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

Abstract

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.

Temporal change in site response caused by earthquake strong motion as revealed from coda spectral ratio measurement

“…A strong motion by the 2003 Tokachi-Oki Earthquake (M W 8.3, 25/09/2003) is recorded at station IBUH03 of KiK-net. There are several reports indicating the change in site amplification just after these strong motions [Yamazoe et al, 2004;Yamanaka et al, 2004]. Hereafter, we refer to earthquakes occurred before and after the mainshock as foreshocks and aftershocks, respectively, even though they are not correct in the strict sense of the words.…”

“…[5] Figure 1 [Yamazoe et al, 2004;Yamanaka et al, 2004]. Hereafter, we refer to earthquakes occurred before and after the mainshock as foreshocks and aftershocks, respectively, even though they are not correct in the strict sense of the words.…”

Recent progress of seismic observation networks in Japan —Hi-net, F-net, K-NET and KiK-net—

“…For example, in the special issue of Earth, Planets and Space 126°E 128°E 130°E 132°E 134°E 136°E 138°E 140°E 142°E 144°E 146°E 24°N 26°N 28°N 30°N 32°N 34°N for the 2003 Tokachi-oki earthquake, five papers are using K-NET and KiK-net data to discuss the source process or local site effects in the source region (Yagi, 2004;Honda et al, 2004;Kamae and Kawabe, 2004;Koketsu et al, 2004;Yamanaka et al, 2004).…”

Kinematic source models for long-period ground motion simulations of megathrust earthquakes: validation against ground motion data for the 2003 Tokachi-oki earthquake

“…The observed Sv at the K-NET and KiK-net (borehole) stations were computed after applying a band-pass filter at 2-20 s and are plotted on the maps for comparison. It should be noted that the observed Sv at the K-NET stations include the effects of surface layers and may have been affected by nonlinear site response because the data are recorded on the ground surface, although we did not use stations where distinctive nonlinear site responses have been reported by several papers (e.g., Yamanaka et al 2004;Morikawa et al 2006). In the southeastern region of Hokkaido, Model H3 generally more closely agrees with the observations than Model C, especially at periods of 2, 3, and 5 s, at which the large values of Sv along the southeastern coast of Hokkaido around Yufutsu, Tokachi, and Kushiro Plains (locations of the plains are shown in Fig.…”