open access

https://doi.org/10.14943/gbhu.69.175

https://hdl.handle.net/2115/21508

Long-period Ground Motions with a Period of About 20 Seconds from the 2003 Tokachi-oki Earthquake

日本語

2006-03-15

北海道大学大学院理学研究科地球惑星科学専攻（地球物理学）

Geophysical bulletin of Hokkaido University

69

175-190

A lot of strong ground motion data from the 2003 Tokachi-oki earthquake (Mw8.3) were obtained by a dense strong motion network including K-NET, KiK-net and so on. We examine the long-period ground motions with a period of about 20 seconds based on these data. First, we estimate propagation directions and phase velocities of long-period Rayleigh waves at 64 seismic array sites in Hokkaido arranged from the dense network; the radius of the array is about 15 Km. The semblance analysis is applied to band-pass filtered, verticalcomponent waveforms at each array; the central frequencies are 0.03 and 0.05 Hz. The propagation directions of these long-period seismic waves basically radiate from the source region of the 2003 Tokachi-oki earthquake. However, the propagation directions seem to turn from the northwest to the west across the Hidaka Mountains, extending from the north to the south in the central Hokkaido. The phase velocities are 3 to 4 Km/sec; the phase velocity of the 0.03 Hz central frequency wave is slightly higher than that of the 0.05 Hz central frequency wave across one array. Next we examine ground velocity waveforms and spatial distribution of peak ground velocities (PGV) based on all data in the eastern Japan from the 2003 Tokachi-oki earthquake. Long-period waves with a period of about 20 seconds predominate on the ground velocity waveforms; at some sites shorter-period seismic waves are overlaid on the longperiod waves. The PGV distribution shows peculiar features; PGV in Hokkaido, the northern part of the epicenter, are larger than those in Tohoku, the south-western part of the epicenter, at the comparable distance. The PGV attenuation relation also shows peculiar features; the decay rate changes around a distance of about 200 Km and the large scatter with one order exists at a distance of about 300 Km. We interpret these features by comparing the observed waveforms with the synthetic ones calculated by using the CMT solution and 1-D velocity structure. The direct S-waves contribute PGV at distances less than about 200 Km, while the long-period surface waves (Rayleigh waves) contribute PGV at distances larger than about 200 Km. The source radiation pattern of long-period surface waves generates the large scatter of PGV at a distance of about 300 Km. This study concludes that the source radiation patterns of long-period S and surface waves mainly contribute the spatial distribution of PGV for large earthquakes such as the 2003 Tokachi-oki earthquake.

紀要論文

Version of Record

0439-3503

AN0008976X