STABILITY AND STRESS MEASUREMENT NEAR THE QINGCHUAN FAULT IN THE NORTHERN LONGMEN MOUNTAINS
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摘要: 汶川8.0级地震后, 余震是否沿青川断裂向汉中方向发展, 是否还会发生强烈余震成为社会关注的问题。为了查明汶川8.0级地震后青川断层库仑应力的变化, 判明余震的发展趋势, 2008年6月2日~28日我们在青川断裂中段阳平关一带开展了断层附近原地应力测量研究, 在3个钻孔中分别用水压致裂法和解除法得到了断层上下盘构造应力的大小、方向和分布特征。根据实测的地应力资料, 断层附近最大水平主压应力方向为NE-NEE向, 与区域构造应力SN-NNE向主压应力方向有一些差异, 说明受断层活动影响, 断层附近的应力状态与区域应力场已经发生了改变。青川断层是一条逆冲兼右行走滑的活动断裂, 在汶川大地震中虽未见其活动, 但其阻隔了余震向北发展, 表明该断层是一条重要的边界断层, 既是区域构造分区的界线, 也是区域应力场的分区标志线。利用地应力实测资料研究了断层的活动性, 在测量深度域内水平和铅直应力的关系为σH>σh>σV, 该应力状态有利于逆断层活动, 与地质资料反映的以逆断层活动及近期在断层附近发生的地质异常现象一致。利用库仑摩擦滑动准则, 摩擦强度取0.6~1.0进行分析, 断层附近的现今地应力状态接近产生逆断层摩擦滑动的临界值, 其中断裂东段发生逆断层活动的可能性较小, 相比而言, 青川断裂带西段危险性更大。Abstract: After the M8.0 Wenchuan earthquake, it becomes an important concern that whether aftershocks direct to Hanzhong along the Qingchuan fault and a strong aftershock also occur.The study on in situ stress measurements was carried out at Ningqiang region in the northeastern section of the fault in order to master the changes of the coulomb stress in the Qingchuan fault after the earthquake and to identify the development trend of aftershocks, using the hydraulic fracturing in situ stress measurement techniques on June 2 to 28, 2008.The size, orientation and distribution of the tectonic stress in the fault were obtained respectively by the hydraulic fracturing and stress relieving in three boreholes.According to the measured stress data, the direction of the maximum level principal stress is NE-NEE near the fault, and does not accord with the SN-NNE direction of the regional tectonic stress.It is shown that the stress state near the fault changes obviously compared to the regional stress field, affected by the impact of fault activity.The relationship between horizontal and vertical stress is σH>σh>σV at the measurement depth.The study reveals that the stress state is conducive to the activities of reverse fault and is basically the same as the geological data.By using sliding friction Coulomb Guidelines and friction strength from 0.6 to 1.0, it can be seen that present stress state near the fault has reached or exceeded the sliding friction threshold of the thrust fault, indicating that present tectonic activity is mainly thrust movement and it is impossible for the strike-slip movement to occur in the measured area.The thrust fault activity is less likely in the eastern section of the fault but much more likely in western part of the Qingchuan fault.
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图 2 龙门山断裂与汶川地震余震分布图
1.彭县-灌县断裂带; 2.映秀-北川断裂带; 3.茂县-汶川断裂带; 4.青川断裂带; 5.青木川断裂带; 6.安县-广元断裂带; 7.北川-南坝-林庵寺断裂
Figure 2. Distribution of Longmenshan fault and Wenchuan earthquake aftershocks
图 3 青川断裂及其邻区的地质构造略图
1.现代堆积; 2.震旦系; 3.元古宙; 4.元古宙花岗岩; 5.断层; 6.河流
Figure 3. Simplified geological map of the Qingchuan fault and adjacent areas
图 4 金山寺附近地貌及水系位错图
Figure 4. Geomorphological map and systematic stream offsets across the Qing chuan fault near Jinshansi Contour line interval being 20m
图 6 宁强5.7级地震震中青木川镇破坏
Figure 6. Damage of Qingmuchuan town in the Ningqiang MS5.7 earthquake
图 7 宁强5.7级地震距震中77km阳平关镇小学破坏
Figure 7. Damage of Yangpingguan primary school in the Ningqiang MS5.7 earthquake
图 8 阳平关嘉陵江河滩长达数公里的地裂缝带和喷砂冒水现象(5.12地震造成)
Figure 8. Blasting water and crack at Yangpingguan along Jialingjiang River
图 9 井下应力解除过程示意图
1.施工钻孔; 2.磨平孔底; 3.孔底磨锥槽; 4.施工小孔; 5.安装地应力探头; 6.套芯解除; 7.套芯结束; 8.取岩芯和应变仪
Figure 9. Stress relieving process under ground
图 11 青川断层地应力实测有效应力-剪应力摩尔圆与临界区图
红线-代家坝实测地应力状态; 蓝线-阳平关实测地应力状态; 黄线-大安镇实测地应力状态; 绿线-解除法实测的地应力状态
Figure 11. Effective stress-Moore Circle with the shear stress and critical areas diagram of Qingchuan fault stress measurement
表 1 地应力测量钻孔概况
Table 1. Borehole profile of in situ stress measurement
表 2 钻孔水压致裂地应力测量结果
Table 2. Hydraulic fracturing stress measurement results
表 3 钻孔空芯包体解除法地应力测量结果
Table 3. Stress relieving measurement results
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