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 ML0.7 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.

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