Seismological evidence for collapse of a deep geological cave—Focal mechanism of a thrust and collapse earthquake in a deep fold in the east Sichuan area

Do large caves exist deep underground? Through seismological investigations, we speculate the existence of a large geological cave about 7 km beneath the surface. The complete moment tensor inversion method (gCAP ) was used to analyze the ML4.7 earthquake that occurred in Rongchang District, Chongqing, on December 27, 2016. The complete moment tensor solution of its focal mechanism was obtained, and the waveform fitting was good. The inversion results showed that this relatively special earthquake had a thrust and collapse type source and that the earthquake centroid depth was 7 km. Based on the focal mechanism results and the fact that the earthquake was located near an anticlinal fold, we infer that this earthquake was attributable to thrusting of a fault in the anticlinal fold, which simultaneously led to destruction of the stability of the geological cavity in the fold, leading to collapse. The focal mechanism results showed that the seismic moment tensor had mainly a horizontal component with less vertical component, which is consistent with the local tectonic stress. The results indicate that the complete moment tensor of the focal mechanism inversion method is an effective means for studying complex earthquakes. In addition, shale gas drilling in this area is very deep and there are hydraulic fracturing operations, suggesting that earthquakes may be induced by the water injection pressure.