Seismic Waveforms At Engineering Bedrocks Beneath Lifelines Suffered By Liquefaction in the 2011 Off the Pacific Coast of Tohoku Earthquake

Abstract eng:
In the 2011 off the Pacific coast of Tohoku earthquake (hereinafter the Tohoku earthquake), buried pipes for water supply, water treatments and telecommunication networks were severely damaged due to severe ground motions and induced liquefaction. Physical damage of buried pipes caused the functional disruption of associated lifeline facilities. In order to clarify the mechanism of the damage by combined actions of severe ground motions and liquefactions, first, we performed series of analyses on the seismic wave propagation in the Tohoku and Kanto areas in the Tohoku earthquake and revealed the trends of seismic waveforms in the affected regions by severe liquefaction: Hitachinaka City and Kamisu City in Ibaraki Prefecture, and Mihama in Chiba City. The synthetic waveforms at engineering bedrocks were computed using the finite difference method with fourth-order spatial and second-order temporal discretization proposed by Aoi and Fujiwara (1999) Second, we focused onto the occurrence of liquefaction at two sites: an inherently liquefiable site (HOR2) and relatively nonliquefiable site (HOR1) in Kamisu City, in which the affected sewer pipes are distributed, based on effective stress analyses using one synthetic waveform and three observed waveforms near the associated liquefied areas in the Tohoku earthquake. For the HOR2 site subjected to the synthetic waveform which wave amplitude at the engineering bedrock shows more than 0.5 m/s and 1.0 m/s2 with the abundance of about 3.0 s long-period components, but that is a relatively smaller amplitude level, the excess pore water pressure ratio gradually increases to the liquefaction criterion of 97% with the long duration of about 100 s excitation. In contrast, for the HOR1 site subjected to observed waveforms with the less long-period components but long duration of more than 100 s showing a relatively larger intensity of about 4.0 m/s2 after the main shocks, the excess pore water pressure ratio also reaches gradually and exceed the liquefaction level. Therefore, the input acceleration to the engineering bedrock with the rich long-period component of more than about 3.0 s and the long duration of more than about 100 s could gradually induce the liquefaction for an inherently liquefiable ground surface such as the Kamisu HOR2 site. From the other aspect, the input acceleration with the long duration of about 100 s after the main shocks showing the maximum intensity of about 4.0 m/s2 also could induce the liquefaction for a relatively non-liquefiable ground surface such as the Kamisu HOR1 site.

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