Recurrent partial resetting of quartz OSL signal by earthquakes: a thermochronological study on fault gouges from the Atotsugawa Fault, Japan

Abstract

Optically stimulated luminescence (OSL) dating utilises the detection of trapped charge in minerals, and has an ultralow closure temperature. There is the potential for direct dating of fault movement using fault gouges, because frictional heating caused by large earthquakes can reduce the OSL signal intensity of minerals within gouges. In this study, we conducted quartz OSL dating on four fault gouge and breccia samples from a surface outcrop of the Atotsugawa Fault, one of the most active dextral strike-slip faults in central Japan, where the last large earthquake occurred in AD1858, with an estimated magnitude of 7. The natural OSL signal intensity of fine-grained quartz was clearly below the signal saturation level, with the fraction of saturation (n/N) between 0.30 ± 0.02 and 0.39 ± 0.03, indicating there was signal resetting by past earthquakes. However, the apparent OSL ages ranged from 22 ± 1 to 72 ± 4 ka, two orders of magnitude older than the age of the last earthquake. To explain the significant age overestimation, we measured the thermal stability of the OSL signal, and used a thermal model with punctuated episodic losses to constrain the average shear heating temperature experienced during an individual faulting event. For an independently known recurrence interval of 2.5 ka and a presumed shear heating duration of 1 s, the observed n/N and the measured thermal stability of the OSL signals correspond to a resetting temperature of ~ 300 °C during a single earthquake event. Graphical Abstract