Evaluating aftershock decay behavior in the Central Himalaya via the modified Omori law

Abstract

Omori’s law (1894) is a fundamental concept that explains the temporal decay of aftershock activity. In this study, we applied a reformulated modified Omori law expressed as a differential equation to quantify the decay rate of aftershock behavior in the Central Himalaya. The key parameter, the deactivation coefficient (σ), quantifies how fast the earthquake sequence decays. Moreover, σ provides insight into post-seismic processes such as stress redistribution and microfracture healing. Aftershock sequences from three 2015 Central Himalayan earthquakes (Gorkha M7.6, Dolakha M7.0, and Kalikot M5.5) were analyzed using data from the International Seismological Center (ISC) and published catalogs. Since σ is a time-dependent function following the Omori epoch, only the initial 5–10 days after the mainshock were considered in this study.

The results show that larger earthquakes exhibit smaller σ values: 0.0115±0.0019 for M7.6, 0.0218± 0.0096 for M7.0, and 0.0388±0.0124 for M5.5, indicating slower and longer-lasting aftershock activity. The relation between mainshock magnitude and σ is consistent with the model σ = A−BM₀, although higher values of A (0.109) and B (0.0134) were obtained. Compared to global observations, the Central Himalaya region exhibits slower post-seismic relaxation. This behavior is likely controlled by the low-angle geometry of the Main Himalayan Thrust, elevated pore fluid pressure, and complex fault structures. The directional migration of aftershocks (eastward in eastern events and westward in western events) may indicate a partially locked or segmented section of the Main Himalayan Thrust, suggesting possible stress accumulation and a seismic gap in the Central Himalaya. These findings provide important implications for seismic hazard assessment, disaster preparedness, and risk mitigation strategies in the Central Himalaya region.