Publication Date:
2002-08-01
Description:
A large data set of ground-velocity time histories from earthquakes that occurred in Friuli-Venezia Giulia (northeastern Italy) was used to define regional predictive relationships for ground motion, in the 0.25- to 14.0-Hz frequency band. The bulk of the data set was provided by the seismic network run by Centro Ricerche Sismologiche (CRS), a department of the Istituto Nazionale di Oceanografia e Geofisica (OGS). A collection of 17,238 selected recordings from 1753 earthquakes was compiled for the years 1995-1998, with magnitudes ranging from M (sub w) approximately 1 to 5.6. Ninety-six three-component strong-motion waveforms belonging to the largest events of the 1976-1977 Friuli seismic sequence were also taken from the ENEA-ENEL accelerogram database and included in our data set. For the strongest event, which occurred on 6 May 1976 at 20:00 local time, an average local magnitude M (sub L) 6.6 was computed by Bonamassa and Rovelli (1986). The inclusion of a large number of acceleration time histories from this earthquake and six others, from magnitudes from M (sub w) 5.2 to magnitude M (sub s) 6.1 (three of them of M (sub s) approximately 6.0), extends the validity of the predictive relationships proposed in this study up to the highest magnitude ever recorded in the region. A total of 10,256 vertical-component and 6982 horizontal-component seismograms were simultaneously regressed for excitation and site characteristics, as well as for the crustal propagation, in the hypocentral distance range 20-200 km. Results are given in terms of excitation, attenuation, and specific site for the vertical ground motion, together with a horizontal-to-vertical ratio for each existing horizontal-component seismometer. The regional propagation was modeled in the 0.5- to 14.0-Hz frequency band by using a frequency-dependent piece wise continuous linear (in a log-log space) geometrical spreading function and a frequency-dependent attenuation parameter: Q(f) = 260(f/1.0) (super 0.55) . The excitation spectra of larger events were modeled by using the regional propagation, a single-corner frequency Brune spectral model characterized by an effective stress parameter, Delta sigma = 60 MPa, and by a regional estimate of the near-surface, distance-independent, network-averaged attenuation parameter kappa (sub 0) = 0.045 sec that was estimated from the rolloff of the empirical source spectra obtained from the regressions. Other studies (De Natale et al., 1987; Cocco and Rovelli, 1989; Singh et al., 2001) suggested large stress drops (Delta sigma congruent to 30-100 MPa,) to explain the high-frequency amplitude levels of the seismic radiation of the largest quakes of the 1976 sequence. Predictions for peak ground acceleration (PGA) and pseudo-spectral velocity (PSV) (5% damping) were computed through the use of the random vibration theory (RVT), with the parameters obtained from the regressions of this study.
Print ISSN:
0037-1106
Electronic ISSN:
1943-3573
Topics:
Geosciences
,
Physics
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