Anatomy of large earthquakes in North China

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-Sigma) 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-Sigma 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.

Automated summary

In this study, detailed 3-D velocity images and velocity azimuthal anisotropy in source areas of 26 large crustal earthquakes in North China from 1966 to 2021 are presented. Most of the earthquakes occurred in high-velocity zones or near the boundary between low-velocity and high-velocity anomalies. Prominent low-velocity and high Poisson's ratio anomalies are observed in the middle and lower crust, indicating the presence of crustal fluids.