Study on Dating of Active Faults by means of Background Dosimetry and Paleomagnetic Measurement

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On the purpose of establishing a geophysical method for age determination of the most recent activity of Quaternary faults, we chose gackground (very low energy) dosimetry and archaeo-magnetic technique as dating tools because at least several times of stick-slip activity occurred within an archaeological time span. Since the Atotsugawa fault in central Japan was already recorded by some historic documentations in Yedo dinasty, that is, the latest activity was occurred in 1858; it was used as one of the most suitable test site for the fault age determination.In the first phase of the project, the fossil earthquake along the Atotsugawa fault were successfully detected from liquified sand beds near the main slip plane using paleomagnetic measurement. In the second phase, the paleomagnetic method has been applied to the other active faults such as Atera fault, the Itoigawa-Shizuoka tectonic line (3 sites), Tan'na fault, Gohmura-yamada fault and so on. The result succeeded to date paleoearthquakes along many of them.The second phase was focused mainly into the dosimetric dating of fault gouges along the earthquake fault, since the method can give more dirtect informations for the fault activity. Here, both thermoluminescense (TL) and elctron spin resonance (ESR) techniques were used. TL glow curves showed that the 1585 event did not completely erase paleodose responsible for the TL output more than 240゜C. Finally, more probable ages of 300-350 years b.p. could be estimated using a initial value for residual dosage because of incompleteness at the time of the slip event, while conventional calculation give rather old ages ranging from 2000-3000 years b.p.