Non-linear transient behavior during soil liquefaction based on re-evaluation of seismic records

• Approach of non-stationary Fourier spectra is applied to soil liquefaction problem.
• Shift of dominant frequency is closely related to transient process during soil liquefaction.
• Underground shear strain enables the computation of soil softening ratio during liquefaction.
• Pulse wave including cyclic mobility consists of primary and secondary modes.

Focusing on soil liquefaction, the seismic records during the Niigata-ken earthquake in 1964, the southern Hyogo prefecture earthquake in 1995 and the 2011 off the Pacific coast of Tohoku earthquake are analyzed by the non-stationary Fourier spectra. The shift of dominant frequency in the seismic record of Kawagishi-cho during the Niigata-ken earthquake is evaluated based on the time-variant property of dominant frequencies. The reduction ratio of the soil stiffness is evaluated from the shift ratio of dominant frequency. It is detected that the pulse wave in the transient process during the progressing liquefaction is composed of the primary mode and the secondary modes. Using the seismic records at the underground of Port Island during the southern Hyogo prefecture earthquake in 1995, the amplification of liquefaction is evaluated by comparing the maximum amplitude spectra of seismic records at GL 0 m and GL-83 m. The average shear strain of underground is calculated from the numerically integrated displacement profiles and the deterioration ratio of stiffness is evaluated from the G-γγ relation. The amplification of liquefaction at Chiba bay area is evaluated by using the ratio of the maximum amplitude spectra of CHB024 and CHBH10 (Borehole) from the mainshock and the aftershock of the 2011 off the Pacific coast of Tohoku earthquake.