Identifying and dating blind thrusting events along the Biwako-seigan fault zone, central Japan, by dense geoslicing

We employed the 'geoslicer' technique to reveal the paleoseismic history of a blind thrust, for which traditional trenching is rarely useful or applicable because of broad surficial deformation and common shallow ground water at potential paleoseismic sites. A geoslicer is a relatively new paleoseismic tool for acquiring a subsurface geological sample; a thin sheet or slice of sediments is obtained to a depth of several meters by using a crane-hung weighted vibrator. Dense geoslicing and drilling across the southern Biwako-seigan fault zone (BSFZ), central Japan, together with a total of 109 radiocarbon dates, successfully revealed that the most recent blind thrusting event on the fault occurred between AD 1060 and 1260, most likely around AD 1170, producing a broad surficial warp more than 200 m wide with a vertical displacement of 5 m or more. Historical accounts strongly support that this event correlates with the 1185 M 7.4 Genryaku Kyoto earthquake. The geoslicing data also suggest four blind thrusting events during the last 18 kyrs and an average earthquake recurrence interval of 4300-5800 years, assuming characteristic behavior. Our results show that dense geoslicing can be a powerful paleoseismic tool for investigating blind thrusts buried at moderate depths, although paleoseismology of deeply buried blind thrusts still presents a big challenge.