Precursory seismic patterns preceding major earthquakes-A case study from Northern Himalayas

Earthquakes are governed by stress accumulation, dissipation, and seismicity along faults. This study examines two major earthquakes in the northern Himalayas: The Mw 7.6 Muzaffarabad (2005) and Mw 7.8 Nepal (2015) events, to assess the role of lower-magnitude earthquakes (≤ 5.9 Mw) in stress evolution through variations in seismic ‘a’ and ‘b’ values. An increasing a-value indicates heightened seismicity, while a decreasing b-value signals stress concentration. Three stress evolution phases are identified: (i) Triggering Phase (TP), with increased low-magnitude seismicity and localized strain months before the mainshock; (ii) Accumulation Phase (AP), with no discernible patterns; and (iii) Release Phase (RP), immediately before the mainshock. Both earthquakes exhibited similar durations for TP (~ 51 days) and RP (~ 25 days), but distinct patterns emerged: Zone 1 (Muzaffarabad) showed no seismic activity during RP, while Zone 2 (Nepal) displayed clear activity, indicating different stress release mechanisms. The seismic stress evolution rate (SSER) was higher in Zone 2, suggesting greater vulnerability. Energy analysis revealed higher releases during TP than AP, with Zone 2 exhibiting the highest energy in RP, indicative of cascading stress dissipation, whereas Zone 1 showed no energy release in RP, reflecting abrupt stress discharge. These findings suggest that continuous monitoring of ‘a’ and ‘b’ value variations and energy release patterns can effectively forecast major earthquakes (Mw ≥ 7.0) in this region.