ON THE MEASUREMENT OF THE PREDOMINANT AND RESONANT FREQUENCIES

The predominant frequency (a source property) and the resonant frequen cies (site effects) have been measured from local seismic events that occurred and were recorded at Campi Flegrei. The predominant frequency is measured as the period of the first half-cycle of the seismogram, which in turn is obtained as six times the standard deviation of the e xpectation value arrival time. Because the width of the seismic pulse is severely affected by attenuation, this effect has been accounted fo r through a new method in time domain, based on the broadening of-a ga ussian pulse propagating in a dispersive medium; the analyzed data sug gest that the errors introduced by not correcting by source directivit y (very difficult to carry out for local low-magnitude events) are not severe. Data also suggest that the first cycle of the seismogram may contain, accurately enough, information on the source properties, and hence allowing for the characterization of the seismic source in terms of a theoretical spectrum. Under the hypotheses (1) that the first cy cles of the seismogram are not severely contaminated by propagation an d (2) that resonant effects are mainly generated at later times, spect ra of seismic records, corrected by attenuation and source term, provi de useful information on the resonant peaks generated by local site ef fects. The present method of analysis shows that the frequency at whic h the resonant peaks are located can be accurately measured, whereas t he amplitude of the peaks suffer some uncertainties.