Peeled cross section of an active fault

An "active fault" is a fault that has caused—and is likely to continue causing—earthquakes repeatedly. To assess the potential for earthquake generation of an active fault, it is necessary to shed light on its past activities. This is a peeled cross section from a trench excavation of the Okaya Fault in Nagano prefecture, part of a central portion of the Itoigawa-Shizuoka Tectonic Line active fault system. The left side of the panel is west, and the right side east. There are displacements along the two almost vertical faults on the lefthand side of the panel, along which the eastern side has moved downward. The top of these faults are covered with the surface layer, and it is evident that the most recent event occurred before its deposition. From detailed analysis of the fault displacement pattern and dating the deposition of the strata, it has been confirmed that there have been four earthquakes within the last 7,200 years. From this exhibit, we can see evidence of the two most recent events.