• DOI: 10.1007/s12040-022-01948-4

The Kopili Fault is one of the most active faults in northeast India, which is the causative source of several earthquakes. The zone is of great importance to the scientific community because of its seismic productivity in past as well in present, which includes two major earthquakes ( M  > 7), several moderate earthquakes, and most recent 28th April 2021 M w 6.5 earthquake. In the present study, shear velocity crustal structure is mapped and the b -value is calculated near the Kopili Fault region using receiver function modelling and Gutenberg–Richter relation, respectively. Joint inversion of receiver function and Rayleigh wave group velocity dispersion data beneath Tezpur (TZR) station shows 40 km thick crust with 18 km upper and 22 km lower crust. However, thin crust (35 km) is observed beneath Mikir Hills. The overall b -value in the Kopili region is estimated to be 0.94. We observed an increase in b -value with depths, which may be due to the strength of crust and upper mantle material caused by the rise in temperature and pressure. The b -value variation with depth shows a sudden increase of b -value at a depth of 40–45 km, which corresponds to the crust–mantle transition, estimated from the receiver function modelling in the region. A sudden reduction of b -value is observed prior to 28th April 2021 M w 6.5 earthquake, which underscores investigations of the b -value and its variations for the purpose of earthquake precursor studies in that area. Research Highlights Moho and b -value in the Kopili Fault region are 40 km and 0.94, respectively. Entire crust and the upper mantle in Kopili Fault are seismogenic. b -value reduction is observed prior to 28th April 2021 M w 6.5 earthquake. Abrupt increase of b -value is well correlated with the Moho discontinuity.