Stress induced near fault-zone breakout rotation: Two case studies in TCDP and JFAST

Abstract:

Within the past decade, two successful rapid-response drilling projects have measured breakouts within the nearfault of a recently ruptured fault. Breakout observation is the direct way to detect the far and near filed stress orientation in drilling. Here we compare those data. In 2006, ICDP performed an inland drilling project to penetrate Chelungpu fault plane in central of Taiwan, which had recently slipped in 1999 Mw 7.6 Chi-Chi earthquake. This drilling project succeeded in full coring and collecting comprehensive logging data in the borehole. The resistivity images run by Formation Micro Imager (FMI) indicated that a breakout rotation in the vicinity of the fault (1111mbf). Leak-off tests on site constrained the magnitude of minimum horizontal principal stress. Here we use these data to determine the stress variation in the fault plane in our breakout dislocation model. Based on the amount of breakout azimuth, rotation and fault geometry, the stress drop can be estimated in this model. In 2012, IODP initiated a rapid drilling project after the 2011 Mw9.0 Tohoku earthquake in Japan Trench. Due to the deep-water depth, only a real-time resistivity image recorded by Logging While Drilling (LWD) and few core samples are recovered by this expedition. However, the breakout azimuth occurred near the plate boundary (820mbsf) represents the stress disturbance after the dramatic slip comparing to TCDP case. In this research, we are attempting to discuss the possible effect factors and reconstruct the geo-mechanical models to interpret the breakout distribution observed from logging data and the stress state after these huge earthquakes.