The Research On Dynamic Stress Field Before And After Lushan Earthquake Based On Gravity Data

As one of the most direct and main force sources of the earth's tectonic movement,gravity is closely related to the tectonic stress field inside the earth,and the study of tectonic stress field is a very important link in the study of geodynamics.Tectonic stress field can reflect the activity degree of earthquake fault,and is an important quantitative index for the stress state of earthquake fault.It is also an important parameter for in-depth study of the mechanical driving mechanism of intraplate earthquakes and inter plate earthquakes.The Western Sichuan area is located in the eastern margin of the Tibetan Plateau,and also located in the North-South seismic belt,including the Longmenshan and Xianshuihe-Anninghe fault zones,which is the key areas to study the eastward flow of the Tibet Plateau,and the interaction between the Tibet Plateau and the Southern China block.The West Sichuan area is also the key region to explore the structural deformation mechanism of the Wenchuan Ms8.0 earthquake and the Lushan Ms7.0earthquake,and to carry out the earthquake risk assessment.We studied the method of computing the static and dynamic tectonic stress field by the earth gravity field,and then the horizontal tectonic stress field in Western Sichuan is computed with the EGM2008 and the ground measured gravity data.The specific contents are as follows:(1)Method to calculate the static and dynamic tectonic stress field by the earth gravity fieldOn the basis of static equilibrium principle,the method of calculating static and dynamic tectonic stress by gravity anomaly and its variation is discussed.Taking the Sichuan Yunnan area(96°~108°E,20°~34°N)as an example,the static tectonic stress field trial is carried out,which is basically the same with the existing research results,which verifies the correctness of the calculation program.(2)Calculation results and analysis of static tectonic stress field in Western SichuanThe static tectonic stress field in West Sichuan area(26.5°~32°N,100°~105°E)is calculated by using EGM2008,the results showed that we can quickly obtain the tectonic stress field by using gravity field data.The horizontal tectonic of the Longmenshan and Xianshuihe-Anninghe fault zones is larger,and the accumulation degree is higher than any other area,which reflect that the tectonic movement is more active in these areas.The horizontal tectonic stress from the Bayan Har block to the Southern China block to the Sichuan-Yunnan block shows clockwise rotation.In the Bayan Har block and Southern China block boundary the strong stop of Sichuan Basin in South China block results in the formation of Longmenshan foreland thrust zone.The tectonic stress field gradually decreases with the increase of depth,and the frequency of seismic activity decreases accordingly.The tectonic stress field is densely distributed with active tectonic activity and frequent seismic activity,which indicates that there is a corresponding relationship between seismic activity and tectonic stress accumulation.(3)Calculation results and analysis of dynamic tectonic stress field in West Sichuan area before and after Lushan earthquakeThe dynamic tectonic stress field of West Sichuan area before and after Lushan earthquake was calculated and analyzed by using the measured data of ground gravity provided by China Earthquake Administration from 2010 to 2015,and the corresponding dynamic tectonic stress field images were drawn.Because of the active tectonic activity in Western Sichuan,the dynamic tectonic stress field obtained by the dynamic tectonic stress field contains several moderate strong earthquake information.The results show that the tectonic stress field can reflect the inoculation process of Lushan earthquake to a certain extent,and the Lushan earthquake occurred in the gradient turning of tectonic stress and the intersection part of which,indicating the Lushan earthquake is related to the local anomaly of the tectonic stress field,the high gradient belt,its turning and intersection parts.the tectonic stress accumulation in the southern section of the Longmen Mountain Fault Zone before Lushan earthquake shows a similar four quadrant distribution characteristics similar to the gravity field,indicating that the earthquake has the characteristics of structural stress change caused by the sinistral strike-slip,and the strong contrast(mutation)of the tectonic stress indicates that the Lushan earthquake area near the epicentral area is more strongly closed before the earthquake.The Wenchuan earthquake has a positive effect on the Lushan earthquake.The post earthquake Lushan earthquake area has obvious post earthquake recovery trend.At the same time,there are many moderate strong earthquakes in the study area before and after the Lushan earthquake,and the dynamic tectonic stress field can reflect the change trend of the corresponding tectonic movement,the characteristics of the earthquake precursors and the characteristics of the recovery trend after the earthquake.