Surface Rupture of the 2008 Iwate-Miyagi Nairiku Earthquake Observed at Oshu and Ichinoseki Cities, Iwate Prefecture, Northeast Japan

The 2008 Iwate-Miyagi Nairiku earthquake (M-w 6.9) struck an area where no active fault was known prior to the earthquake. The earthquake was shallow and large enough to produce scattered and discontinuous surface ruptures. To better understand the source processes and seismotectonic setting of the earthquake, we mapped the coseismic surface ruptures observed at Oshu and Ichinoseki Cities, Iwate Prefecture. The NNE-to NNW-trending surface ruptures appeared at three localities: (1) between Mochikorobashi and Ochiai, (2) Itagawa, and (3) southwest of Shinyu hot spring. Most of the surface ruptures are fold scarps up on the west, which are consistent with source fault models based on geodetic and seismological data. These surface ruptures appear to be associated with a reverse slip on west-dipping faults imaged by seismic reflection profiling, which originated as Miocene normal faults and have been reactivated as reverse faults since the Pliocene. Some of the ruptures appeared along preexisting tectonic scarps, although these scarps are short and discontinuous. Geomorphic expression of the causative faults of the 2008 earthquake is weak. Integration of geomorphic, geologic and geophysical data is necessary to identify active faults in areas where crustal strain is distributed to multiple fault strands and thus the tectonic landform of each fault is not well developed.