Crust–mantle velocity structure and seismogenic environment in the NE margin of the Tibetan Plateau and central-western North China constrained by deep-learning-based phase picking

The northeastern (NE) margin of the Tibetan Plateau and central–western North China are located in the tectonic transition zone between eastern and western China and constitute a key linkage between the two domains. Therefore, research on the deep structure of this region is important for understanding the interactions between tectonic blocks and the seismogenic mechanisms of earthquakes. This study used seismic phase data from the China Earthquake Networks Center during 2009–2023 and artificial intelligence (AI)-picked body-wave phases from ChinArray-II/III portable arrays. After uniform quality control, we built P-wave, S-wave, and S–P traveltime datasets and applied a double-difference tomography method incorporating a P-wave to S-wave velocity ratio (v_P/v_S) consistency constraint to invert three-dimensional (3D) P-wave velocity (v_P), S-wave velocity (v_S), and v_P/v_S structures for depths from 0–60 km. The results show that the crust–upper mantle velocity structures beneath the NE margin of the Tibetan Plateau and central–western North China exhibit pronounced lateral heterogeneity and are closely related to the regional tectonics. Low-velocity anomalies in the shallow crust are primarily controlled by sedimentary layers in faulted basins, whereas the velocity and v_P/v_S structures from the middle–lower crust to the top of the upper mantle reflect the effects of deep material movement and lithospheric thinning and weakening. The 2023 M_S6.2 Jishishan and 1303 M8.0 Hongdong earthquakes both occurred in fault-bounded zones with distinct velocity and v_P/v_S contrasts. The Jishishan mainshock occurred in a velocity transition zone, with aftershocks in the brittle upper crust with high v_P, v_S, and low v_P/v_S. The mid-lower crust shows low velocities and high v_P/v_S, which are likely related to the northeastward flow and compression from the Tibetan Plateau. In contrast, the Hongdong source region features high v_P, v_S, and low v_P/v_S at shallow depths and high v_P/v_S at depth and is controlled by distant Tibetan Plateau compression, deep upwelling, and basin extension. The obtained high-resolution 3D velocity model reveals significant heterogeneity in the crust–mantle structure beneath the NE margin of the Tibetan Plateau and central–western North China, as well as its intrinsic relationship with seismic activity. These results provide important geophysical constraints for understanding regional tectonic dynamics and the seismogenic mechanisms of strong earthquakes in this domain.