川甘陕交汇地区现今地应力环境与地震危险性

摘要

The Sichuan-Gansu-Shaanxi border area has experienced many great earthquakes with complicated tectonic deformation and tectonic activities, which is the reason why this area is important for study of seismic risk. In order to better understand in−situ stress environment of Tianshui area and to evaluate the seismic risk, it is necessary to conduct deep borehole in−situ stress measurement. In this study, data were gathered from 9 in−situ stress sensors installed in deep borehole (600 m) in the southwest of Gangu County, Gansu Province and 14 in−situ stress sensors installed in deep borehole (400.12 m) at Sandui Town, Sichuan Province. The in−situ stress data reveal that the geostress level of the region is relatively high and both horizontal principal stresses in two tests are larger than the vertical stress (SH>Sh>Sv). The direction of the current maximum horizontal principal stress is N41°E in Gangu borehole and is N85°W in Sandui borehole. The directions of the current maximum horizontal principal stress reflect that the present activity property of the north margin of west Qinling fault is thrust and sinistral slip, and that the present activity property of the Qingchuan fault is thrust and dextral slip. The activity of the north margin of west Qinling fault and the Qingchuan dault was analyzed by Coulomb friction sliding criterion, and the results reveal that the current in−situ stress levels have reached the critical condition of sliding instability. Based on the in−situ stress state and earthquake migration data, it is suggested that more attention should be paid to the Sichuan-Gansu-Shaanxi border area. The results obtained by the authors have great significance for analysis of active faults, long−term monitoring of in−situ stresses, assessment of the regional geological environment and geological disasters prevention.%川甘陕交汇地区新构造活动强烈、地震频发，具有复杂多样的构造变形模式和构造强烈活动特征，为一潜在地震危险性研究的关键构造部位。为了查明川甘陕交汇关键构造部位地壳浅表层现今地应力环境和潜在地震危险性，在甘肃省水市甘谷县及四川省广元市三堆镇实施机械岩心钻探工程和水压致裂地应力测量。地应力测量结果表明，甘谷钻孔3个主应力关系为SH>Sh>Sv，甘谷地区现今水平主应力起主导作用，且具有较高地应力值，钻孔附近最大水平主压应力方位平均为N41°E，易于钻孔附近北西西向西秦岭北缘断裂产生左旋走滑兼逆冲活动；三堆钻孔3个主应力关系为SH>Sh>Sv，该地区现今水平主应力起主导作用，钻孔附近最大水平主压应力方位平均为N85°W，利于钻孔附近北东向青川断裂产生右旋走滑兼逆冲活动。利用库仑摩擦滑动准则对断裂活动进行分析，结果表明天水和广元地区的地应力大小均已经达到了使地壳浅部断层产生滑动失稳的临界条件，需加强地应力实时监测和分析。该研究成果为川甘陕交汇关键构造部位的断裂活动性分析和地质环境安全评价提供科学依据。