利用汶川地震断裂带科学钻探3号井(WFSD-3)成像测井资料分析震源区应力场特征

Analysis of stress field characteristics in source region using the image logging in borehole3 of Wenchuan earthquake fault zone scientific drilling(WFSD-3)

为了得到最接近于汶川地震过程中的构造应力场,本文利用汶川地震断裂带科学钻探3号井(WFSD-3)的成像测井资料,从研究钻孔崩落和裂缝特征的角度出发,对汶川地震震源区构造应力场在汶川地震前后的变化进行了分析.结果显示,该孔两处钻孔崩落分别位于1411.8～1413.6 m及1431.8～1433.4 m深度处,由崩落形态推断该区最大主压应力方向为近NWW向,之后对比前人利用震源机制解和原地应力测量得到最大主压应力方向结果,发现汶川地震震源区水平最大主压应力方向发生了明显变化,向由震前及震后短时间内的近NWW向变为震后一段时间的近NNE-NEE向.分析认为:汶川地震正是在近NWW向的区域最大水平主压应力的作用下,在倾向为NW向的断层面上产生逆倾滑而发生的纯逆冲型地震,而汶川地震的发生又使得区域水平最大主压应力方向发生了明显变化.最大主压应力方向的变化特征能很好地分析和解释余震震源机制,说明变化前后的区域构造应力场对汶川地震序列的发生均具有重要影响.

In order to get the closest tectonic stress field during the Wenchuan earthquake, basing on the studying perspective of borehole collapse and fracture characteristics, this paper used the imaging logging data of Borehole 3 of Wenchuan Earthquake Fault Zone Scientific Drilling(WFSD-3) to analysis the changes of the tectonic stress field in the epicenter area before and after the Wenchuan earthquake. The results showed that the two borehole breakouts were located at the depths of 1411.8 to 1413.6 m and 1431.8 to 1433.4 m of WFSD-3 respectively. From shapes of the two borehole breakouts, it was concluded that the direction of the maximum principal stress in this area was nearly NWW. Compared with the previous results of maximum principal stress direction which obtained by the methods of focal mechanism solution and in-situ stress measurement, it showed that the maximum principal stress direction of the epicenter area changed obviously, which is from about NWW before and in the short time after the Wenchuan earthquake to near NNE-NEE in the period after the earthquake. According to the analysis, on the fault plane whose dip direction is NW, the Wenchuan earthquake was a purely thrusting-type earthquake that occurred under the action of the regional maximum principal compressive stress(the direction is near NWW). In turn, the occurrence of the Wenchuan earthquake caused a significant change in the direction of the regional maximum principal compressive stress. The change characteristics of the direction of the maximum principal compressive stress can well analyze and explain the focal mechanism of the aftershocks. It showed that the regional tectonic stress field after the change had the same impact on the Wenchuan earthquake sequence as it did before the change.