The Stability of the Horizontal-to-Vertical Spectral Ratio of Triggered Noise and Earthquake Recordings

The aim of this work is to analyze the stability of the amplification function obtained by the horizontal-to-vertical spectral ratio (HVSR) for a sedimentary site with a simple geomorphological situation. We have estimated the stability of the HVSR analyzing two years of data, composed of 674 triggered noise records (man-made seismic waves strong enough to reach the threshold set for earthquake detection) and 132 earthquakes (local, regional, and teleseismic events). The resonance peaks obtained with the two different data sets converge on an average both in frequency and amplitude. We examined and rejected the possible presence of periodicity of the fundamental frequency in the time sequences and its relevant amplification obtained by the HVSR of both triggered noises and earthquakes. Then, we performed a correlation analysis between these sequences and other parameters. In particular they have been correlated with signal amplitude, rainfall, and magnitude (for earthquakes only). A weak, negative correlation has been estimated between the rainfall and fundamental frequency and between the fundamental peakʹs amplification and magnitude. Finally, we validated the HVSR transfer function with a 1D model using a Vs profile obtained with the noise analysis of surface wave technique (Louie, 2001).