Stress measurements around earthquake fault and shallow subduction zone using core samples retrieved from deep boreholes

点击翻译按钮获取中文摘要

Objectives of this study are, i) to examine whether core-based methods, especially the anelastic strain recovery (ASR) method, can be applied for determining the in-situ stress in cases of deep ocean drilling such as the coming "The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE)" at great depths, and ii) to understand basic features of stress distribution in the vicinity of an active fault by means of application the stress measurements on a scientific drilling project, Taiwan Chelungpu-fault Drilling Project (TCDP).First, we analyzed three-dimensional in-situ stress by using anelastic strains measured in six independent directions of oriented cores which were taken from a 5,000 m-depth continental drilling project. For all the core samples measured, the expansional anelastic strains were obtained ; and the strain values in various directions was sufficiently high for the accuracy of the measurement system used. Then, these strains were successfully used for a three-dimensiona … More l analysis of the directions of three principal stresses and estimation of the magnitudes of the stresses. As second case study, in order to understand the feature of rock stress change at different depths above, within, and beneath the Chelungpu fault after the Chi-Chi earthquake, we employed the three-dimensional ASR method to determine current stress using drill core samples retrieved from TCDP. Acquired anelastic strains were of high quality ; and the strain data could be used for three-dimensional analysis resulting in the determination of directions and the estimation of magnitudes of the principal stresses. Preliminary results showed that the directions of principal stresses changed between the shallower depth above the fault and the deeper depth beneath it, that is, the current stress distribution in the TCDP hole might be influenced by the Chelungpu-fault rupture. Consequently, it can be said that the ASR method is well suited for use in directly determining the directions of principal stresses in three dimensions and in estimating their magnitude in isotropic rocks at large/great depths. Less