Spectral Characterization of Digital Ground Displacement Records in the Low 0.03–0.30 Hz Range: Low Predominant Frequency Corroboration and Seismic Structural Resonance Application

The spectral content of ground displacement records in the lowest frequency range is investigated. For such a study, digital records are mandatory because any Fourier analysis of the older analog displacement records is meaningless below 0.2 Hz (or 0.3 Hz, depending on the filter frequency employed), but those newer digital records were not widely available until recently, at least for major inland earthquakes (MW > 7) and for the near-field. If analog displacement records are selected carefully, clear predominant frequencies can be found in the 0.05–0.25 Hz range, but these important results do require confirmation by means of analyzing exclusively digital records, where the important signal-to-noise predicament at long periods is much less problematic than in the old analog records. The low-frequency spectral analysis proposed has lately become possible because (1) processing or filtering of these newer digital records implies high-pass frequencies of less than 0.1 Hz, and (2) crucial strong-motion earthquakes around the world are now predominantly recorded digitally, which was not the case 20 years ago; that is, there is now a relatively large set of near-field digital records of events with magnitudes larger than, say, seven. The first main conclusion is the corroboration or demonstration that ground displacement has one predominant frequency in the 0.05–0.25 Hz range. This seismological result is subsequently applied in Earthquake Engineering/Structural Dynamics; in particular, it will be shown that mathematical structural resonance can be observed or experienced in a building model. That is, clear displacement amplification will be demonstrated when there is a concrete match between the predominant frequency and the fundamental structural frequency; needless to say, this vibration-based or mathematical resonance analysis is not possible if there is no defined predominant frequency of seismic action.