Foreshocks of the 2018 ML 4.0 Shimian Earthquake in the Anninghe Fault and Its Implications for Earthquake Nucleation

Foreshock activity sometimes precedes large earthquakes, but how foreshocks relate to mainshock nucleation is still unclear with limited case studies existing. One way to further the understanding of the foreshock occurrence mechanism is to maximize the resolution of the foreshock characteristics by waveform-based earthquake detection and location. Here, we apply the match and locate method to scan continuous waveforms 30 days before and 44 days after the 2018 ML 4.0 Shimian earthquake in Sichuan, China, and obtain approximately three times more events than reported in a local catalog. The augmented seismicity suggests the existence of a blind small strike-slip fault deep in the east of the Anninghe fault. Forty-one foreshocks of magnitude ranging from ML−0.7 to 3.4 occurred within 4 hr before the mainshock and did not show an accelerating pattern leading up to the mainshock. Focal mechanisms are consistent between the mainshock and foreshocks, implying that the mainshock and foreshock hypocenters are located on the same fault plane. The high-precision relative locations reveal that most of the foreshocks rupture adjacent source patches along the fault plane, with little or partial overlap, which is consistent with cascade stress triggering from foreshocks to foreshocks to the mainshock. Our research is one of the few to focus on the foreshock sequence of moderate mainshocks and provides a new case for studying the mechanism of foreshocks of intraplate earthquakes with a low incidence of foreshocks.

Soil gas CO2 emissions and fault locking along the Anninghe fault and the Zemuhe fault, in China Seismic Experimental Site

<p>The Anninghe fault (ANHF) and the Zemuhe fault (ZMHF) with high level of seismic hazards in the China Seismic Experimental Site, located in southeastern of Tibet, are some of the most active faults in China. Measurement of the soil gas CO<sub>2</sub> has been conducted in three sites along the ANHF and the ZMHF for the first time. Totally, 394 sampling points along 15 profiles were measured. The fault locking degree of different segments of the ANHF and the ZMHF were inverted by the negative dislocation model using GPS velocity data  since 2013 to 2017. The measurements results show that the average and maximum value of CO<sub>2</sub> in the ZMHF is significantly higher than that in the ANHF. Soil gas CO<sub>2</sub> geochemistry yielded different spatial anomalous features, indicating the different properties and permeability of the faults. The inversion results reveal that the level of coupling including the locking depth and intensity along the southern segment of the ANHF was significantly larger than the northern segment of the ZMHF. Combining the CO<sub>2</sub> emission results, we concluded that the intensive locking of the segments reduced their permeability due to the self-sealing process, results in less gas to escape from the deep. Correspondingly, the creeping fault with low level of coupling can maintain high permeability which is more favorable to gas CO<sub>2</sub> migration.</p>

Dynamic Mechanism Research on the Tectonic Activation of Anninghe Rift

Anninghe rift is located on the western edge of Yangtze Block next to Tibetan Plateau, along the axis of a continental paleorift zone, Panxi paleorift. Recent studies have found that an upward mantle convection system existed since the late Pliocene in the deep lithosphere of a long and narrow area controlled by Anninghe fault. Lithospheric temperature distribution in the area has characteristics similar to that in Baikal and other modern rifts. A mantle upwelling area was in a constant state of “pull-subsidence.” Brittle rock mass of the shallow crust cracked into the new secondary subsidence blocks. A thick lacustrine sedimentary sequence of continental subsidence type developed. These all indicate that Anninghe rift is in an obvious tectonic activation state. It is believed that the tectonic activation of Anninghe rift has been produced by both horizontal squeeze from a plastic flow of the upper crust and expansion from mantle uplift. The pressure from the plastic flow of the upper crust is slightly greater than the expansion stress from the uplifting of lithosphere. Under this specific geodynamic environment, whether the tectonic activation of Anninghe rift can continue depends on the thermal motion rate of deep mantle materials and the eastward migration of the crustal materials of Tibetan Plateau.