Fan deposits amplifying ground motions by the 2004 Mid-Niigata Prefecture earthquake: An example from the Tamugiyama basin, Kawaguchi Town, Niigata Prefecture, central Japan

Abstract

The Tamugiyama district in Kawaguchi Town, Niigata Prefecture is one of the most affected areas by the 2004 Mid-Niigata Prefecture earthquake. Though this district is located in a small (ca. 2 km²) basin, some colonies were damaged lightly and others severely. The worst-hit colony is located on the central part of the Tamugiyama fan underlain by thick (ca.20 m) muddy deposits with the N-values ranging from 5 to 20. On the other hand, lightly damaged colonies are located on the Tamugiyama-IV terrace underlain by gravel rich deposits with the N-values of over 50 and on the marginal area of the Tamugiyama fan. The Tamugiyama fan deposits were supplied from surrounding hill areas where weakly consolidated Pliocene to Lower Pleistocene sedimentary rocks occurred and a large number of landslides are recognized. The deposits filled a basin-like incised valley during the latest Pleistocene time. The close relationship between damages and thickness of the muddy fan deposits indicates that the fan deposits intensely amplified ground motions resulting in local severe damages. This fact is noteworthy for making seismic hazard maps.