Evaluation of site amplification factors in northwest Morioka area by microtremor array surveys

Abstract

In the northwestern part of Morioka City, Iwate Prefecture, in the northern part of Honshu, Japan, microtremor array observations consisting of a sensor spacing of 0.6 m and those consisting of a sensor spacing of 10-20 m were carried out. Phase velocity dispersion curves were used to estimate the S-wave velocity structure down to a depth of 30 m. The outline of the underground along the north-south direction observation line 1 and the east-west direction observation line 2 was grasped from the relationship between the basement depth and the peak period. S-wave velocity structure models were estimated from the phase velocity dispersion curve. By arranging this S-wave velocity structure model for each observation line, a pseudo two-dimensional S-wave velocity structure model was estimated. In addition, the 30m average S wave velocity was calculated from the S wave velocity structure model, and the site amplification factors from the engineering bedrock was calculated. The results are shown as follows. Observed average S-wave velocities (AVSs) in the Iwate athletics park and the area around the Morokuzu River were 226 m/s to 489 m/s, which were smaller than those obtained by Japan Seismic Hazard Information Station (J-SHIS). At 12 of the 17 points, the observed AVSs were slower than those by J-SHIS. The observed amplification factors in the Iwate athletics park and the area around the Morokuzu River were 1.25 to 2.03, which were larger than those by J-SHIS. The results shows that site amplification factors by J-SHIS were underestimated in this area, demonstrating the importance of miniature/small array observations.