Fluids Involved in the Occurrence of the 1975 Ms 7.3 Haicheng Earthquake Evidenced From Seismic Velocity Anomalies

The 1975 Ms 7.3 Haicheng earthquake in the Liaodong Peninsula is well-known worldwide to be the first successful short-term earthquake prediction. To understand the physical basis why this earthquake can be predicted and to elucidate the detailed seismogenic structure of the Haicheng earthquake, in this study we incorporate the variation of information constraints (VI) into body-wave travel-time tomography to determine high-resolution Vp, Vs and Vp/Vs models around mainshock and aftershocks. Compared to previous tomographic methods, the VI-based method can enhance the intrinsic correlation between Vp and Vp/Vs models, thus better resolving regional geological processes and lithological compositions. We assembled seismic arrival times recorded by permanent and temporary seismic stations in the region. Our results reveal pronounced low-velocity and high Vp/Vs anomalies in the middle to lower crust beneath the seismogenic fault, indicating a mechanically weakened zone likely associated with fluids. Seismic velocities along the fault plane further show that the mainshock nucleated within a transitional zone between brittle, competent granitic rocks featured by low Vp/Vs values and adjacent fluid-rich domains associated with high Vp/Vs values. The spatial distribution of aftershocks along the seismogenic fault shows a strong correlation with zones of high Vp/Vs anomalies. We propose that deep-sourced fluids, most likely originating from the upper mantle upwelling, migrated upward along pre-existing lithospheric-scale fault systems. This progressive fluid infiltration reduced the effective normal stress and mechanically weakened the fault zone. Under sustained tectonic loading, stress became locally concentrated on the strong blocks in the fault plane until fluid overpressure acted as an efficient trigger for rupture initiation. Before the mainshock, the infiltration of fluids can induce intensive foreshocks, which were used as precursors for the prediction of the Haicheng earthquake.This study highlights the coupled effects of stress evolution, fluid migration, and fault structure in controlling intraplate earthquake occurrence, providing new insights into the physical mechanisms governing seismic hazard in continental interiors.