Spatiotemporal Characterization of Stress-Coupled Seismicity and Preparatory Processes Along the Main Himalayan Thrust

This study presents a retrospective analysis of the 1991 Uttarkashi earthquake (Mw 6.8) and the 1999 Chamoli earthquake (Mw 6.6), two major Himalayan earthquakes occurring in close spatial proximity along the Main Himalayan Thrust (MHT). To investigate the possible preparatory processes preceding these mainshocks, seismic events with magnitudes ≤ 5.9 were analyzed. Initially, the Seismic Recursive Genealogy Algorithm (SRGA) was applied within a 330 km radius to identify stress-coupled seismic events associated with each earthquake sequence. Subsequently, the refined SRGA catalogue was integrated with the novel Cohesive Strain Consolidation Algorithm (CSCA), developed using Dobrovolsky’s strain-radius concept, to delineate dominant head clusters during the three-year preparatory period prior to each mainshock. The spatial distribution of the head clusters revealed two distinct strain hotspot zones, namely SHS-1 located near the epicenter and SHS-2 situated relatively farther away. While the Uttarkashi sequence exhibited stronger strain concentration within SHS-1, the Chamoli sequence showed enhanced strain accumulation within SHS-2. Further temporal analysis demonstrated that SHS-1 experienced prolonged seismic quiescence, whereas SHS-2 was characterized by repeated stress accumulation and episodic strain-energy release. In addition, a simultaneous increase in Gutenberg–Richter a-value accompanied by a decrease in b-value was observed immediately prior to both mainshocks, indicating elevated stress accumulation. Comparative evaluation between the raw and refined catalogues demonstrated improved reliability in detecting anomalous seismic behavior. Moreover, the Chamoli sequence exhibited relatively linear preparatory trends, whereas the Uttarkashi sequence displayed fluctuating and heterogeneous rupture behavior, suggesting a more complex fault preparation process preceding the 1991 event.