Weiren Lin, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) (Japan)
EnChao Yeh, National Taiwan University (Taiwan)
Hisao Ito, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) (Japan)
JihHao Hung, National Central University (Taiwan)
Wonn Soh, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) (Japan)
Determination of current stress is important and necessary in cases of active fault drilling programs. Based on investigation of existent stress measurement methods, unfortunately, it was preliminarily concluded that there is not a perfect method by which the magnitudes and orientations of the three-dimensional stress can be reliably measured at large depth. We suggest that combinational applications of borehole methods and core-based methods, considered being possible and practical approach for deep drilling projects, should be employed. As an example of stress determination in active fault drilling project, we will show the results from TCDP which was aimed at investigating physical and chemical properties of the active Chelungpu-fault that slipped during the 1999 Chi-Chi, Taiwan earthquake.
To determine current stresses post the earthquake, we employed two stress measurement techniques in the TCDP holes. The firs method employed is a core-based, anelastic strain recovery (ASR) technique. It was used to determine the orientations of current three-dimensional stress using drill core samples retrieved from TCDP hole-A. Acquired anelastic strains were of high quality and reached several hundred microstrains which is sufficiently high for the accuracy of measurement system used. Thus, the strain data could be used for a three-dimensional analysis resulting in the determination of orientations of the principal stresses. The preliminary results showed that the orientations of principal stresses changed between the shallower depth above the fault and the deeper depth beneath it, that is, the present stress distribution in the TCDP holes might be influenced by the Chelungpu-fault rupture. Using the second approach, that included the analysis of both borehole breakouts and drilling-induced tensile fractures from electrical resistivity-based images, reliable orientations of in-situ horizontal principal stresses were determined in TCDP hole-B which is located just 40m away from the hole-A. Overall, the orientation of the maximum horizontal principal stress SHmax in the depth range of 930-1330 m coincides with the downdip direction of the rock formations. However, the SHmax at the depth of around 1130 m is oriented at right angle to the dip of the strata; that is, Shmin orientation coincides with the dip azimuth. This observation fact suggests that this fault zone ruptured during the 1999 earthquake.
Comparison of the ASR measurement and breakout analysis showed that the orientations of horizontal principal stress are well consistent with each other. Moreover, we can obtain a constraint for the magnitude of the maximum horizontal principal stress SHmax according to the combination of the estimated stress magnitudes and utilization of the Anderson?fs faulting law. The results of this study suggested the both the orientation and magnitude of current stress in the vicinity of the Chelungpu fault in TCDP holes might be influenced by fault rupture.