Relationship between damages by the 2004 mid-Niigata Prefecture Earthquake and thickness of fan deposits in Tamugiyama area, Kawaguchi Town, Niigata Prefecture, central Japan(<Special issue>Damage and geological setting of The Mid Niigata Prefecture Earthquake 2004)

Abstract

The Tamugiyama district in Kawaguchi Town, Niigata Prefecture is one of the most affected areas by the 2004 mid-Niigata prefecture Earthquake. However, the damage distribution shows regional difference within the Tamugiyama district. The worst-hit colony is located on the alluvial fan underlain by thick (ca.20m) muddy deposits. The fan deposits are soft with the N values from 5 to 20 and the low S wave velocity (<200m/sec.). On the other hand, the basement rocks and the terrace deposits are hard with the N values over 50 and the high S wave velocity (>600m/sec.). The peculiar cycle of the fan deposits is calculated at about 0.5 seconds. This is mostly same as that of wooden houses. The serious damages, therefore, seem to have caused by this peculiar cycle. It is generally thought that there is little amplification of the seismic motion on alluvial fans because deposits beneath most alluvial fan consist of gravels. However, Chuetsu area including the Tamugiyama district, the peculiar fan deposits composed mainly of soft muddy deposits much amplified seismic motion of the earthquake.