Abstract:

The occurrence of earthquake is closely related to the crustal stress field. Earthquakes are caused by the failure of faults, driven by tectonic stress build-up in the Earth’s crust. The change of the stress field before a large earthquake is directly related to the earthquake preparation process. In order to understand the relationship between the tectonic stress field and the low-level seismicity of the Longmenshan Fault and adjacent region before the 2008 Wenchuan earthquake, the composite focal mechanism method based on P wave first motions of small and medium earthquakes is used to determine the tectonic stress field before the Wenchuan earthquake and analyze the temporal and spatial characteristics of the composite focal mechanisms.
Accurate earthquake location is a necessary factor to determine the focal mechanism and the stress field, especially to invert the focal mechanism and the stress field using P wave first motion of the near-field and local earthquake. Firstly, we estimated the hypocentral location and its uncertainty of a large number of small and medium earthquakes in Sichuan, China with a relatively accurate earthquake location method by considering the arrival time uncertainty. Secondly, the azimuth and take-off angle of the P wave first motion of a large number of small and medium earthquakes were calculated, whose focal mechanisms usually cannot be determined from small amount of P wave first motions, and the different weight values were given to the P wave first motion according to the hypocentral distance. Then we determine the composite focal mechanisms on the 0.5°×0.5° grid point in Sichuan area before the Wenchuan earthquake by using the composite focal mechanism method. The results show that the principal compressive stress(P)axes and principal tensile stress(T)axes of the composite focal mechanisms have obvious zoning characteristics, divided roughly by the Longmenshan Fault, the Xianshuihe Fault, and the Huayingshan Fault. The direction of the compressive axis of the northern Sichuan block from the west of the Longmenshan fault zone to the Longriba Fault is near EES-WWN, and that of the extension axis is nearly vertical, which results in the movement pattern of thrusting with right-lateral strike-slip in the Longmenshan fault zone and promoted the accumulation of stress field before the Wenchuan earthquake. The composite focal mechanisms in the south of the Xianshuihe Fault show a strike-slip pattern, which perfectly explains the sliding behavior of a series of major strike-slip earthquakes on the Xianshuihe Fault. The southeast segment of Huayingshan Fault presents a thrust pattern, which is consistent with the paleostress model proposed by predecessors. Thirdly, in order to understand the temporal variation of the crustal stress field before the Wenchuan earthquake, we calculate the focal mechanism rotation angles(FMOAs)of the annual composite focal mechanisms taking the Wenchuan earthquake as the time end to the focal mechanism of the Wenchuan earthquake obtained by different authors and institutions before the Wenchuan earthquake. It is found that the FMOAs of all the focal mechanisms of different authors and institutions reached its minimum value and were lower than its standard deviation 1 year before the Wenchuan earthquake. In view of the large rupture scale of the Wenchuan earthquake, we calculate the FMOAs of the annual composite focal mechanisms to the focal mechanisms of the Yingxiu-Hongkou initial rupture segment and Beichuan rupture segment before the Wenchuan earthquake. The results show that the FMOA of the Yingxiu Hongkou section decreased obviously, which indicates that this method can predict the location of future earthquake to some extent. Finally, in order to verify the uniqueness of convergence of stress field before the Wenchuan earthquake, we calculated the FMOAs of the annual composite focal mechanisms to the focal mechanisms of the other four reference points except the location of the Wenchuan earthquake in Sichuan area, and the results did not show the phenomenon that the stress direction of the four points tends to be consistent.
Above all, the temporal and spatial variation characteristics of the FMOAs of the stress field show that the focal mechanism and location of the Wenchuan earthquake are closely related to the convergence of the composite focal mechanism around the epicenter before the Wenchuan earthquake, which illustrates that the convergence tendency of the stress field to the Wenchuan earthquake rupture may provide a new idea to explore large earthquake precursor from tectonic stress field.

Key words: Wenchuan earthquake, composite focal mechanism, crustal stress field, convergence of stress direction.

摘要：

地震的发生与地壳应力场有密切的联系, 地震是应力积累到一定程度使得断层破裂的结果, 而大地震之前的应力场变化与大地震的孕育和发生有着最直接的联系。为理解地震活动较低的2008年汶川地震之前的构造应力场与该地震发生的关系, 文中采用能综合利用小地震P波初动信息的综合震源机制方法求解汶川地震之前的应力场。首先采用2001年1月—2008年5月12日汶川8.0级地震前的P波初动符号求解了四川地区大量中小地震的综合震源机制, 发现综合震源机制的压轴和张轴方向以龙门山断裂带、 鲜水河断裂带和华蓥山断裂带为界。龙门山断裂带以西至龙日坝断裂的川北地块压轴近SEE-NWW向, 张轴近垂直, 致使该地块在龙门山断裂带上表现为以逆冲为主兼右旋走滑的运动方式, 有益于汶川地震孕育过程中的应力积累。鲜水河断裂南段及附近地区的震源力学机制呈现为走滑模式, 完美地解释了鲜水河断裂上一系列走滑大地震的滑动行为。华蓥山断裂南东段则呈现为逆冲模式, 与前人提出的该地区的古应力模式一致。为研究汶川地震之前地壳应力场随时间的变化, 文中还分时段计算了汶川地震前综合震源机制旋转到汶川地震震源机制的旋转角变化, 发现采用不同作者和机构得到的汶川地震震源机制均可得到旋转角在震前1年达到最小值的结论, 且低于旋转角的标准差。以上结果表明, 汶川地震之前出现了应力方向趋于促使主震破裂方向的现象。

关键词: 汶川地震, 综合震源机制, 地壳应力场, 应力方向集中