Source Fault of the Iwate-Miyagi Nairiku Earthquake in 2008 Estimated by Distribution of Heights of Fluvial Terraces

Abstract

The Iwate-Miyagi Nairiku Earthquake in 2008 occurred at June 14, 2008 in the area where active faults have not been mapped. We mapped fluvial terraces and measured the relative height between L1 and M1 surfaces around the source region of the Iwate-Miyagi Nairiku Earthquake, for the purpose of estimating location and nature of the source fault. Relative height between L1 and M1 surfaces is equivalent to amount of incision in the period between ages of L1 and M1 surfaces. We showed that amount of incision varies markedly around the focal region and that the variation of amount of incision is related to the activity of the Iwate-Miyagi Nairiku Earthquake in 2008. Difference of amount of incision between both sides of the source faults is about 60-65 meters. Assuming that incision and uplift are in balance, difference of incision is approximate to vertical displacement of the source fault. Supposing that L1 and M1 surfaces were formed in Marine Oxygen Isotope Stage 2 and 6, respectively, we can estimate the vertical displacement rate of the source fault at about 0.5 mm/yr. This value is similar to that of the Kitakami Lowland Western Margin Faults. We concluded that the location and nature of (blind) active fault can be estimated roughly even in areas with no clear fault scarp, by clarifying the distribution of amount of incision using heights of fluvial terraces.