We present detailed 3-D isotropic P and S wave velocity (Vp, Vs) images and Vp azimuthal anisotropy in source areas of 26 large crustal earthquakes (M5.6–7.8) that occurred in North China during 1966 to 2021. Most of the large earthquakes are located in high-velocity (high-V) zones or close to the boundary between low-velocity (low-V) and high-V anomalies. Prominent low-V and high Poisson's ratio (high-σ) anomalies are revealed in the middle and lower crust under almost all the source areas, which may reflect crustal fluids. The low-V and high-σ anomalies may cause local thinning and weakening of the brittle seismogenic layer above them. The crustal fluids may originate from hot and wet upwelling flows in the big mantle wedge (BMW) beneath East Asia due to corner flows in the BMW and deep dehydration reactions of the stagnant Pacific slab in the mantle transition zone.
- Crustal structure
- Earthquake dynamics
- Intraplate seismicity and tectonics
- North China
- Seismic tomography