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

S U M M A R Y We systematically analyse temporal changes of fault zone (FZ) site response along the Karadere-Düzce branch of the North Anatolian fault that ruptured during the 1999 İzmit and Düzce earthquake 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 Düzce 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 consistent with a reduction of S-wave velocities in the top 100–300 m during the Düzce main 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.