A Case Study of Three-dimensional Determination of Stress Orientation to Crystalline Rock Samples in Wenchuan Earthquake Fault Scientific Drilling Project Hole-2

Stress state around an earthquake fault is a key parameter to understand mechanisms of the fault rupturing. We tried to determine three-dimensional in-site stress orientations by anelastic strain recovery (ASR) measurements. As a case study, we applied this core based ASR method to two drill core samples retrieved from Wenchuan Earthquake Fault Scientific Drilling Project Hole-2. The core samples used for ASR experiments are chloritized diorite classified as a crystalline rock and retrieved from depths of 1,444 and 1,469 m in the hole-2, respectively. Anelastic strains of a core sample in nine directions, including six independent directions, were measured after its in situ stress was released by drilling. We obtained anelastic strain variation with time due to relaxation after its stress release. Then, the three-dimensional principal orientations of the in situ stress tensor at the two depths were successfully determined by determining the three dimensional principal orientations of the anelastic strain tensor. Our preliminary results showed both the stress regimes at the two depths are nearly strike slip faulting stress regime, and the maximum horizontal stress orientations are northwest-southeast or north-northwest-south-southeast. In addition, the results also suggested that the ASR method is applicable and useful for such in situ stress measurements in deep drilling projects.