Research On The Stress Field In 2003 Dayao Earthquake Source Region

The Yunnan Dayao earthquake with Ms6.2 occurred on 21th July 2003, followed by a major Ms6.1 earthquake some 88 days later in the same region. The two epicenters between each other only 8km, the hypocenters is almost coincidence in the same place. Up to now the activities of small and moderate earthquakes are still frequent in Dayao region. Many seismologists made studies on earthquake sequence, double-difference earthquake location and velocity structure. In recent years, with the development of many observation networks, especially digital seismic observation system, rapidly layout the mobile observation stations surrounding the epicenter after earthquake greatly enriching the observation data of small and moderate earthquakes. To further understanding the scientific theory and seismic activities provides a good database platform and provides excellent seismic data for this article.This article based on the background of seismic observation data made by mobile seismic network using the new version of grid points test method (Yu Chunquan et al.,2009) solve the composite plane solution of aftershock zone and get the main compressive stress direction. Using the stress value calculation method (Wan Yongge,2006a) get the underground stress value in the focal area, further revealing the local stress field characteristics in the region of Dayao.The major content for this paper consists of three parts:In the first part, the geologic tectonic background and the spatial distribution of small and moderate earthquakes in Dayao area are described briefly. The result shows that Dayao earthquake did not occurred in the main fault zone, the epicenters of the two mainshocks is very close, the spatial distribution of aftershocks is obvious linear distribution and the faults are almost vertical strike-slip fault. This Dayao double earthquakes are different from other double earthquakes occurred in Yunnan, which is almost a straight line and there is some overlap between each other, indicates that two earthquakes are controlled by the same fault. Relative to the mainshock, the distributions of aftershocks are nonuniform and are almost a kind of unilateral distribution. The aftershocks occurred by Ms6.2 earthquake concentrated in the northwest of the mainshock. The aftershocks occurred by Ms6.1 earthquake concentrated in the southeast of the mainshock. Ms6.1 earthquake is another rupture in the same fault extending to the south. In this paper we use the double difference earthquake location result (Hua Wei et al.,2006) to reduce the error influence of the absolute positioning methods on velocity structure. Finally, we use the method (Yongge Wan et al.,2008) to determine the strike and dip of the fault plane.In the second part, using the P wave first motion information of two aftershocks sequence recorded by mobile observation stations on 22th July to 21th August and 17th October to 17th November and the new version of grid points test method, we study the stress field in the aftershocks zone. The results indicate that the direction of maximum principal stress axis of aftershocks zone is close to level trending in north northwest or west southwest. The intermediate principal stress axis is almost perpendicular. The minimum principal stress axis is close to level trending in east northeast or west southwest. The maximum’ principal stress axis of aftershocks zone has some deviation from level trending in northwest or southeast after mainshock occurred on 16th October. The intermediate principal stress deviate from the vertical. The minimum principal stress axis has some deviation from level trending in northeast or southwest. Comparing to the stress field of aftershocks zone in previous period, the maximum, intermediate and minimum principal stress axis generate deflection. Combining the P wave first motion of the two aftershock sequences, we get the stress field direction in Dayao region. The type of composite plane solution is strike-slip mode, the obtained composite plane solutions and the fault plane solutions have good consistency.In the third part, we calculate the stress magnitude of the source region of Dayao earthquake. The expressions for pre-seismic and post-seismic stress direction in geographic coordinate were given first, then we converted the pre-seismic stress field and the stress drop to the post-seismic principal stress coordinate, and the mathematic relationship among them was established. According to the principal that the shear stress equals to zero in the post-seismic principal stress coordinate, the equation whose unknown parameter is the pre-seismic stress value is established. Solving the equations we can get the three maximum, intermediate and minimum principle stress magnitude before earthquake occurrence, and then projected onto the fault plane, calculate the normal stress and shear stress generated on fault before and after earthquake occurrence.The results of this part show that the pre-seismic shear stress is larger than that of post-seismic, and their difference corresponds to the stress drop approximately. The underground compress stress is very large, but the difference stress is small. Finally we analysis the influence of calculation parameters biases to stress magnitude.