Stress field observation and modeling from the NanTroSEIZE scientific drillings in the Nankai Trough system, SW Japan

Many studies have investigated the stress state in terms of drilling projects in the Nankai Trough region, the potential area for next devastating earthquakes. To understand the stress state and the geologic properties in Nankai, several drilling projects have been conducted over time. Among these projects, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) included studies regarding stress orientations and magnitude estimations by logging data from the Nankai Trough. Due to limitations in technology of drilling and the data processing, the actual stress orientation and the magnitude of the Nankai Trough at depth remains controversial. In this study, we discussed the stress state in each borehole and evaluated the stress state using various models. Our models evaluated stress states contributed by a slip deficit model in the Nankai Trough system and the coseismic dislocations of the two September 5th, 2004 earthquakes. Compared to the stress estimation from logging data, the simulated stress orientation could more or less explain the logging data at the NanTroSEIZE drilling boreholes. Based on these models, the stress tensor in Nankai can be extended from the boreholes to more dimensional area, even the locations where no drilling has been reached.