Contrast of rheology in the crust and mantle near Moho revealed by depth variation of earthquake mechanism in continental China

Rheology of continental lithosphere is an important, but not well resolved basic issue in geodynamics. There are observational and experimental evidences to support a model of strong mantle-ductile weak lower crust near the Moho; there Are also evidences to support a contrast model of weak ductile mantle-strong crust. In this study, we use CMT data of Harvard with corrected focal depth to analyze the depth variation of plunge angle of principal stresses in continental China. It is found that the plunge angles of earthquakes are either nearly horizontal or close to vertical for very shallow earthquakes, while there are great varieties in the plunge angle in the middle and lower crust and in the mantle. This feature is distinctively different from that in the regions with weak ductile lower crust, where the plunge angles of principal stresses at the base of earthquake generation layer are found nearly horizontal or vertical similar to the case of very shallow earthquakes. It may indicate that both kinds of crust-mantle rheology models exist in China. Therefore, detailed integrated study on regional litho-spheric rheology is necessary.