SEGJ14th

Presentation information

Poster session (core time)

Poster

Tue. Oct 19, 2021 3:30 PM - 5:30 PM Area 01 (Remo)

[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.

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