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Temporal variation of seismic b‑value in the Himalayas and foreland region and its implications on crustal stress variability

Chetia Monisha, Gogoi Priti Rekha, Lahiri Siddhartha Kumar

Acta Geophysica

2023

Vol. 71, no. 4

1675--1692

The temporal variation of seismic b-values during 1964–2020 was investigated for the Himalayas and foreland lying between 69°E-98°E and 21°N-36°N covering a range of more than 3000 km in five different time windows; 1964–1974, 1975–1985, 1986–1996, 1997–2007, and 2008–2020. The b-values show a very significant variation from 0.4 to 3.3. Seismically active areas are either in the phase of incubation or the phase of trigger indicating stress accumulation punctuated release. Since each jump in the magnitude of earthquakes is associated with a logarithmic decrease in frequency, an incubation period can be treated as the occurrence of a large number of low-magnitude earthquakes (cumulative energy released is much smaller than a single big trigger); hence, large b-value and the vice-versa. Thus the low b-value anomaly zones may be regarded as high-stress accumulation zones approaching the phase of triggering. The study area was divided into six type zones based on geological, gravity and DEM (Digital Elevation Model) data. As expected, most of the large-magnitude earthquakes were seen to have occurred in the low b-value regions. A comparative study of variation in b-values with depth for two windows, the western Himalayan syntaxis and the Indo-Burma range shows differences in the stress accumulation and the triggering potential for different ranges of depths. The study reveals that in the central Himalayas and its adjoining region large-magnitude earthquakes are due in the near future as crustal stress accumulation is high in these zones.