Abstract
AbstractAn inland crustal earthquake (Mj6.1) occurred on December 14, 2004 in the northern part of Hokkaido, Japan. A large ground acceleration of 1127 cm/s2and a large pseudo-velocity response of over 100 cm/s were recorded at the nearest strong-motion station, HKD020, about 10 km from the hypocenter. This large ground motion is considered to be attributable to the source effect and the site effect. The site effect is investigated using the traditional spectral ratio technique and the theoretical evaluation based on the subsurface structure model. The results imply that the site effect has an insignificant effect on the large ground motion at HKD020. The source effect is investigated by constructing the source model that explains the broad-band strong-motion records at four stations around the epicenter using the empirical Green’s function method. The estimated source model satisfies the empirical relationship between the strong motion generation areas and the seismic moment for inland crustal earthquakes. The high-frequency level of the acceleration source spectrum is also consistent with the empirical relationship. These results suggest that this earthquake is a normal crustal event and that the large ground motion at HKD020 is mainly attributable to the source effect, short distance from the strong motion generation area and the forward directivity effect. Finally, the temporal change of the site response at HKD020 is examined using long duration records including the main shock and several aftershocks. The site response based on theS-wave horizontal-to-vertical spectral ratio method shows the nonlinearity for the main shock and an aftershock occurring about 20 s after the main shock. However, the site response shows linearity for other later aftershocks. This site response change is attributed to the difference in ground motion amplitude.
Highlights
A damaging crustal earthquake occurred in northernHokkaido, Japan at 14:56 on December 14, 2004 (JST)
We investigated the factors causing severe ground motions at the K-NET HKD020 station during the 2004 Rumoi earthquake (Mj 6.1), which occurred on an unrecognized shallow fault
The site effect was investigated using the traditional spectral ratio technique and the theoretical evaluation based on the subsurface structure model
Summary
Japan at 14:56 on December 14, 2004 (JST). The hypocenter is located in a southern Rumoi district; we call this earthquake the 2004 Rumoi earthquake in this paper. The focal depth and magnitude (Mj) determined by the Japan Meteorological Agency (JMA) is 8.6 km and 6.1, respectively This is the first Mj 6 class crustal earthquake in Hokkaido since high density strong motion networks, such as K-NET and KiK-net operated by NIED, were deployed over Japan. The northern island of Japan, is situated in the plate convergence zone where the Amurian (Eurasia) plate is in collision with the Okhotsk (North American) plate, and the Pacific plate is subducting along the Kurile trench (Fig. 1) These plate motions accompany many interplate and intraslab earthquakes with magnitude 7 or more around Hokkaido, while large (>Mj 6) inland crustal earthquakes have been rare.
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