InSAR evidence indicates a link between fluid injection for salt mining and the 2019 Changning (China) earthquake sequence

Abstract:

In June 2019, an earthquake sequence comprising five M>5 events stuck a region of Changning located in the Sichuan basin (southwest China) with fluid injection for both shale gas hydraulic fracturing and solution mining of salt. Its occurrence raised an extensive controversy on the cause. There have been three published studies that used seismological data to relocate the Changning earthquake sequence and also to invert for focal mechanism solutions of some large earthquakes. However, due to the sparse distribution of seismic stations and the failure of a key station closest to injection wells for salt mining before the sequence, available seismic data is not enough to constrain the spatial locations of events in the earthquake sequence. These poorly-determined parameters are unfavorable to be used as evidence to investigate the cause of the sequence. Here we use near-field interferometric synthetic aperture radar (InSAR) observations to determine the source parameters of the sequence and to investigate the relationship with local injection activities. Both Sentinel-1 and ALOS-2 SAR images are collected to measure co- and pre-seismic surface deformation. We find that: (1) Co-seismic InSAR observations show distinct surface deformation occurred near the salt mine; (2) Nonlinear and linear inversions reveal that the earthquake sequence ruptured a southwest-dipping thrust fault above the 3 km depth near the salt mine; (3) The inferred seismogenic fault intersects with the open-hole sections of injection wells for salt mining; (4) There were 1-2 cm cumulative displacements in the LOS direction near the salt mine in about four months before the sequence. These four lines of evidence reveal that the Changning earthquake sequence was most likely induced by salt mining. It is an unique case with an unprecedented magnitude of Ms 6.0, which is even larger than the historical records of earthquakes induced by wastewater disposal and hydraulic fracturing.