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

Abstract

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.

Apparent Low‐Velocity Belt in the Shallow Anninghe Fault Zone in SW China and Its Implications for Seismotectonics and Earthquake Hazard Assessment

“…Based on the checkerboard analysis (Figure 4) and recovery test (Text S2 and Figure S2 in Supporting Information S1), the east‐shifted low‐velocity belt can be resolvable. Additionally, it correlates with the location of seismicity detected by the Xichang array (Feng et al., 2021).…”

“…The gray dots represent the locations of microseismic events detected from the Xichang array by Feng et al. (2021). A schematic diagram of the interpretation is shown in the lower right panel.…”

“…The two red triangles (denoted by AS013 and AS123) indicate the example stations used in Figure 2. Gray dots with varying sizes represent microseismic events detected by the Xichang array between 13 January 2013 and 28 January 2019 with magnitudes ranging from M L 0.5 to M L 2.5 (Feng et al., 2021). The black arrows indicate the GPS velocity vectors (Zhao et al., 2015).…”

“…(a-i) Vertical profiles of Vs along fixed latitudes of 28.15°, 28.2°, 28.25°, 28.3°, 28.35°, 28.4°, 28.45°, 28.5°, 28.55°, and 28.6°, respectively. The gray dots represent the locations of microseismic events detected from the Xichang array byFeng et al (2021).…”

Big Data Seismology

“…In real fault systems, the cascade and preslip models of nucleation are not mutually exclusive and indeed may feedback on one another (Cattania & Segall, 2021; McLaskey, 2019; Noda et al., 2013). Recent observations from high‐resolution earthquake catalogs around the world (Cabrera et al., 2022; Durand et al., 2020; Ellsworth & Bulut, 2018; Feng et al., 2021; Gardonio et al., 2020; Malin et al., 2018; H. Meng & Fan, 2021; Moutote et al., 2021; Sánchez‐Reyes et al., 2021; Shelly, 2020; Trugman & Ross, 2019; M. P. A. van den Ende & Ampuero, 2020; Yao et al., 2020; Yoon et al., 2019) are beginning to bridge the gap between laboratory and field scales by providing more complete and holistic observations of the nucleation process. The diverse range of physical processes highlighted by these recent studies suggest that earthquake nucleation does not follow a simple, uniform trajectory common to all earthquakes, but instead may be complex and highly dependent on the details of the faulting environment and stress regime.…”