Strong ground motions from an Mj 6.1 inland crustal earthquake in Hokkaido, Japan: the 2004 Rumoi earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2009
Received: 7 April 2008
Accepted: 30 October 2008
Published: 27 July 2009
An inland crustal earthquake (Mj 6.1) occurred on December 14, 2004 in the northern part of Hokkaido, Japan. A large ground acceleration of 1127 cm/s2 and 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 the S-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.