InSAR Evidence Indicates a Link Between Fluid Injection for Salt Mining and the 2019 Changning (China) Earthquake Sequence

In June 2019, an earthquake sequence comprising five M > 5 events occurred in a region of southwest China with fluid injection for both hydraulic fracturing and salt mining, which raised an extensive controversy on the cause. Here we use 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 Advanced Land Observing Satellite 2 SAR images are collected to measure coseismic and preseismic surface deformation. Geodetic inversions with coseismic observations show that the sequence ruptured a previously unmapped southwest-dipping thrust fault above 3 km depth, which intersects with the open-hole sections of wells for solution mining of salt. In the 4 months before the sequence, cumulative line-of-sight displacements near the wells are around 1-2 cm after correcting seasonal-like deformation. Our results indicate that water injection likely enhanced pore pressure within the fault zone and thus contributed to inducing the sequence. Plain Language Summary Earthquakes are seldom reported in relation to fluid injection for salt mining. Here, we present a unique case of an earthquake sequence probably induced by such human activity in Changning (southwest China). The sequence began with a Ms 6.0 earthquake, which is an unprecedented magnitude and larger than the historical records of earthquakes induced by wastewater disposal and hydraulic fracturing. Due to the sparse distribution of seismic stations and the failure of a key station closest to injection wells for salt mining before the Ms 6.0 earthquake, available seismic data are not enough to constrain the spatial locations of events in the earthquake sequence. Here we instead use high spatial-resolution interferometric synthetic aperture radar observations of coseismic surface deformation to image the causative fault of the sequence. We find that the open-hole sections of injection wells intersect the seismogenic fault. Moreover, the cumulative surface displacements near the salt mine in the similar to 4 months before the sequence are 1-2 cm, which was mainly caused by ground uplift. Accordingly, we hypothesize that fluid injection increased pore pressure within the fault zone and brought it to rupture. Our interferometric synthetic aperture radar observations and inversion results present significant evidence toward the causality between salt mining and the earthquake sequence.