Junichi Goto

Enigmatic, highly active left-lateral shear zone in southwest Japan explained by aseismic ridge collision

Global positioning system (GPS) site velocities and earthquake focal mechanisms reveal an active left-lateral shear zone cutting across Kyushu in southwest Japan. Surprisingly, no active faults have been identified in association with this zone of rapid contemporary deformation. To explain the existence of this shear zone, we propose a model comprising subduction of an aseismic ridge (Kyushu-Palau Ridge) at the southwest end of the Nankai Trough. Because of rapid (~40 mm/yr) along-strike migration of the ridge, we suggest that the ridge subduction point (and resulting left-lateral shear zone) is never in one place long enough to enable the development of a through-going fault zone that can be identified at the ground surface, reconciling the mismatch between the GPS, seismological, and geological data in this region. Our conceptual model is supported by numerical modeling results. We also suggest that the along-strike change in subducting plate buoyancy explains the recent counterclockwise rotation of the Kyushu forearc documented in paleomagnetic studies, as is found in many other western Pacific subduction margins.

Development of Characterization Technology for Fault Zone Hydrology

Several deep trenches were cut, and a number of geophysical surveys were conducted across the Wildcat Fault in the hills east of Berkeley, California. The Wildcat Fault is believed to be a strike-slip fault and a member of the Hayward Fault System, with over 10 km of displacement. So far, three boreholes of ~;; 150m deep have been core-drilled and borehole geophysical logs were conducted. The rocks are extensively sheared and fractured; gouges were observed at several depths and a thick cataclasitic zone was also observed. While confirming some earlier, published conclusions from shallow observations about Wildcat, some unexpected findings were encountered. Preliminary analysis indicates that Wildcat near the field site consists of multiple faults. The hydraulic test data suggest the dual properties of the hydrologic structure of the fault zone. A fourth borehole is planned to penetrate the main fault believed to lie in-between the holes. The main philosophy behind our approach for the hydrologic characterization of such a complex fractured system is to let the system take its own average and monitor a long term behavior instead of collecting a multitude of data at small length and time scales, or at a discrete fracture scale and to ?up-scale,? which is extremely tenuous.

Tectonic Risk Forecasting Through Expert Elicitation for Geological Repositories: The TOPAZ Project

This paper describes the development of a probabilistic methodology for the evaluation of tectonic hazards to geological repositories in Japan. The approach is a development of NUMO’s ITM methodology, which produced probabilistic hazard maps for volcanism and rock deformation for periods up to about 100,000 years in a set of Case Studies that covered a large area of the country. To address potential regulatory requirements, the TOPAZ project has extended the ITM methodology to look into the period between 100,000 and 1 million years, where significant uncertainties begin to emerge about the tectonic framework within which quantitative forecasting can be made. Part of this methodology extension has been to adopt expert elicitation techniques to capture differing expert views as a means of addressing such uncertainties. This paper briefly outlines progress in this development work to date.Copyright