Observed Evidences of Frequency-Dependent Site Amplification Due to Structural Control of Active Reverse Faults at Different Localities in Japan

Abstract

Observed active fault zone related site amplification is calculated based on Fourier acceleration spectrum (FAS) at three different localities in Japan. The FASs are calculated using 26432 earthquakes recorded at 126 K-NET and KiK-net seismic stations, which are distributed on the fault zones, upthrown and downthrown sides. This observed amplification is strongly frequency-dependent because of the presence of the near-surface low-velocity flower fault structure and the deeper fault zone. Moreover, the amplification patterns at each study area are tectonic-specific patterns. Sources inside the active fault zones could produce amplification at high frequencies at stations on both fault zone and far away from the fault zone, because of the impact of the near-surface fault zone. Sources outside the active fault zones yield remarkable high amplification at low frequencies exhibiting a gradual increase through stations on hanging walls, fault zones, and footwalls. Interestingly, the peaks of the low-frequency amplification are corresponding to wavelengths that approximately equalize the width of the fault zone. The presence of fault zone low-velocity layers could be successfully detected by the diffuse field theory inversion.