Abstract
The Hokusetsu area located in the northern part of the Osaka Basin, Japan, has the complicated geological structure including graben, called the Imogawa lowland, between the Hokusetsu Mountains and the Senri hill. We estimated the S-wave velocity structures in both the graben site and the hill site by carrying out microtremors array surveys with the maximum radius of around 100 m (S-array) and 500 m (L-array), respectively. The observed phase velocity up to 2000 m/s was obtained by these microtremor array surveys. On the other hand, the estimated depth of basement rock (Vs > around 3000 m/s) is not accurate enough in the microtremor array surveys. To estimate the deep structures, therefore, we reanalyzed data of two different seismic reflection surveys conducted at the Hokusetsu area and integrated the results of the microtremors array surveys and the seismic reflection surveys. To explain well both the observed phase velocity dispersion curves from the microtremor array survey and the depth of basement rock from the reflection seismic survey, we combined the structures shallower than about 500 m estimated by the microtremor array survey and the ones deeper than about 500 m estimated by the reflection seismic survey. As the result, the depth of basement rock (Vs > around 3000 m/s) of the integrated structural models is about 1000 m at the Imogawa lowland and about 600 m at the Senri hill, respectively, indicating the sharply inclined bedrock spread in the Hokusetsu area. Around the temporary seismic observation stations set up by the Railway Technical Research Institute (RTRI) at the Hokusetsu area, microtremor array surveys with the maximum radius of around 30 m (XS-array) were also carried out to estimate a shallow S-wave velocity structures. These surveys revealed that the sedimental layer (Vs < 500 m/s) has a thickness of about 200 m at the Imogawa lowland and about 100 m at the Senri hill, respectively.
