Stepover of Strike-Slip Faults and Overpressured Fluid Favor Occurrence Short-Term Foreshocks: Insights from the 2021 Yangbi Foreshock–Mainshock–Aftershock Sequence, China

ABSTRACT This study comprehensively and in detail analyzed the spatial and temporal distributions of earthquakes, tidal responses, focal mechanisms, rupture directions, coseismic stress transfer, and stress field characteristics associated with the Mw 6.0 Yangbi earthquake that occurred in May 2021. This earthquake constitutes a typical foreshock–mainshock–aftershock sequence, monitored by permanent and dense temporary seismic networks. Detailed and precise hypocenter determinations revealed complex and previously unknown seismogenic fault structures located several kilometers away from the well-documented active faults in the region. The foreshocks were found to be closely associated with both the main fault and the conjugate faults surrounding a fault stepover. Notably, the onset timing of three foreshock episodes and the occurrence time of the mainshock were clearly coincident with the peak of dilational volumetric tidal strain, indicating a potential tidal triggering effect. By synthesizing findings from other research teams, this study proposes that the Yangbi earthquake foreshock swarms were the result of the combined effects of deep overpressured fluids, transitional slow-slip events, and cascade triggering. Among these factors, deep fluids are inferred to have served as the initial driver and the most critical controlling factor. Similar to the Haicheng earthquake, for which a successful short-term and imminent prediction was issued, the Yangbi earthquake likewise demonstrates that tectonic complexity (primarily stepovers in this case) and the action of deep overpressured fluids facilitate the occurrence of short-term and imminent foreshocks, thereby potentially enhancing earthquake predictability.