A reappraisal of the 2005 Kashmir (Mw 7.6) earthquake and its aftershocks: Seismotectonics of NW Himalaya

• Published in: Tectonophysics 2020-08, Vol.789, p.228501, Article 228501
• Main Authors: Powali, Debarchan, Sharma, Shubham, Mandal, Riddhi, Mitra, Supriyo
• Format: Article
• Language: English
• Subjects: Aftershocks; Cambrian; Centroids; Coulomb failure stress; Dimensions; Earthquakes; Energy; High frequency back-projection; Kashmir earthquake; Propagation velocity; Rupture; Seismic activity; Seismo-tectonics; Source mechanism; Time functions; Velocity; Waveforms
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2020.228501

We study the source properties of the 2005 Kashmir earthquake and its aftershocks to unravel the seismotectonics of the NW Himalayan syntaxis. The mainshock and larger aftershocks have been simultaneously relocated using phase data. We use back-projection of high-frequency energy from multiple teleseismic arrays to model the spatio-temporal evolution of the mainshock rupture. Our analysis reveal a bilateral rupture, which initially propagated SE and then NW of the epicenter, with an average rupture velocity of ~2 kms−1. The area of maximum energy release is parallel to and bound by the surface rupture. Incorporating rupture propagation and velocity, we model the mainshock as a line source using P- and SH-waveform inversion. Our result confirms that the mainshock occurred on a NE dipping (~35°) fault plane, with centroid depth of ~10 km. Integrated source time function show that majority of the energy was released in the first ~20 s, and was confined above the hypocenter. From waveform inverted fault dimension and seismic moment, we argue that the mainshock had an additional ~25 km blind rupture beyond the NW Himalayan syntaxis. Combining this with findings from previous studies, we conjecture that the blind rupture propagated NW of the syntaxis underneath a weak detachment, overlain by infra-Cambrian salt layer, and terminated in a wedge thrust. All moderate-to-large aftershocks, NW of the mainshock rupture, are concentrated at the edge of the blind rupture termination. Source modeling of these aftershocks reveal thrust mechanism with centroid depths of 2–10 km, and fault planes oriented sub-parallel to the mainshock rupture. To study the influence of mainshock rupture on aftershock occurrence, we compute Coulomb failure stress on aftershock faults. All these aftershocks lie in the positive Coulomb stress change region. This suggest that the aftershocks have been triggered by either co-seismic or post-seismic slip on the mainshock fault. •2005 Kashmir mainshock had a bilateral rupture on NE dipping thrust plane with energy released within 10 km of the surface.•The Mainshock had a surface rupture of ~75 km, and was blind for ~25 km NW of the Himalayan syntaxis beneath a wedge thrust.•The aftershocks NW of the mainshock surface rupture were triggered by co-seismic or post-seismic slip on the mainshock fault.