Observation of ground motions of aftershocks of the 2007 Noto Hanto earthquake in Monzenmachi, the Wajima city, for estimation of site amplification

Abstract

Heavy damage occurred in Monzenmachi, the Wajima city, in the northern part of the Noto peninsula during the 2007 Noto Hanto earthquake with an M_j of 6.9. The area is located along a river and sandwiched by mountains at the northern and southern ends of a Quaternary narrow valley 1–2 km wide. The seismic intensity observed in the area was 6 upper during the main shock. In this study, we conducted observations of ground motions during aftershocks in Monzenmachi to understand local site effects in the vicinity of the seismic intensity station. We installed 11 accelerometers in three areas of Monzenmachi, Hashiride, Touge, and Kuroshima. In each area, three or five stations were chosen for a deployment of a linear array across the areas with heavy damage. The analysis was performed using aftershock records observed during events with M_j of 2.3–4.4. The spectral ratio of records from the station near the seismic intensity station to a reference station on Tertiary layers shows a dominant peak at a period of 1 s. We also found distinct peaks at periods of 0.4–0.7 s in the spectral ratios at the sites in the areas with heavy damage. In contrast, no dominant peaks were identified at the stations at the foot of the mountains. This clearly indicates that site amplification effects play an important role in defining the damage distribution in an area. Similar peaks were found in the H/V spectra of microtremors at the aftershock observation sites, though the peak amplitudes of the microtremor H/Vs are smaller than those of the S-wave spectral ratios.