Direct ESR dating of fault gouge using clay minerals and the assessment of fault activity

The ESR dating method has been applied to the fault gouge of the Nojima Fault, the outcrop of which arose due to the southern Hyogo Prefecture Earthquake on January 17th, 1995, and to the fault gouge of the Median Tectonic Line (MTL) which is a major active fault in Japan, without considering the resetting of ESR signals. The ESR ages obtained for clay minerals in the fault gouges are considered to indicate the age of the beginning of the fault movements at the outcrop. Assuming no radon loss and the general values of water content and alpha-ray efficiency, the ESR ages of the fault gouges of the Nojima Fault and MTL are estimated as 0.55-1.16 and 6.76-9.28 Ma, respectively. The ESR age of the Nojima Fault is consistent with the age of the beginning of the Rokko Movements in the Kinki districts estimated geologically. On the other hand, the ESR age obtained from the MTL is younger than the K-Ar age (about 11 Ma), which is presumably overestimated due to the existence of the source minerals in the K-Ar dating sample. Since it is geologically known that the MTL at the outcrop had moved until the Late Pleistocene-Holocene, the fault movement of the MTL at this outcrop probably began at 6.76-9.28 Ma B.P. after the intrusion of the felsitic dyke (12-16 Ma) which is the source rock of the fault gouge into the fault boundary. The ESR dating of clay minerals in the fault gouge may be effective for assessing fault activity if no Quaternary deposit exists at the outcrop. The younger the ESR age of the fault gouge is, the higher the fault activity is. If not indurated and the freshest clay gouge can be collected at the outcrop, the ESR age of the most recent fault movement may be obtained from the clay gouge.