The region of large site amplification in Northern Hokkaido, Japan, extends to the eastern Soya Region

Abstract

AbstractIn the document titled “predominant period of long-period ground motions and the natural period of deep subsoil” by the Central Disaster Management Council, the west part of Northern Hokkaido facing the Sea of Japan is categorized as a region, where long-period (> 6 s) ground motions are predominant. In contrast, the document categorizes the east part of Northern Hokkaido as a region where short-period (< 1 s) ground motions are predominant. The J-SHIS velocity structure model also implies that a deep sedimentary basin is located in the west part of Northern Hokkaido, but not in the east part. However, the result of a past generalized inversion analysis was inconsistent with these general understanding and suggested that the region of large site amplification in the low-frequency range in Northern Hokkaido extends to the eastern Soya Region. It was preferable to check this result with an independent dataset. Therefore, in this study, the records of the May 24, 2013, MW8.3, Sea of Okhotsk earthquake were used to investigate the distributions of site amplification factors in Northern Hokkaido. Because this is a distant earthquake and the epicentral distance was greater than 1300 km even at the closest station in Hokkaido, it was assumed that the difference of ground motions at different stations represents the difference of the site effects. The Fourier spectra for the S wave part of this earthquake at K-NET and KiK-net stations clearly indicated that the region of large site amplification in the low-frequency range in Northern Hokkaido does extend to the eastern Soya Region. It is desirable in a future study to reveal the cause of the discrepancy between this amplification pattern and the existing velocity structure models. Graphical Abstract