| The Great Wenchuan Earthquake, measured at8.0Ms, occurred on Monday, May12in Wenchuan County, Sichuan province. The earthquake epicenter was located in Yingxiu (N31.00, E103.40).The main seismogenic fault was the Longmen Mountain central fault. The Wenchuan earthquake is another major geological movement occurring in Longmen Mountain fault zone in nearly three centuries. With the large earthquake magnitude, shallow focus, numerous activefaults and causing a large number of geological disasters including landslides,avalanches, and debris flow, it showed a serious devastation and a large rangeof impact.This paper is based on the Dujiangyan-Wenchuan highway tunnel repairingand construction project and Guanyuan-Gansu highway construction in Longmen Mountain region. Through the ground stress measurement field research andnumerical simulation, it fully revealed the characteristics of ground stress in Longmen Mountain region. Analyzed the geostress of the bedding faults strike and reverse faults strike and the rule of displacement mutation at different distances from the earthquake fracture. Propose the different characteristics and the rule of displacement mutation of the geostress before and after the earthquake in Longmen Mountain region. According to the research on the characteristics and rule of the greostress in Longmen Mountain region, it has an important reference value and guiding significance on the projects under construction or under plan in this region. The region area nearily5.2×105km2and200~300m deeps. The main fidings and the major works are as followed: (1)Before the earthquake, The highest concentrations of the maximum main stress and shear stress was near the Longmen Mountain fault, maximum stress occurs in the central fracture and regions like Beichuan, Maoxian, Wenchuan, Yingxiu before the back mountain fracture. Maximum main nearily11.1MPa.After the earthquake, Maximum main nearily8.4MPa,the stress decreased significantly near the fault which introduced that nearby faults released a certain amount of energy. It had a higher shear stress in the south-west and north-eastof Longmen Mountain fault. Shear stress nearly0.48MPa.The Shear stress ishigher than the surrounding places in the Longmen central fault near Beichuan.(2)The maximum main stress had an overall directions from NEE to SEEboth before and after the earthquake and only had some stress field changes insome areas. While the direction of displacement vector is from east to west both before and after the earthquake.Specific changes is that behind the longmenshan stress direction changed S45°E to N45°E before and after the earthquake.In the middle of Longmen mountain stress direction changed S50°E to N50°E before and after the earthquake.(3)The change rule of the stress before and after the earthquake. Before the earthquake, the stress showed a trend of high in the middle and low at theends. After the earthquake, it showed a trend of high at the ends and low in the middle. That is to say, the central fault of Longmen Mountain released energy affected by the earthquake.(4)The change rule of the displacement before and after the earthquake. Ithas a small average displacement3~4mm before the earthquake while a significant increasing15~18mm after the earthquake in the research area. It means that the velocity increased in the entire region after the earthquake and Longmen Mountain fault was more active than before. The overall displacement movement trend showed a progressively larger trend from southeast to northwest.(5)For the future engineering activities in the Longmen Mountain region, itneeds to focus on Sichuan-Yunnan land masses in the south-central part of Longmen Mountain fault zone and areas near the Xianshui River fault. The stress in this two areas are both higher and the stress direction was trending there.Besides, the moving velocity was higher than other areas and they belong to the areas with strong activity. It is worth our attention as the stress value is relatively high in the north-east region of the study area after the earthquake. |
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