Seismotectonics and Spatio-temporal Variations in Seismicity Rates along Andaman Trench

The earthquake Frequency Magnitude Distribution (FMD) or b values have been meticulously used in the last two decades as a deterministic earthquake precursor or indicator, in earthquake forecasting or probabilistic seismic hazard assessment studies, in a variety of tectonic and stress regimes. In the present study, temporal fluctuations of b values were analysed by means of 185 well located (Mw ≥ 5) earthquakes, in the region of Andaman and Nicobar Islands (N10° to 14°) for a time period of ∼100 years (1918 to 2018). Variation in b value rates before and after the 2004 great Sumatra-Andaman earthquake (Mw 9.3) was analysed, preceded by a comprehensive geodynamic and seismotectonic- crustal deformation overview along the Andaman trench section. The analysis suggest a precursory drop in b values from 1.05 ± 0.05 (1918 to 2004) to 0.90 ± 0.05 post 2004 earthquake (2004 to 2018) reflecting a greater slip deficit and strain localization, that exceeds considerably the pattern of energy release in this region. Further, the region of Andaman and Nicobar Islands have witnessed a poor slip distribution (on-fault reactivation) from the great earthquakes that nucleated near the Sumatra segment, and have ruptured few large earthquakes, with no accounts of great earthquake (>Mw 8) during the last century. This emphasizes that, strain accumulation has been concentrated over a long period of time, and can contribute a destructive deformation event in near future.