Earthquakes induced by overpressure methane-bearing fluid in northwest Sichuan Basin, China.

Earthquakes are a major geohazard worldwide, and understanding their triggering mechanisms remains a key research objective. Currently, many noteworthy studies have proposed mechanisms to explain the triggering of earthquakes, including a special mechanism: the release of overpressure fluid. However, direct evidence is lacking for the induction of earthquakes by the overpressure of fluids. Fortunately, we have discovered many overpressure methane-bearing inclusions in the calcite vein of the Xujiahe Formation in northwest Sichuan, which effectively record the information of overpressure fluids. The trapping pressure of pure methane inclusions in the latest conical calcite veins were obtained, ranging from 114.8 MPa–157.1 MPa, with an average of 130.1 MPa. The calculated pressure coefficient ranges from 1.88 to 2.57, with an average of 2.17, indicating a typical abnormally overpressure. The conical calcite veins locally pierce the earlier fibrous calcite on the opposite wall, which suggests an abrupt pressure drop and fracture closure. The sudden release of overpressure fluid inevitably disturbs the regional stress field, generating localized earthquakes recorded by extensive seismites preserved in the Triassic Xujiahe Formation. Brittle deformation features and seismites provide independent geological evidence for post-depositional seismic shaking. All indications suggest that the accumulation and subsequent release of overpressure methane-bearing fluid in the fractures of the Xujiahe Formation in northwest Sichuan Basin can indeed induce localized earthquakes.