关键词: 汶川地震, 综合震源机制, 地壳应力场, 应力方向集中

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.