A surface-wave method and micro-tremor array measurements have been carried out at Central Anamizu, Ishikawa Prefecture, Japan in order to investigate local site effect of earthquake in terms of near surface S-wave velocity model. Many houses were collapsed or severely damaged by the Noto Hanto Earthquake in 2007 at the central Anamizu. Collapsed or damaged houses were concentrated around the K-NET Anamizu strong ground motion observation station. It seems that the near surface heterogeneity may strongly affect surface ground motion during the earthquake. The investigation result shows that S-wave velocity down to the depth of 20m has strong heterogeneity at the site. Low velocity layer, in which Swave velocity is 60m/s, beneath the K-NET Anamizu disappears at the other side of the park only 50m away from the K-NET. Swedish ram soundings were also carried out around the K-NET and the results agreed with one of the surface-wave method very well. Two-dimensional site amplification was calculated by a finite-difference method using the S-wave velocity model obtained through the investigation. The calculation shows that near-surface heterogeneity strongly affects amplification of the site.
The Noto Hanto Earthquake in 2007 occurred on March 25, 2007 had a magnitude (Mj) of 6.9 on the magnitude scale of Japan Meteorological Agency. The earthquake killed one person and injured more than 250 people. The earthquake caused the collapse of old wooden houses with heavy roofs. The highways, railways and the Noto airport runway were also damaged. The earthquake induced many slope failures along the steep shores and mountainous area. Liquefaction was also observed at several locations. Many residential houses were collapsed or damaged in the central area of Anamizu town, placed 15km away from the epicenter (Figure 1). Collapsed or damaged houses were concentrated around the K-NET Anamizu strong ground motion observation station. The K-NET is the Japanese strong ground motion observation network operated by the National Research Institute for Earth Science and Disaster Prevention (NIED) and 1,000 stations are deployed all over Japan. At the K-NET Anamizu, maximum horizontal acceleration of about 800 gal (velocity of 100kine) was observed during the earthquake. Dominant frequency at the Anamizu is about one second that is very important for the earthquake resistance of two story wooden houses. Figure 2 shows the S-wave velocity model and soil column at the KNET Anamizu. We can see that a peat layer with extremely low S-wave velocity of 60m/s places down to the depth of 10m. People focuses on the effect of such thick low velocity layer and long period strong ground motion on the heavy damage of houses. Figure 1 also shows the velocity seismograms obtained by K-NET stations in the Noto peninsula. It is clear that the ground motion at the K-NET Anamizu was extremely large. In order to delineate nearsurface structure and evaluate the local site effect at the K-NET Anamizu, we have carried out a surface-wave method, micro-tremors array measurements, Swedish ram soundings and Swedish weighted soundings in the central Anamizu.
