Seismicity and Detailed Fault Zone Structure Beneath Haicheng Seismic Zone, NE China: Implications for the Fluid‐Induced Fault Reactivation

Abstract The 1975 Ms 7.3 Haicheng earthquake occurred in northwestern Liaodong Peninsula, which was successfully forecasted and marked a landmark achievement in seismology. However, the seismogenic mechanism of this earthquake remains poorly known. In this study, we obtain high‐precision earthquake catalog and 3‐D models of crustal P‐ and S‐wave velocities (Vp, Vs) and their ratio (Vp/Vs) in the Haicheng seismic zone (HSZ) from an unprecedentedly dense seismic array that we recently deployed. Our results show the seismicity pattern in the HSZ is predominantly controlled by an interconnected conjugate fault system, comprising the WNW‐striking Haichenghe fault and its NE‐striking concealed secondary fault. Furthermore, a prominent fluid‐rich zone characterized by low Vp, low Vs and high Vp/Vs ratio is clearly revealed in the middle crust at ∼15 km depth, directly beneath the hypocentral region of the 1975 Haicheng earthquake. A distinct seismic cluster is also observed adjacent to the main shock hypocenter, exhibiting spatial correlation with the underlying crustal fluid reservoir. This geometric correspondence indicates a fluid‐driven triggering mechanism that contributes to the localized seismicity. We deem that the reactivation of preexisting seismogenic faults induced by crustal fluid migration could be a possible mechanism for the nucleation of such a large intraplate earthquake.