Borehole Records Observed at the Port Island in Kobe During the Hyogo-Ken Nanbu Earthquake of 1995 and Its Simulation

Strong motion records of the Hyogo-ken Nanbu earthquake of 1995 observed by the borehole array at the Port Island in Kobe are analyzed to see the effects of soil nonlinearity. The horizontal peak accelerations and the predominant period on the surface are smaller and longer than those in the borehole. We try to simulate the observed accelerograms by using 1-D linear, equivalent linear, and effective stress analyses with the reasonable soil constants derived from the boring survey. The analyses show that a) linear analysis yields much higher accelerations than the observed, b) equivalent linear analysis yields good agreement up to the depth GL-16m but fails to reproduce the surface records, and c) the nonlinear analysis considering excess pore water pressure can reproduce the observed records both on the surface and in the boreholes. The pore pressure ratios of the reclaimed and holocene sand layers reach to almost 100% within 10 seconds from the start of strong shaking. Thanks to this strong liquefaction the peak ground acceleration and velocity at the reclaimed land would be reduced considerably which should be related to the damage distribution in Kobe.