Investigation on the active fault related to the 2008 Iwate-Miyagi Nairiku Earthquake

Abstract

The 2008 Iwate-Miyagi Nairiku Earthquake occurred the area where any intensive investigations on active faults had not yet been conducted, and thus no active faults had been identified to be existing. It was essential to examine whether no active faults actually exist in the area as the fundamental basis for discussion on the cause of the earthquake. We conducted aerial photograph interpretation using large-scale photos taken both in 1976 and just after the earthquake, as well as a field survey to identify surface ruptures. Then we found that some probable deformed landforms due to active faulting were sporadically spread along the Koinooka river in Genbi-cho, Ichinoseki city, and that surface ruptures appeared coincidently with such deformed landforms. Additionally, we excavated trenches across the earthquake fault, and clarified that the fault had been repeating activations several times after the deposition of the lower terrace sediment. Therefore, it suggests that an active fault was located in this area, and the earthquake was strongly related to this newly found active fault.