Assessment of fault activity by ESR dating of fault gouge; an example of the 500 m core samples drilled into the Nojima earthquake fault in Japan

Fault gouge in the 500 m core samples drilled into the Nojima Fault, which caused the 1995 Kobe Earthquake, was used for electron spin resonance (ESR) dating. The fault gouge was located at a depth of 388.4 m, however, all the ESR signals in the fault gouge had been incompletely reset by the 1995 faulting. As a result of ESR dating using quartet ESR signals in montmorillonite, the youngest ESR age, about 0.15-0.29 Ma, is obtained from the 0-3 mm sheet part of the fault gouge just on the fault plane. This age corresponds to Middle-Late Pleistocene. For protection against earthquake disaster, active faults which have moved since Late Pleistocene have high seismic risk, so it is important to evaluate the ESR age of the fault gouge even though it does not always mean the age of the latest fault movement. Meanwhile, the oldest age is about 0.39-0.73 Ma obtained from the 33-50 mm part. This age may mean not only the production age, but also the age of the first fault movement along the fault plane. (C) 2000 Elsevier Science Ltd. All rights reserved.