Seismotectonic characteristics of the 1976 MS 7.8 Tangshan earthquake sequence revealed by high-resolution tomography

The fine shallow velocity model of the Tangshan Fault Zone (TSFZ) is of paramount importance in understanding the intricate geological structure and seismogenesis in this area. In this study, a dense seismic array consisting of 110 short-period seismometers was deployed in the TSFZ for ∼2 months. With this dataset, a 3-D shallow structure model deep to 4 km with the highest resolution up to now are obtained by using an ambient noise tomography, which can provide clear seismotectonic characteristics of the 1976 M_S 7.8 Tangshan earthquake sequence in the upper crust. The results show that the $V_S$ model is influenced by sedimentary strata and characterized by a conspicuous high-velocity anomaly in the northern region and a contrasting low-velocity anomaly in the southern region of the TSFZ. The velocity structure correlates well with the geological structure, and a significant velocity contrast can be observed at depths greater than 1 km on both sides of the TSFZ. By combining previous research findings with our novel discoveries, we suggest that the distribution of low-velocity sediments in the TSFZ is not only consistent with the shallow fault system but also related to local transtensional effects. Earthquakes in the middle and upper crust are primarily concentrated near the high-resistivity and high-velocity body in the northeastern part of the Tangshan Fault. Under the influence of right-lateral strike-slip motion, this high-resistivity and high-velocity body will rupture when the accumulated stress reaches a critical threshold.