长白山天池火山西南麓水压致裂法地应力测量及其实测数据在断裂稳定性分析中的应用

Hydraulic fracturing tests for stress measurements and analysis of stability for frictional sliding of faults at the Southwest foot area of Tianchi volcano in Changbaishan Mountains

长白山天池火山是一座新生代多成因复合火山,其活动性一直受到地学界的高度重视.而地应力测量数据对于了解火山的活动情况等具有重要意义,但目前长白山天池火山地区实测地应力资料较少.为了更准确地了解长白山天池火山地区现今地应力状态及断裂稳定性,在长白山天池火山西南麓MJZ测点0~300 m孔深开展了水压致裂法地应力测量.利用此次实测地应力数据,本文首先分析了该地区地壳浅表层应力状态,结果表明:①测区以水平构造应力为主导,应力值与其他地区相比,相对较低;②SH(最大水平主应力)、Sh(最小水平主应力)和Sv(垂向主应力)均随深度增加而基本呈线性增大趋势;在60~180 m深度,三者展现为逆断层应力状态,即SH>Sh>Sv;在180~300 m,则表现为SH>Sv>Sh,该应力关系有利于走滑断层活动;③钻孔上部SH方向为N38°E,与东北地区构造应力场方向基本一致.钻孔下部由于受区内NW向断裂和其他因素影响,存在与区域主应力方向有偏差的局部应力场.其次,基于实测应力数据,应用库仑摩擦滑动准则,并取摩擦系数为0.6和1.0,初步评价了区域断裂的稳定性.结果显示,研究区现今应力状态尚未达到断裂失稳滑动水平,推测测区断裂目前相对稳定.结合其较低的应力积累水平,一定程度上反映出长白山天池火山区目前没有明显活动,地壳总体是稳定的.本文的研究结果可为长白山天池火山的活动性、地震地质等研究提供地应力方面的基础数据,同时也为测区地质环境安全评价提供参考依据.

The Tianchi volcano in Changbaishan Mountains is a polygenetic composite volcano which formed in Cenozoic era,and its activity has been highly concerned in the earth science field.The data of in-situ stress measured are of great significance to understand the volcanic activity,but at present there are few observed data of crustal stress in the area of Tianchi volcano,Changbaishan Mountains.In order to accurately learn about the current state of stress and fault stability in the above-mentioned area,hydraulic fracturing tests for stress measurements were carried out at MJZ borehole ranging from 0~300 m depth in the Southwest of the Tianchi volcano in Changbaishan Mountains.Those data derived from the in-situ stress measurements reveal that the in-situ stress field is dominated by horizontal tectonic stress in the study area,and the ground stress values are lower as compared with that obtained elsewhere in China;SH(the maximum horizontal principal stress)、Sh(the minimum horizontal principal stress)and Sv(the vertical principal stress)change basically linearly with the depth of MJZ borehole;in the depth region between 60 and 180 m,the relationship of the three principal stresses is SH>Sh>Sv,which indicates the stress state is conducive to the thrusting of the fault;in the depth range of 180 to 300 m,the relationship changes to SH>Sv>Sh,which represents the stress regime of the study area is favorable for the strike-slip faulting;The direction of the SH in the upper part of borehole is N38°E which is basically consistent with those of the tectonic stress field of Northeast China;the direction of the SH in the lower part of borehole is inconsistent with those of regional stress field,it is inferred that the NW fault and some other factors probably have an effect on the SH,that is there is a local stress field at MJZ borehole.In addition,slip stability of faults in this study area is preliminarily estimated according to Column friction criterion together with the frictional coefficients being assumed to be 0.6 and 1.0,and the result shows that the present-day crustal stress state does not yet meet the sliding critical condition of the shallow fault and it is presumed that the fault is in a relatively stable state in this area;Combined with the low level of regional stress accumulation,to a certain extent,it demonstrates that there is no obvious tectonic activity in the area of Tianchi Volcano in Changbaishan Mountains,and the crust basically keeps stable.Conclusions in this paper would be of great significance for studies on activity of the Tianchi volcano and seismogeology as well as assessment of the regional geological environment and geological disasters prevention.