報告書番号

N06019

タイトル（和文）

露頭岩盤上強震観測点およびその周辺地域の地盤構造の解明‐根室観測点における詳細調査‐

タイトル（英文）

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

概要 （図表や脚注は「報告書全文」に掲載しております）

当所の露頭岩盤上強震観測点である根室観測点において，サイト特性の影響を定量的に評価する目的で，観測点直下および周辺の弾性波速度構造を明らかにするための各種調査を実施した．その結果，観測点は同地方の広域的な基盤層である根室累層に貫入した，厚さ50m以上の粗面粗粒玄武岩からなる岩床の上に乗っていると解釈された．また常時微動のH/Vスペクトル比は，調査結果に基づく1次元速度構造モデルに地震基盤相当層を付加したモデルで部分的にではあるが説明できることを明らかにした．

概要 （英文）

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.

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