Seismic b-Value and the Assessment of Ambient Stress in Northeast India

Seismicity data of northeast India, recorded between 1986 and 1999 by a local network, are analysed for estimation of b-values. Based on the obtained values, viz. low (b a parts per thousand currency sign 0.5), moderate (0.5 < b a parts per thousand currency sign 0.7) and high (b > 0.7), the study area is classified into different seismic-domains. An assessment of stress level is also carried out in identifying seismic-domains. Seismic activities, though mostly confined in some sectors, are presumably triggered by mutual interaction of the Shillong Plateau, Mikir Hills, Indo-Burman Ranges and the easternmost part of the Himalayas, and the contributions from deep-seated fractures cannot be ignored. The results resemble the seismic character of a foreland setting adjacent to a convergent margin. The b-values estimated for 240 square grids of dimension 0.6A degrees A xA 0.6A degrees over five seismic domains indicate wide variation. An analysis of cumulative seismic moment release (M (O)) in different layers also indicates an anomaly in reference to the total seismic-energy budget of the five zones. The lower b-value and higher M (O) recorded at relatively lower depth (similar to 30 km) towards the southwest of the study area might be associated with upward bulging of a strong lithosphere. The bulging is perhaps regionally compensated by the downward flexing of the descending Indian lithosphere beneath the Upper Assam area; features unequivocally observed in any foreland setup. Towards the north and east of the study area, random variations of in both b-value and M (O) along the converging zone suggest a varied tectonic environment with active interaction between the tectonic elements in these areas.