Relationship between seismicity and crustal thermal structure in North China

This study relocates 6225 earthquakes recorded in North China during 2009 to 2015 using the double-difference location algorithm. The result shows that the relocated earthquakes are more concentrated around the faults and most focal depths range between 5 and 15 km. Then the Curie-point depths are estimated by using the centroid spectral method based on 3D fractal magnetization model, and the average thermal gradients of the magnetic layer are calculated. Furthermore, crustal temperatures are estimated based on the 1D steady thermal conduction equation with temperature-dependent conductivities. Our results show that most of the earthquakes occurred in the regions with low thermal gradients except for those in the central-eastern part of the Zhangjiakou-Bohai seismic zone. Both the 1966 M_S7.2 Xingtai earthquake and 1976 M_S7.8 Tang-shan earthquake occurred in the low thermal gradient regions with estimated tem-peratures ranging between 200℃ and 300℃. The evaluated temperature range for most of the earthquakes with M≥2.0 is between 100℃ and 500℃, and the temperature range is between 200℃ and 400℃ for large earthquakes with M≥4.0. All these temperatures are in line with that of the crustal brittle-ductile transition observed in laboratory studies, suggesting that most earthquakes in North China occurred in the crustal brittle-ductile transition zone.