Probabilistic assessment of earthquake hazard in the Andaman–Nicobar–Sumatra region

The Andaman–Nicobar–Sumatra (ANS) region is a very hazardous area on the globe, which has witnessed a megathrust earthquake of Mw 9.2 on 26 December 2004 and several dozen large earthquakes in the past. We estimate earthquake hazard parameters (i.e. seismic a- and b-values, maximum expected earthquake magnitudes, mean return periods and probabilities of earthquakes) in 11 shallow and 4 intermediate to deep depth seismogenic zones of the ANS region using a uniform and comprehensive earthquake data for the duration 1906–2018. The earthquake hazard scenarios for all seismogenic zones are calculated using the Gutenberg–Richter frequency–magnitude relation and the Gumbel’s extreme value theory. The low b-values (< 1.0) for both types of zones in the entire region suggest that the region is very active, under high stress and capable to generate large to great earthquakes. The estimated maximum magnitudes in different time periods using the extreme value theory show that shallow–depth zones 7, 8 and 11 (west to the Sumatra) have capabilities to generate an earthquake of magnitude Mw ≥ 8.0 in the next 50 and 100 years, while all intermediate to deep zones can generate magnitude less than 8.0. The mean return periods of earthquakes of magnitude Mw 7.0 in shallow zones 4–9 and 11 (the Sumatra and Nicobar Islands) exhibit less than 25 years. It is less than 80 years in shallow zones 4–11 for magnitude Mw 7.5, while higher return periods have been observed in the intermediate to deep zones (except for zone 4). The high probabilities (> 0.90) for the earthquake of Mw 7.0 in the next 50 years and 100 years are observed in shallow zones 4–11 (the Sumatra and Nicobar Islands), while only intermediate to deep zone 4 (Sumatra) shows high probabilities. The low return periods (< 25 years) and high probabilities (> 0.90) for the earthquake of Mw 7.0 are observed in shallow zones 5–11 (the Nicobar Islands and Sumatra regions), which suggest high earthquake hazard in these zones. The spatial variations of earthquake hazard parameters from one zone to another suggest a large grade of crustal heterogeneity and seismotectonic complexity present in this area.