Investigation of subsurface structure beneath the strong-motion station on rock outcrop. Detailed survey at Nemuro, Hokkaido, Japan

Subsurface velocity structure beneath the strong-motion station installed on the rock outcrop is investigated to explain the local site effect found on the observed response spectra. PS logging and density logging are applied to the borehole drilled nearby the station to estimate the one-dimensional accurate velocity structure. Laboratory experiment for P and S wave velocity of the sampled borehole core is also carried out. Furthermore the refraction survey and microtremor measurement are done for the mapping of two-dimensional subsurface velocity structure, since the Nemuro station is located on the narrow area between the hillock and the seaside. As a result of PS logging the velocity structure just beneath the station is found to be almost homogeneous from 0.8 m to 35 m depth, which consists of trachybasalt with the S-wave velocity of around 2 km/s. The basaltic layer appears again from 42 to 50 m depth after the thin layer of mudstone. Observed basaltic layers are considered to be intrusive rocks in to the basement of mudstone. On the other hand the 2D seismic exploration revealed rather complex velocity structure. Lateral heterogeneity of seismic velocity is recognized not only in the direction connecting from the hillock to the coast, but also in the direction parallel to the coast line. Finally theoretical H/V spectral ratio of Rayleigh wave, which are calculated from the estimated velocity structure model, are shown to be consistent with the H/V spectral ratio of observed microtremors when seismic basement with S-wave velocity of 3.2 km/s is assumed at the depth of 150 m. (author)