Determination of three-dimensional in situ stresses by anelastic strain recovery in Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Abstract

Abstract The Wenchuan Earthquake Fault Scientific Drilling Project was implemented rapidly after the great 12 May 2008 earthquake (Mw 7.9) to better understand rupture mechanisms of the seismic faults. The first borehole of the project, WFSD-1 was located in Hongkou Township, Dujiangyan City, Sichuan Province, China on the hanging wall of the Yingxiu–Beichuan fault, which underwent large dextral and vertical displacement during the earthquake. The near-vertical borehole was 1201 m long. In-situ stresses were measured in rock samples from depths between 424 and 1173 m drilling depth by the anelastic strain recovery (ASR) method. The average trend of the maximum principal stress σ1 was N309° (ranging from N291° to N325°), rotated with increasing borehole depth from NW–SE to WNW–ESE. The magnitude of the in-situ maximum principal stress was estimated to be 35.3 MPa at the depth of 1173 m. The relations between horizontal and vertical stresses are vertical stress σv > maximum horizontal stress σH > minimum horizontal stress σh at depths above 424 m, σH > σh > σv at depths from 424 m to 800 m, and σH > σv > σh below the depth of 800 m. These indicate that the stress states are in a normal faulting stress regime above 424 m; a reverse faulting regime from 424 m to 800 m and a dextral strike-slip regime below 800 m. The stress measurements from WFSD-1 are consistent with the focal mechanism solution in which the mainshock was dominated by thrusting accompanied by dextral strike-slip motion. The orientations of σ1 are roughly consistent with the tectonic displacement direction of the Longmenshan area, which provides further evidence of NW–SE movements representing compression of the Songpan–Ganzi block toward the Sichuan basin.