Seismogenic structure of the 26 March 2025 Yongqing ML4.6 earthquake and its implication for seismic hazard assessment

Abstract

Despite being an economically developed sedimentary basin with significant oil and gas production, the North China Plain exhibits anomalously high intraplate seismicity for a densely populated region. On 26 March 2025, a ML4.6 earthquake struck Yongqing County, following a similar ML4.8 event in 2018. Both earthquakes occurred near the epicentral region of the devastating 1679 M8 Sanhe-Pinggu earthquake and the seismically active Tianjin Gap, thus drew widespread public attention due to their tectonic implications. This paper conducted a comprehensive investigation of the 2025 Yongqing earthquake's seismogenic structure through the following points. (1) the first one is focal mechanisms inversion for the mainshock and three ML≥2.5 aftershocks, (2) the second one is the earthquake relocation by waveform cross-correlation, and (3) the third one is rupture direction estimation by centroid relative relocation and the spectral ratio technique. The results reveal that the 2025 ML4.6 earthquake ruptured a NE-striking, steeply dipping right-lateral strike-slip fault, with predominant SW-to-NE rupture propagation. This kinematic behavior shows striking similarity to the 2018 ML4.8 event, suggesting potential structural linkage. While both seismogenic faults remain unmapped in regional geological surveys, they potentially link with the Xiadian fault zone at mid-crustal levels. Nevertheless, our analysis reveals no conclusive evidence of structural continuity between these faults and the 1679 Sanhe-Pinggu earthquake rupture zone. The nearby Late Pleistocene-active Hexiwu Fault, which has historically produced only one M~5.5 earthquake recorded but exhibits persistent moderate seismicity, may be capable of generating larger ruptures in the future, highlighting the necessity for intensified seismic monitoring within this area.