Regional patterns of seismic b-values variations in the Himalayan region (71.6°E - 95.5°E and 37.5°N - 26.6°N)

Abstract

This study conducts a detailed seismic hazard assessment of the Himalayan region. It focuses on studying how b-values, based on the Gutenberg-Richter law, vary throughout location and time. These fluctuations assist measuring tectonic stress and provide insights into the region’s seismic activity. This research focuses on five Himalayan sub-regions: Far Western, Western, Central-I, Central-II, and Eastern. It incorporates earthquake data spanning 1964 - 2023 obtained from the International Seismological Centre. The data were de-clustered using the Reasenberg method and examined by Maximum Likelihood Estimation. The results demonstrated considerable spatial variability in b-values across the Himalayan sub-regions. The Far Western Himalayas displayed the greatest b-value (0.93 ± 0.02), indicating frequent smaller earthquakes and lesser tectonic stress. In contrast, the Eastern (0.68 ± 0.02) and Central-I (0.69 ± 0.03) regions had the lowest b-values, implying more stress accumulation and a greater risk of future strong earthquakes. Temporal fluctuations, as a decrease in b-values preceding to the 2015 Gorkha earthquake (Mw 7.8) and a subsequent increase in Central-II (1.19 ± 0.03), highlighted the retention and release cycles. The Eastern Himalayas, particularly the Dhubri-Chungthang fault zone seismic gap in Bhutan, are considered a key high-risk zone. This region, with b-values ranging from 0.65 to 0.75, has remained unruptured since the 1934 Bihar-Nepal earthquake (Mw 8.4). The findings showed the influence of the continual convergence of the Indian and Eurasian plates (~20 mm/year) on strain heterogeneity. This study underlines the vital demand for intensive seismic monitoring, resilient infrastructure, and disaster readiness in low b-value areas to alleviate catastrophic risks in one of the globe’s most tectonically active regions.

Keywords

b-value / Gutenberg-Richter law / Himalayan region