Poster

[DM-P-04] Evaluation of site amplification factors in north-west morioka area by microtremor array surveys

*Hidekazu Yamamoto¹, Toshinobu Wada¹, Tsuyoshi Saito¹ (1. Iwate University (Japan))

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 and peak periods of microtremor H/V spectral ratios were used to estimate the S-wave velocity structure down to a depth of 30 m. The outline of the underground ground along the north-south direction survey line 1 and the east-west direction survey 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 survey 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. Furthermore, the amplification function of the SH wave as a function of the frequency was calculated from the S wave velocity structure model.

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. In addition, the maximum values of amplification factors as a function of frequency were about 5 times at frequencies of 2, 3 and 7 Hz around the Iwate athletics park. On the other hand, the maximum value of amplification factor function around Morokuzu River was about 4 times at a frequency of 1 Hz. The results shows that site amplification factors by J-SHIS were underestimated in this area, demonstrating the importance of micro/small array observations.