中国大陆震源机制深度变化反映的地壳-地幔流变特征

大陆岩石层流变性质是一个重要而又没有完善解决的问题：既存在支持Moho面附近为强地幔-流动弱地壳模式的实验和观测事实，也存在支持流动弱地幔-强地壳模式的观测事实. 本文利用哈佛震源机制矩张量解及有关震源深度资料，对我国震源机制随深度的变化进行分析，发现虽然表浅地震震源机制主应力轴倾角接近水平或竖直，但中、下地壳和上地幔震源机制主应力轴倾角则多样化. 这与某些存在柔性下地壳软弱层地区具有孕震层下缘震源机制主应力轴也接近水平或竖直的特征明显不同. 它可能表明上述两种壳-幔流变模式在我国均存在，对我国岩石层流变性质还需要分区进行更细致的多方面研究.

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 lithospheric rheology is necessary.