Temporal Change in Propagation Time of Seismic Waves Observed at Vertical Array in the Onagawa Nuclear Power Plant during, before, and after the 2011 Off the Pacific Coast of Tohoku Earthquake

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The Onagawa Nuclear Power Plant, operated by Tohoku Electric Power Co., Inc., is situated near the focal region of the 2011 off the Pacific coast of Tohoku Earthquake. The array records observed at the site during the earthquake provide valuable information about the wave propagation at a large strain level. In this study, the temporal changes in the propagation velocities of the seismic waves were examined based on the vertical array records during the mainshock and the events before and after it. It was revealed that (1) the shear rigidity of the soil layers between sensors G-3 and G-4 (from G.L.-27.3 to -1.7 m) decreased by 60-70% at the maximum strain of 1×10-3 due to the principal motion of the mainshock, whereas (2) the shear rigidity between sensors G-1 and G-2 (from G.L.-147.1 to -61.5 m) decreased only by 10-20% at the maximum shear strain of 5×10-5.