Progress of active fault studies in Japan in the past 25 years and future prospects

Abstract

We reviewed studies of active faults in Japan during the past 25 years, particularly since the 1995 Hyogo-ken Nanbu earthquake that drastically changed the system and budget for such studies. Active fault studies have been central to the national seismic hazard reduction program, led by the Headquarters for Earthquake Research Promotion (HERP). Many studies have examined ~100 major active fault zones, employing paleoseismic trenching and high-resolution shallow seismic reflection profiling. Several types of large-scale maps of active faults have been published, including digital maps with three-dimensional base maps. These data are fundamental to the long-term evaluation of earthquake occurrences on major active fault zones. New research techniques have been introduced to active fault studies, such as detailed digital elevation models (DEMs), light detection and ranging (LiDAR), photogrammetry using unmanned aerial vehicles (UAV), shallow subsurface geophysics, and OxCal age modelling of paleoseismic events. Since the 1995 earthquake, there have been several surface-rupturing inland earthquakes with highly variable earthquake magnitude and coseismic faulting behavior. These earthquakes illustrate the need for new criteria for long-term evaluation of seismic hazard, including the incorporation of non-characteristic earthquakes into seismic hazard models, rather than assuming only characteristic earthquakes for a given fault zone.