The Zhangjiakou–Bohai Seismic Belt （ZBSB） is one of the most active intraplate seismic zones in eastern China and constitutes an important tectonic boundary between the Yanshan Uplift and the northern Bohai Bay Basin. Extending northwestward from Zhangjiakou to the Bohai Sea, the belt is characterized by a complex system of NW- and NE-trending conjugate faults and has experienced several devastating earthquakes, including the 1679 Sanhe–Pinggu M8.0 earthquake and the 1976 Tangshan M_S7.8 earthquake. Despite extensive geological and geophysical investigations, the deep geodynamic processes controlling seismicity within the ZBSB remain controversial. Previous seismic tomography, gravity, and magnetotelluric （MT） studies have revealed significant lithospheric heterogeneity beneath the region, including low-velocity, high-conductivity, high-v_P/v_S, and high-density anomalies in the middle–lower crust and upper mantle. However, the relationship between these deep structural anomalies and the mechanical properties of the lithosphere remains poorly understood. Since lithospheric rheology exerts fundamental control over stress accumulation, deformation localization, and earthquake generation, constraining the rheological structure of the ZBSB is crucial to understanding its seismogenic mechanisms.

In this study, we estimate the lithospheric effective viscosity structure beneath the central segment of the ZBSB using a three-dimensional electrical resistivity model derived from broadband magnetotelluric observations. The MT dataset consists of measurements from 100 stations acquired in 2023 using Phoenix MTU-5A instruments. The average station spacing is approximately 10 km, with denser coverage of about 5 km in the Beijing and Tangshan regions. Data were processed using robust estimation and remote-reference techniques to suppress cultural noise and improve impedance quality. The final impedance dataset spans periods from 0.01 to 3000 s. A three-dimensional resistivity model was obtained using the ModEM inversion package, yielding an overall root-mean-square misfit of 1.55 and reliable imaging down to approximately 60 km depth.

The resistivity model reveals strong lateral and vertical heterogeneity beneath the study area. In the Tangshan region, a prominent high-resistivity body （R1）, with resistivity exceeding 1000 Ω·m, extends from the surface to depths of approximately 30 km on the eastern side of the Tangshan Fault. The 1976 Tangshan earthquake occurred near the boundary of this resistive body. West of the Tangshan Fault, a large conductive anomaly （C1） is observed from depths of approximately 20 km to the upper mantle, with resistivities ranging from 20 to 50 Ω·m. Geological and geophysical evidence suggests that this anomaly is unlikely to be caused by interconnected graphite films or sulfides. Instead, considering the elevated geothermal gradient, high heat flow, gravity anomalies, and regional tectonic activity, C1 is interpreted as partially molten mantle-derived material, possibly accompanied by fluids. In contrast, the Sanhe–Pinggu region is dominated by a large resistive body （R2） extending from the surface to the Moho discontinuity, corresponding to a preserved crystalline basement. The 1679 Sanhe–Pinggu earthquake and most recent small-to-moderate earthquakes occurred within or near the margins of this rigid block.

To investigate lithospheric rheology, we employ the empirical resistivity–viscosity relationship proposed by Liu and Hasterok （2016）, which links electrical resistivity and effective viscosity through their common dependence on temperature, pressure, grain size, and activation energy. Because this method is valid only when conductivity anomalies are associated with rheologically weak materials, such as melts or fluids, interpreting C1 as partially molten material provides a necessary physical basis for viscosity estimation. Model parameters were calibrated by comparing resistivity-derived surface strain rates with geodetically observed maximum shear strain rates from GNSS measurements. The optimal parameters were determined to be C₀＝1.3 and C₁＝2.3, yielding a negligible misfit between the modeled and observed strain rates.

The resulting viscosity model reveals pronounced rheological contrasts across the central ZBSB. High-viscosity domains （＞10²² Pa·s） are concentrated within the lithosphere beneath the Sanhe–Pinggu seismic zone and the upper crust of the Tangshan region, indicating the presence of mechanically strong crystalline basement blocks. In contrast, extensive low-viscosity zones （10¹⁹–10²⁰ Pa·s） occur in the middle–lower crust and upper mantle between the two seismic regions and spatially coincide with the conductive anomaly C₁. Beneath Tangshan, the lower crust exhibits significant viscosity reduction, whereas the lower crust beneath Sanhe–Pinggu remains mechanically strong. Several vertically continuous low-viscosity channels are identified beneath the Fengtai–Yejituo fault and Xiadian fault, extending from the upper mantle toward shallow crustal levels. These channels likely represent pathways for fluid and heat transport and correspond well with the distribution of geothermal manifestations.

Based on the rheological structure and regional geodynamic framework, we propose distinct seismogenic models for the Tangshan and Sanhe–Pinggu earthquake zones. In the Tangshan region, low-viscosity mantle-derived material ascends obliquely toward the northeast along pre-existing weak zones in the lithosphere at the boundary between the Bohai Bay Basin and the Yanshan Uplift. The upwelling material mechanically pushes the overlying rigid crustal block, weakens the Tangshan Fault through fluid infiltration, and thermally erodes the lower crust. These processes reduce the mechanical stability of the lithosphere and facilitate stress concentration within the overlying high-viscosity block, ultimately contributing to fault rupture and the 1976 Tangshan M_S7.8 earthquake. Ongoing interaction between deep upwelling material and the crust may also explain the persistent seismic activity observed in the region today.

In the Sanhe–Pinggu region, the lithosphere is characterized by a relatively intact, mechanically strong crust–mantle structure. The rigid crystalline basement serves as the principal stress accumulation zone, whereas the low-viscosity material beneath its eastern margin accommodates deformation through ductile flow, thereby reducing stress accumulation rates. Fluid migration along the Xiadian Fault likely decreases fault strength by elevating pore-fluid pressure and reducing effective normal stress. Under the influence of regional ENE–WSW compressional stress, potentially enhanced by eastward migration of mantle-derived thermal material from the Datong volcanic region, stress became concentrated within the rigid block and eventually triggered right-lateral strike-slip failure along the Xiadian Fault, resulting in the 1679 Sanhe–Pinggu M8.0 earthquake.

Our results demonstrate that the central segment of the ZBSB exhibits strong lithospheric rheological heterogeneity and that deep low-viscosity mantle-derived materials play a fundamental role in controlling crustal deformation and earthquake generation. The combination of three-dimensional MT imaging and resistivity-derived viscosity estimation provides new constraints on the seismogenic environment of major intraplate earthquakes in North China and offers valuable insights into the interaction between deep mantle processes, lithospheric rheology, and seismic hazard in the Zhangjiakou–Bohai Seismic Belt.