The dynamic implication of focal mechanism solutions of Wenchuan earthquake sequence

At 14:28 CST on May 12, 2008, the disastrous MS8.0 Wenchuan earthquake took place in Sichuan province of China, followed by tens of thousands of aftershocks. It occurred on the Longmenshan fault zone which is a high-angle inland over-thrust fault. What is the corresponding tectonic implication behind the focal mechanism is an important issue to realize the dynamic mechanism of Wenchuan earthquake sequence. In order to reveal it, our research as following is carried out. 1) We extensively collected digital waveform records, seriously and strictly read out P wave first motion, used more accurate locating results, employed the improved grid point test method, computed the focal mechanism solutions of Wenchuan earthquake sequence, and gave out 125 reliable focal mechanism solutions (M≥4.0,including a M3.9 earthquake result). The result shows that most of the earthquakes are thrust or strike-slip. The focal mechanism solutions have characteristic of subsection distribution. The P axis mainly distribute in direction of E-W within a certain range. 2) According to the main faults information and fine medium parameters, using ANSYS software, we established a three-dimensional elastic finite element model of the Longmenshan fault zone and its surrounding areas. According to Global Position System observations, considering three representative patterns of tectonic deformation velocity varying with depth, we loaded three different boundary conditions and got each numerical simulation result of the tectonic stress field in this region. The simulation results of the three patterns consistently show that the orientation of the principal compressive stress is almost E-W in Longmenshan fault zone and its surrounding area. And from north to south, there is a clockwise rotation within a narrow range. The result is consistent with the previous research in these areas. 3) Based on the numerical simulation results of tectonic stress field of three structural deformation patterns, employing two different methods, we computed theoretical focal mechanism solutions. The results show that the tectonic stress field, corresponding to the pattern that lower crust in plateau has greater deformation velocity, is most consistent with the distribution of the focal mechanism solutions of Wenchuan earthquake sequence. 4) Through the research of theoretical focal mechanism solutions, we found that, as to the focal mechanism solutions of Wenchuan earthquake sequence, regional tectonic stress is an important dynamic factor to their distribution, while seismogenic fault structure have great effect to their specific structural shape. A small part of the aftershocks have inconsistent focal mechanisms with the regional tectonic stress field, and their mechanisms are worth further exploring.