GROUND MOTION ESTIMATION IN THE HIGASHINADA WARD IN KOBE DURING THE HYOGO-KEN NAMBU EARTHQUAKE OF 1995 BASED ON AFTERSHOCK RECORDS

Abstract

Aftershock records of the Hyogo-ken Nanbu earthquake of 1995 observed in the Higashinada Ward in Kobe are analyzed to find the appropriate ground models at each sites. Then main shock ground motions there are estimated based on these models. Spectra at the sites on sand layers 10m to 20m thick show considerable amplification compared to those on hard rock. Spectral ratios of horizontal components between them can be modeled very well by the 1-D ground models derived from the boring data (N-values) and geophyisical exploration. We try to reproduce ground motions at these sites by using equivalent linear or effective stress analyses with these soil models and the main shock records observed on the hard rock. We found that equivalent linear analysis gives us estimated peak accelerations of 670-1,130 Gals and peak velocities of 140-170 cm/sec and that effective stress analysis yields peak accelerations of 630-1,000 Gals and peak velocities of about 130 cm/sec. Thus we can say that ground motions in the heavily damaged area in the Higashinada Ward must reach to the level of 600 Gals in acceleration and 130 cm/sec in velocity at least.