Evaluation on spatial variation of subsurface velocity structures and earthquake ground motions along the closed Takachiho railway line

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  • Iwata Naoyasu
    Railway Technical Research Institute, Center for Railway Earthquake Engineering Research, Seismic Data Analysis
  • Tsuno Seiji
    Railway Technical Research Institute, Center for Railway Earthquake Engineering Research, Seismic Data Analysis

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Other Title
  • 旧高千穂鉄道沿線における地下速度構造および地震動の空間変動評価
  • キュウ タカチホ テツドウ エンセン ニ オケル チカ ソクド コウゾウ オヨビ ジシンドウ ノ クウカン ヘンドウ ヒョウカ

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Abstract

　In case that strong ground force affects railway facilities during earthquakes, the reduction of the safety performance of railway structures and running vehicle is concerned. When strong ground motions are observed, railway operators stop trains as soon as possible to secure the safety of trains. After the operator stop trains, a cautious inspection patrol along the railway section following the rule decided in advance is implemented. After the safety of railway facilities are ensured, the train operation is resumed. To confirm visually the damage of railway facilities and deformation of railway tracks in the inspection, it may take a lot of time. To comprehend exactly the characteristics of earthquake ground motions along the railway line, therefore, the rapid evaluation of the earthquake ground motions are strongly required not only at the points where the seismographs are installed but also at the other points on the railway line.<br> 　We proposed the method for estimating the subsurface velocity structures and earthquake ground motions along a railway line. In this study, we tried to apply the method to a railway embankment section in the closed Takachiho railway line, of which length is approximately 600m. The adopted section consists of an embankment in a valley, ridge in an excavation and other embankment in a valley. We installed the seismographs at two embankment sites and an excavation site. At first, we estimated S-wave velocity structures at two embankment sites, using the geophysical exploration such as surface wave exploration and array microtremor survey. Secondly, we estimated the S-wave velocity structures along the railway section using the H/V spectral ratios obtained by the high-dense microtremor measurement based on the S-wave velocity structures estimated. Finally, we estimated the earthquake ground motions along the closed Takachiho railway section by the one-dimensional multiple reflection theory using an observed seismic motion at site of the excavation section. We confirmed the validity of the S-wave velocity structures and earthquake ground motions estimated by the proposed method, through the agreements of the observations and estimations along the railway section.<br>

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