Surface deformation and tectonic background of the 2019 Ms 6.0 Changning earthquake, Sichuan basin, SW China

The June 17, 2019 Ms 6.0 Changning earthquake at a depth of 3-5 km struck the Changning anticline fold system. No active faults have been observed in the region. Some studies have attributed the earthquake to the regional shale gas exploitation. Following the earthquake, we conducted a field survey to investigate co-seismic surface deformation in the area, and observed many surface fractures and uplifted areas. Our work indicated that the deformation followed a Riedel shear pattern. The fractures and uplifted areas represent T-fractures and Sextrusions, respectively. The observed Riedel shear pattern is indicative of left-lateral strike-slip motion. To ascertain the relationship between the Riedel shear pattern and regional tectonics, we investigated the trends in layer striations and fault scratches. The results indicated that there were two stages in the evolution of the anticline. In the first stage, the principal compressive stress that dominated the original formation of the anticline trended NE-SW. In the second stage, the regional principal compressive stress shifted to the current E-W trend, generating oblique motion that included reverse and left-lateral components. We propose that the tectonic stress field in this region had a strong bearing on the focal mechanism of the mainshock and the co-seismic surface deformation. Both the eastward expansion of the Xianshuihe area and southeastward expansion of the Longmen Shan have affected the current Changning anticline fold system.