2019 M-w 5.9 Mirpur, Pakistan Earthquake: Insights from Integrating Geodetic, Seismic, and Field Observations

摘要

On 24 September 2019, an M-w 5.9 earthquake struck near the Mangla reservoir in northeastern Pakistan and resulted in 39 fatalities and 746 serious injuries, making it the deadliest earthquake in the region since the 2005 M-w 7.6 Kashmir earthquake. Here, we integrate geodetic, seismic, and field observations to characterize the source properties and impact of the Mirpur earthquake as well as investigate whether it might be a reservoir-induced event. From inverting Interferometric Synthetic Aperture Radar data, we find that a fault with strike similar to 310 degrees, dip similar to 6 degrees, and rake similar to 117 degrees is the optimal source, with slip concentrated between 5 and 6 km depth. This is consistent with our relocated aftershocks depth distribution and the lack of surface rupture observed in the field. Therefore, we infer that the earthquake ruptured the Main Himalayan Thrust (MHT). The event's shallow depth might explain the extensive damage caused despite its moderate magnitude, with a maximum shaking intensity of VIII based on our field survey. The survey also revealed extensive damages associated with earthquake-induced liquefaction. Our modeling shows that loading due to increased reservoir water level in the three months before the Mirpur earthquake led to Coulomb stress increase of similar to 7-10 kPa on the rupture plane. However, this effect is - 10 times smaller than the Coulomb stress increase due to the 2006 Mangla earthquake, and the Mirpur earthquake only occurred similar to 1-2 weeks after peak water level. These suggest that pore pressure diffusion contributed to promoting the fault rupture at a time when it is close to failure due to accumulated stress from inter-seismic loading. Because the Mirpur earthquake resulted in a stress increase of >0.2 MPa on the surrounding sections of the MHT and nearby faults, future rupture of these faults is a significant hazard and proper management of reservoir operations is necessary to prevent further elevating the seismic risk.