ESTIMATION OF SUBSURFACE STRUCTURE BASED ON MICROTREMOR AND SEISMIC OBSERVATIONS IN AREA DAMAGED BY 2018 HOKKAIDO EASTERN IBURI EARTHQUAKE, HOKKAIDO, JAPAN

Abstract

To investigate the cause of serious damage during the 2018 Hokkaido Eastern Iburi Earthquake, we observed microtremors and aftershocks at a landslide area and around strong ground motion observation stations. Subsurface velocity structures were determined using a heuristic approach based on a forward calculation using the phase velocities and H/V spectra. As a result, S-wave velocity structures were estimated, and the predominant frequencies of the H/V spectra were obtained. The predominant frequencies of the H/V spectra of the microtremor and seismic waves are 0.8–5 Hz. The S-wave velocity of the uppermost layer is 75–130 m/s, and its thickness is approximately 10 m. In particular, it has been suggested that the uppermost layer of volcanic ash caused the landslide in the town of Atsuma. The predominant frequency corresponds to the thickness of the alluvial layers with S-wave velocities of 75–300 m/s. It was also found that the predominant direction of the horizontal earthquake ground motion of the landslide was affected by the topography of the mountain.