Propagation Time of Seismic Waves of Surface Ground Estimated from Acceleration Records on the Slab Foundation and in the Supporting Layer

Abstract

The propagation time of the seismic waves from the bottom of the slab foundation to GL−40 m was estimated using Normalized Input-Output Minimization (NIOM) analysis for earthquake records obtained from seismic observations on the slab foundation of the buildings and in the supporting Layer (GL−40 m) at the Toyosu campus of the Shibaura Institute of Technology. The results showed that the observed propagation time varied, but this variation was related to the distance between the plane waves passing through the upper and lower seismographs, assuming that the seismic waves were plane waves. The propagation time evaluated from the borehole data showed relatively good agreement with the average of the observed propagation time, indicating that the averaging process may be able to eliminate some of the above-mentioned variation. Next, we applied the NIOM analysis to the observation records of the 2011 Great East Japan Earthquake to examine the nonlinear characteristics of the ground. The results indicated that the maximum shear strain in the ground was about 5 to 6 × 10^-4 and that the shear stiffness ratio G/G₀ may have decreased to about 0.8 to 0.6.