Kentaro Motoki

Observation of aftershocks of the 2003 Tokachi-Oki earthquake for estimation of local site effects

Observation of aftershocks of the 2003 Tokachi-Oki earthquake was conducted in the southern part of the Tokachi basin in Hokkaido, Japan for estimation of local site effects. We installed accelerographs at 12 sites in Chokubetsu, Toyokoro, and Taiki areas, where large strong motion records were obtained during the main shock at stations of the K-NET and KiK-net. The stations of the aftershock observation are situated with different geological conditions and some of the sites were installed on Pleistocene layers as reference sites. The site amplifications are investigated using spectral ratio of S-waves from the aftershocks. The S-wave amplification factor is dominant at a period of about 1 second at the site near the KiK-net site in Toyokoro. This amplification fits well with calculated 1D amplification of S-wave in alluvial layers with a thickness of 50 meters. In addition to the site effects, we detected nonlinear amplification of the soft soils only during the main shock. The site effects at the strong motion site of the K-NET at Chokubetsu have a dominate peak at a period of 0.4 seconds. This amplification is due to soft soils having a thickness of about 13 meters. Contrary to the results at the two areas, site effects are not significantly different at the stations in the Taiki area, because of similarity on surface geological conditions.

Estimation of local site effects in Ojiya city using aftershock records of the 2004 Mid Niigata Prefecture earthquake and microtremors

Aftershock observations of the 2004 Mid Niigata Prefecture earthquake were conducted in the central part of Ojiya city, the Niigata prefecture, central Japan, to investigate local site effects. We installed eight accelerographs in the vicinity of the K-NET and JMA stations in the area. The stations of the aftershock observations are situated under different geological conditions including one installed in a mountainous area on Tertiary layers to serve as a reference site. We examined the ground-motion characteristics of the records for a Mj 6.1 aftershock focusing on local site effects. The amplification, at a period of less than 1 sec, is the largest in the vicinity of the K-NET station. The amplification at periods longer than 2 sec is larger in the western part of the city than those in the east. We estimated the S-wave velocity structure in the sediments above the basement with an S-wave velocity of 3.4 km/sec from the inversion of phase velocities measured by the array observations of vertical microtremors. We discuss the amplification factors using the S-wave velocity profile and show that shallow soils over the layer with an S-wave velocity of 0.49 km/sec are responsible for the amplification at periods shorter than 0.4 sec. Deeper sedimentary layers are needed to explain amplification at periods of 1 sec.

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

Heavy damage occurred in Monzenmachi, the Wajima city, in the northern part of the Noto peninsula during the 2007 Noto Hanto earthquake with an Mj 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 Mj 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.