The Zhangjiakou–Penglai fault zone is an important active tectonic belt in the eastern North China Craton. Its tectonic activity is closely related to the destruction of the North China Craton, lithospheric thinning, crustal deformation, and the preparation of strong earthquakes. The central segment of this fault zone hosted the 1976 Tangshan M_S7.8 earthquake and the 1679 Sanhe–Pinggu M_S8.0 earthquake. Although these two strong earthquakes occurred in close spatial proximity, they differ markedly in seismogenic fault properties, rupture processes, and postseismic stress adjustment, indicating that their deep seismogenic environments may have been controlled by different structural factors. Owing to the limited spatial resolution and coverage of previous observations, the fine-scale upper-crustal velocity structure of the central Zhangjiakou–Penglai fault zone, the deep geometry of the major faults, and the deep dynamic linkage between the Sanhe–Pinggu and Tangshan earthquakes remain insufficiently understood.

To further reveal the upper-crustal velocity structure and seismogenic tectonic background of strong earthquakes in this region, this study uses one month of continuous waveform records from 229 newly deployed dense temporary seismic stations covering the central segment of the Zhangjiakou–Penglai fault zone. Rayleigh-wave phase and group velocity dispersion curves were extracted using ambient noise cross-correlation, and a high-resolution three-dimensional shear-wave velocity model of the study area down to 10 km depth was obtained using direct inversion tomography. The dense station spacing and short-period surface-wave data provide improved constraints on shallow crustal heterogeneity, which is essential for resolving the structural differences between adjacent earthquake regions. The results show that the upper-crustal velocity structure exhibits significant lateral heterogeneity, with an overall pattern of higher velocities in the north and lower velocities in the south, which corresponds well to the tectonic framework of the Yanshan uplift in the north and the Jizhong depression in the south. In the east–west direction, the velocity structure displays clear segmentation and can be divided into western, central, and eastern velocity domains. In general, the high- and low-velocity anomalies are mainly distributed along NE or NWW trends, consistent with the strikes of the major fault zones and tectonic units.

The subsurface velocity structures beneath the two strong earthquake regions show distinct characteristics. In the Sanhe–Pinggu earthquake region, a pronounced Xiadian-north low-velocity anomaly is observed at depths of 6–10 km. This anomaly is located north of the Xiadian fault and is consistent with previous evidence for deep low-velocity, high-conductivity structures and mantle-derived fluid activity. It may reflect the modification of upper-crustal materials by residual fluids and hydrothermal processes associated with mantle material upwelling. In contrast, the shallow structure of the Tangshan earthquake region is mainly controlled by a high-velocity anomaly. The Tangshan high-velocity body may represent ancient rigid basement rocks with relatively high strength and brittleness, capable of sustaining significant stress accumulation. The high- to low-velocity transition zones at its base and margins may represent important sites for stress concentration and rupture initiation. Therefore, the contrasting velocity structures beneath the two earthquake regions suggest that their seismogenic processes were influenced not only by fault geometry, but also by the mechanical properties and spatial configuration of crustal materials. The two low-velocity features may have originated from the same source, namely mantle material upwelling related to the stagnation effect during Pacific Plate subduction.

Integrating the velocity structure, fault distribution, historical seismicity, and regional geodynamic background, we suggest that the low-velocity anomaly beneath the Sanhe–Pinggu earthquake region and the relatively low-velocity feature at depth beneath the Tangshan earthquake region may both be associated with mantle material upwelling induced by the stagnation of the subducting Pacific Plate. Although the Sanhe–Pinggu and Tangshan earthquakes exhibit different shallow tectonic styles, their seismogenic processes may have been jointly controlled by deep thermal activity, fluid processes, and the spatial distribution of rigid high-velocity bodies. Specifically, upwelling mantle-derived thermal material and fluid activity may have promoted regional stress adjustment and fault weakening, whereas rigid high-velocity bodies provided favorable sites for stress accumulation. Differences in the distribution patterns and depth-dependent coupling of these high-velocity bodies further led to differences in the seismogenic structures of the Sanhe–Pinggu M_S8.0 and Tangshan M_S7.8 earthquakes.