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.
Progress in Geophysics
maximum principal compressive stress