Acceleration records of the 1995 Kobe earthquake and its first aftershocks, provided by vertical arrays at Port Island (PI), SGK, and TKS sites, were used to estimate changes in the shear moduli of subsurface soils caused by strong ground motion. Shear-moduli estimates are based on stress-strain relations determined for consecutive time intervals during the mainshock and aftershocks. We found that in the upper 0-13 m at PI (reclaimed fill above the groundwater level), the shear modulus decreased by ∼80%-90% of its initial value owing to liquefaction. Signs of its recovery appeared 3-5 min after the decrease in the intensity of strong motion. At depths of 13-18 m (reclaimed fill below the water level) and 27-32.5 m (water-saturated sand) at PI, 0-11 m at SGK, and 0-14 m at TKS (water-saturated alluvium), the shear modulus decrease was estimated at about ∼80%, 50%-60%, 60%-70%, and ∼50%, respectively. Recovery began immediately after the decrease in the intensity of strong motion and was completed within a few minutes. The results show that the behavior of the upper layers at sites PI and SGK, where the highest accelerations were recorded, was dynamically unstable. So far as the method can be applied to any site where vertical-array records and profiling data are available, accumulation of representative data on nonlinear soil behavior during strong motion for various soil conditions is desirable. Summarizing and generalizing these data, we may be able to predict soil behavior in future earthquakes.

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