Non-linearity and temporal changes of fault zone site response associated with strong ground motion

Abstract

SUMMARY We systematically analyse temporal changes of fault zone (FZ) site response along the Karadere-Dbranch of the North Anatolian fault that ruptured during the 1999 u and Dearthquake sequences. The study is based primarily on spectral ratios of strong motion seismic data recorded by a FZ station and a station ∼400 m away from the fault and augmented by analysis of weak motion records. The observations are used to track non-linear behaviour and temporal changes of the FZ site response. The peak spectral ratio increases 80-150 per cent and the peak frequency drops 20-40 per cent at the time of the D¨ uzce main shock. These co-main shock changes are followed by a logarithmic recovery over an apparent timescale of ∼1 d. However, analysis of temporal changes at each individual station using weak motion waveforms generated by repeating earthquakes show lower-amplitude longer-duration logarithmic recoveries that are not detected by the spectral ratio analysis. The results are con- sistent with a reduction of S-wave velocities in the top 100-300 m during the Dmain shock of 20-50 per cent or more and logarithmic post-main shock recovery on a timescale of 3 months or more. The observations support previous suggestions that non-linear wave propagation effects and temporal changes of seismic properties are generated in the shallow material by strong ground motion of nearby major earthquakes.