川西北跨度23a的原地应力实测数据特征及其地壳动力学意义分析

CHARACTERISTICS OF IN-SITU STRESS MEASUREMENT IN NORTHWEST SICHUAN BASIN WITH TIMESPAN OF 23 YEARS AND ITS CRUSTAL DYNAMICS SIGNIFICANCE

研究区位于龙门山断裂带北段的中央断裂带和前山断裂带之间,该区地质结构主要为准原地体系和逆冲推覆体系,岩性以志留系下统的泥岩、粉砂泥岩分布最广,遍及全区。为了满足该区一工程建设的需要,在研究范围内开展了4个钻孔的水压致裂原地应力测量工作,测试时间跨度达23 a。测试数据显示,虽然测试时间跨度达23 a,但是原地应力状态基本没有变化,研究区最大水平主应力方向的优势方向为N24.4°W,最大、最小水平主应力随深度线性增加的梯度分别为0.036和0.025。基于实测数据计算的特征参数kH max,kh min,m和R在23 a内没有变化。该区域位于龙门山断裂带中央断裂的下盘,为被动盘,断层活动性和地震活动性都极其微弱,这种特殊的区域地壳动力学条件决定了研究区稳定的区域应力场特征。通过与以往研究成果的对比分析可知,m较好地反映了该区域地壳的应力积累能力,即从应力积累的角度反映了区域断层的强度,这一研究与以往研究成果十分吻合。在未来的研究中,或许可以利用实测应力资料计算的m值,再结合区域断层和地震活动性的比较分析来评价特定区域的地震危险性。

The research area,in Lizhou district,Guanyuan City in the northwest Sichuan Basin,is located between the central faults and front faults of north section of Longmenshan fault belt.The major geological structural units in the research area are semi-platform unit and thrust nappe unit.The lithology is mainly mudstone and silt mudstone formed during the lower Silurian era,which are present almost all over the research area.In order to meet the requirements of one engineering establishment,the in-situ stress measurements by hydraulic fracturing method in four boreholes were accomplished;and the measurement time span was up to 23 years.The measured results indicated that：the in-situ stress state did not change in the past 23 years;the dominant orientation of the horizontal principal stresses in the research area was N24.4°W;and the stress gradients of the maximum and minimum horizontal principal stresses vs.depth were 0.036 and 0.025,respectively.The four feature parameters,kH max,kh min,？m and R,calculated through measured principal stresses,did not change during the 23 years.The research area is in the foot wall of the central faults of the Longmenshan fault belt,and is a passive wall.At the same time,the activities of fault movements and seismic events in the research area are extremely weak.Such a specific regional geodynamic conditions have determined the very stable stress field of this area.Comparing with the other research findings,the calculated ？m reflects the stress accumulative capability of the regional crust in this area,i.e.the magnitude of ？m reflects the strength of regional faults from the point of stress accumulation.Such a finding is consistent with the others′.In the future research,it is possible to use ？m values determined with the measured stresses and the corresponding data from the analysis of activities of fault movements and seismic events to evaluate the earthquake risk of one certain area.