| 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 mainland interior over-thrust fault, and is different from other great thrust-type earthquakes in recorded history. The scientific issues of Wenchuan earthquake sequence, such as seismogenic mechanism, evolution regularity and tectonic background et al, attracted wide attention from geosciences field at home and abroad. Tectonic stress is the direct cause of earthquake preparation and evolution, while focal mechanism solution can reveal the fault properties and the seismic mechanism. What is the corresponding tectonic implication behind the focal mechanism solutions is an important issue to realize the dynamic mechanism of Wenchuan earthquake sequence.In order to reveal the dynamic mechanism of Wenchuan earthquake sequence, this research is carried out, using plentiful seismic data of this earthquake sequence, mainly in terms of focal mechanism solutions and tectonic stress field. Firstly, we determined the focal mechanism solutions using the P wave first motion, and analyzed its characteristics. Secondly, employing finite element method, we simulated the tectonic stress field of Longmenshan fault zone and its surrounding areas. Thirdly, we calculated the theoretical focal mechanism solutions based on the simulation result. Finally, through comparing the theoretical focal mechanism solutions with practical results, we tentatively analyzed the quantitative relationship between the character of focal mechanism solutions of Wenchuan earthquake sequence and regional tectonic stress field as well as fault structure.As the idea above, our main research and conclusions include the following four parts:1) We deeply analyzed the theory and key issues of determining focal mechanism solution by using P wave first motion. Based on the grid point test method given by Xu Zhong-Huai, we improved the inversion method mainly from four aspects:①The weight of P wave first motion data is determined by its credibility and intensity. In this way, the solution error caused by the uneven distribution of data can be eliminated to some extent.②Clustering analysis is appended to the inversion process. We cluster solutions for all optional ones, and give the average solutions for each cluster. If there are multiple average solutions, the mechanism solution is limited to the few alternatives solutions.③Employing jackknife technique, the inversion quality improves. This technique not only increases the possibility of finding out true solutions, but also makes us comprehend the quality of focal mechanism solutions unambiguously.④The B-axis search method is ameliorated. It becomes simple and more uniform. After these, through theoretical data verification, we validated the reliability and validity of the improved method. In addition, we provided a new scheme for evaluating the quality of focal mechanism solutions based on the dispersion of optional solutions as well as weighted inconsistency ratio.2) 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 sequence, and gave out 125 reliable focal mechanism solutions (M≥4.0,including a M3.9 earthquake result). The results show that the initial rupture of mainshock is purely thrust, 124 aftershocks are mostly thrust or strike-slip. The focal mechanism solutions have characteristic of subsection distribution. The P axis mainly distribute in the direction of E-W within a certain range.3) 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 changing 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. It indicates that the velocity difference of the tectonic deformation on the model's boundaries is the main factor to control tectonic stress field in this region. In addition, the direction of the principal compressive stress and the rotation trend are consistent with the previous research in these areas, which could verify the rationality of our simulation results to a certain extent.4) 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.5) 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. |
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