サハリン北東部の活断層と古地震

抄録

We have mapped a series of north-trending faults along the northeastern coast of Sakhalin, Russia, based on aerial photograph interpretation and field observations. These faults include the Upper Pil'tun fault which ruptured during the 1995, M_w 7.0 Neftegorsk earthquake, and the Ekhabi-Pil'tun and Garomay faults east of the surface ruptures. The coseismic displacement associated with the 1995 earthquake shows that the Upper Pil'tun fault is a right-lateral strike-slip fault. Right-lateral stream offsets as much as 80m demonstrate that similar earthquakes have occurred repeatedly along the fault in the late Quaternary. The fault extends for 10 km further to the south beyond the southern termination of the 1995 surface breaks, which is consistent with the coseismic ground deformation detected by SAR interferometry. The Ekhabi-Pil'tun fault trends N10° W to N10°E with a series of fault scarps down-to-the-west across low-relief hilly terrain eroded byperiglacial processes. Right-laterally offset streams and lake shores suggest thatthe Ekhabi-Pil'tun fault is also a right-lateral strike-slip fault. A 3-m-deep trench across the fault north of the Sabo River contains geologic evidence of two episodes of pre -historic surface faulting. The most recent event occurred after 4000 yr B.P. and the penultimate earthquake occurred at about 6200 yr B.P. The interval between the events is greater than 2000 years, which is significantly longer than that obtained for the Upper Pil'tunfault by previous trench excavations.