The Zhangjiakou–Bohai Seismic Belt is an important NW-trending seismic zone in eastern China, located at the junction of the North China basin, the Yanshan uplift, and the Taihang uplift. It is characterized by complex fault systems, strong crustal deformation, and frequent seismic activity. Many strong earthquakes have occurred along this belt, including the 1976 M_S7.8 Tangshan earthquake. Previous geophysical studies have revealed remarkable low-velocity anomalies in the middle and lower crust beneath the Tangshan region and other parts of the Zhangjiakou–Bohai Seismic Belt. These anomalies are generally considered to be related to mantle-derived material intrusion, magmatic underplating, and lithospheric modification. However, the deep origin of these low-velocity anomalies remains unclear. In particular, whether the mantle material beneath the Zhangjiakou–Bohai Seismic Belt is mainly derived from the Datong volcanic field, the Bohai Bay region, or a broader mantle upwelling system beneath the North China Craton is still an important scientific question.

To investigate this issue, this study collected teleseismic waveform data recorded by permanent and temporary seismic stations across North China. The data set includes records from 757 broadband seismic stations and 965 teleseismic events. The selected events have suitable epicentral distances, magnitudes, and recording quality. After waveform preprocessing, P-wave relative travel-time residuals were picked using waveform cross-correlation in the frequency band of 0.02–0.1 Hz. A total of 93 416 reliable P-wave relative travel-time residuals were obtained. Because relative travel-time residuals were used, the common origin-time error of each earthquake could be largely removed, reducing the influence of source-side uncertainties and making the inversion more sensitive to lateral velocity variations beneath the study area.

Finite-frequency body-wave tomography was applied to image the three-dimensional P-wave velocity structure from 0 to 800 km depth. Compared with conventional ray-theoretical tomography, finite-frequency tomography considers the finite-frequency sensitivity of seismic waves and can better describe wave propagation in heterogeneous media. The model was parameterized with a horizontal grid spacing of about 0.5°×0.5° and a vertical spacing of 50 km. The IASP91 model was used as the initial reference model. A damped least-squares inversion method was adopted. By analyzing the trade-off curve between data variance and model variance, a damping factor of 30 was selected for the final inversion, which provides a reasonable balance between travel-time fitting and model smoothness.

Resolution tests were carried out to evaluate the reliability of the tomographic model. Checkerboard tests show that the study area has good horizontal resolution from 50 km to about 700 km depth, especially beneath the North China Craton, the Datong volcanic field, the Bohai Bay region, the Ordos block, and the Yinshan–Yanshan tectonic belt. The horizontal resolution can reach approximately 0.5° in well-covered areas, whereas the vertical resolution is generally about 100 km and locally about 200 km. The tests also indicate that horizontal resolution is better than vertical resolution, which is consistent with the general characteristics of body-wave tomography. Additional synthetic tests and comparisons using different initial models suggest that the main deep anomalies are stable and are unlikely to be artifacts caused by downward smearing of shallow structures.

The tomographic results reveal strong lateral heterogeneity in the crust and upper mantle beneath North China. At depths of 50–200 km, the western Ordos block is mainly characterized by high-velocity anomalies, whereas the central Shanxi block, especially around the Datong volcanic field, shows a broad low-velocity anomaly. In the eastern North China Plain, both high- and low-velocity anomalies are observed, with a significant low-velocity anomaly beneath the Bohai Bay region. The Yanshan region is dominated by high velocities. At depths of 250–400 km, the high-velocity anomaly beneath the Ordos block gradually weakens, while low-velocity anomalies expand in the northern and eastern parts of the study area. In the mantle transition zone, low-velocity anomalies become more widespread beneath the eastern block, indicating complex mantle structure and possible deep material migration.

Four main low-velocity anomalies are identified in the model. The L₁ anomaly is located beneath the Datong volcanic field and surrounding areas. It is prominent from 50 km to about 200 km depth and can be traced locally to 350–400 km. The L₂ anomaly occurs along the southern margin of the Xing’an–Mongolia orogenic belt and extends to about 400 km depth. The _L3 anomaly is mainly distributed along the southern segment of the TanLu fault zone and is visible above about 300 km depth. The L₄ anomaly lies beneath the Bohai Bay region and extends to at least 200 km depth. These low-velocity anomalies are interpreted as the response of hot mantle materials, asthenospheric upwelling, or zones modified by mantle-derived intrusion.

A key result of this study is that the Datong and Bohai Bay low-velocity anomalies are separated in the shallow upper mantle but become connected below about 200 km depth and extend downward toward the mantle transition zone. This geometry suggests that they may not be completely independent shallow anomalies, but may be related to a broader mantle upwelling or material migration system beneath North China. The low-velocity anomaly beneath the Datong volcanic field may be associated with asthenospheric upwelling and mantle processes related to the stagnant Pacific slab in the mantle transition zone. However, the present results do not show a clear continuous low-velocity channel penetrating through the mantle transition zone into the lower mantle. Therefore, the Datong-related anomaly is more likely controlled by upper-mantle and transition-zone processes rather than by a simple lower-mantle plume. The Bohai Bay low-velocity anomaly may reflect asthenospheric upwelling beneath the eastern North China Craton and may be closely related to lithospheric thinning, thermal erosion, and craton destruction.

By integrating teleseismic P-wave tomography with previous local earthquake tomography and v_P/v_S results, this study further discusses the deep background of the Zhangjiakou–Bohai Seismic Belt. Previous local earthquake tomography shows that beneath the Tangshan region, a continuous low-velocity body exists below about 20 km depth and extends into the lower crust. This feature may represent a channel for mantle-derived material entering the crust. v_P/v_S results indicate that the middle–lower crust of North China commonly has relatively high v_P/v_S values, mostly greater than 1.74 and locally reaching about 1.80. These values are higher than those of typical continental crust and are closer to mafic or mantle-derived materials, suggesting that the crust has been modified by mafic intrusion, mantle underplating, fluids, or thermal alteration. However, the v_P/v_S values do not indicate widespread large-scale partial melting in the crust.

Overall, the low-velocity anomaly beneath the Zhangjiakou–Bohai Seismic Belt may include a mantle-derived upwelling pathway, especially beneath the Tangshan region. This pathway is probably more closely connected with the Bohai Bay low-velocity anomaly than with a direct vertical source beneath the Datong volcanic field. In other parts of the belt, low-velocity anomalies may mainly reflect dispersed magmatic underplating and mantle-derived intrusion into the lower crust. These processes likely caused thermal weakening, compositional modification, and partial destruction of the lithosphere, thereby influencing the crustal stress field and the seismogenic environment.

In conclusion, this study provides a finite-frequency P-wave tomographic model of North China from 0 to 800 km depth and offers new constraints on the deep origin of low-velocity anomalies beneath the Zhangjiakou–Bohai Seismic Belt. The results show that the Datong volcanic field and Bohai Bay both have significant low-velocity anomalies extending deeper than 200 km. These anomalies are connected below about 200 km and continue toward the mantle transition zone. The Zhangjiakou–Bohai Seismic Belt may contain a mantle upwelling channel beneath the Tangshan region, which is likely related to the Bohai Bay low-velocity anomaly. The study suggests that mantle upwelling, magmatic underplating, and lithospheric modification have played important roles in shaping the deep structure and seismic environment of the region.