Seismic characteristics and tectonic processes of the 2015 Gorkha and Kodari earthquakes: insights from seismicity, b-value, stress field, and crustal heterogeneity

Abstract

This study investigates the tectonic processes driving the 2015 Gorkha (Mw 7.9) and Kodari (Mw 7.2) earthquakes in central Nepal, focusing on seismic parameters and subsurface structures to improve understanding of earthquake generation in the Himalayan region. We analyzed seismicity patterns, fault plane solutions, temporal b-value variations, and stress field dynamics using a comprehensive dataset compiled from multiple global and national catalogues. The spatial and depth distribution of seismicity revealed clustering at 10–20 km depth, primarily along the Main Himalayan Thrust (MHT), and was closely associated with low P-wave velocities (< 6.0 km/s), indicating a brittle, seismically active crust. Since the earthquake locations are from a non-relocated catalogue, future use of a relocated dataset is expected to improve the accuracy of these observations. Temporal analysis of the b-value, a statistical proxy for stress level and heterogeneity, showed a significant decrease from 0.98 to 0.89 immediately before and during the mainshock sequence, followed by an increase to 1.12 in the post-seismic period. This pattern aligns with global observations of stress accumulation and release, supporting the hypothesis of increased stress concentrations prior to bigger earthquakes. Empirical relationships derived between local magnitude (M_L) and body-wave magnitude (m_b) indicate a region-specific scaling, with m_b ≈ 1.6 × M_L, likely reflecting strong crustal heterogeneity and seismic attenuation in the central Himalaya. Focal mechanism solutions reveal dominant thrust faulting, with localized strike-slip and normal faulting components attributed to structural complexities and crustal anisotropy. Stress inversion results show a persistent northeast orientation of the maximum principal stress axis (σ₁), consistent with the India-Eurasia plate convergence direction. Notably, σ₁ became more horizontal and less scattered following the mainshock, while the minimum principal stress (σ₃) transitioned from a diffuse pattern to a predominantly vertical orientation. These changes indicate a redistribution and stabilization of the regional stress field post-rupture. These findings underscore the dynamic interplay between crustal properties, tectonic stress regimes, and earthquake generation processes in the Himalaya. Moreover, the reported precursory drop in b-value and reorganization of the stress field provide critical insights for refining earthquake forecasting models and integrating geophysical observations into future Earthquake Early Warning (EEW) strategies in the region.