GNGTS 2022 - Atti del 40° Convegno Nazionale

192 GNGTS 2022 Sessione 2.1 coda analysis. The robustness of the coda calibration procedure was tested by progressively reducing the number of calibration events and stations and applying each new calibration to the validation dataset. Even in the case of a reduction of 75% of the number of events and stations with respect to the best case, Mw and corner frequency estimates remained substantially stable. This confirmed the ability of the coda approach to obtain stable source parameters even with few calibration events and stations. The comparison between CCT and GIT results in terms of source spectral shape demonstrated their similarity over a large frequency range (0.4-25-Hz is the common range covered by both calibration approaches). Coda-derived spectra are within the 5th and 95th percentile of GIT apparent source spectra in all cases, and in general overlap the median GIT-derived spectra. The CCT and GIT calibrated models and parameters have been applied to the holdout dataset composed of 247 events (~1.7 <Mw < ~ 5.0) recorded in the same region. This comparison had a dual function: first, to validate the results of the coda calibration approach and second, to confirm that the CCT can be used in near real-time analysis to create catalogs characterized by homogeneous and reliable estimates of source parameters for small and large events using a common method. In term of source scaling, both GIT and CCT source parameters provided indication of increasing apparent stress with increasing magnitude, in particular below magnitude 5, with apparent stress varying in the range from ~0.01 to ~1 MPa. Beside the source term, the CCT and GIT calibrations provide estimates of relative site amplification effects. In order to compare the site terms for stations used in both calibration procedures, the single station results were normalized to the respective network average (i.e., the evaluated amplifications were relative to the network average considering the set of stations used by each approach). The relative, weak-motion site terms obtained from for 14 stations analyzedwith CCT and GIT approaches showed an overall good agreement for common pass bands, as expected by the dominance of S -waves in the scattered field (Aki, 1992; Zeng, 1993). The only striking amplification difference is observed below 2-Hz at one of the stations installed on unconsolidated sediments. Although several studies investigating the consistency of the S-wave and coda-wave site amplifications found a similarity of the results within a factor between 1.5 and 2 (Kato et al. , 1995; Bonilla et al. , 1997), a few larger discrepancies were observed either for stations installed over deep sediments (Margheriti et al. , 1994) or when recordings at epicentral distances smaller than the focal depth were analyzed (Su et al. , 1996). In our talk, a summary of the analysis and results obtained by Bindi et al. (2020) and Morasca et al. (2022) is presented. References Abercrombie, R.E., (1995). Earthquake source scaling relationship from 1 to 5 ML using seismograms recorded at 2.5 km depth , J. Geophys. Res., 100, 24 015–24 036. Abercrombie, R.E., (2015). Investigating uncertainties in empirical Green’s function analysis of earthquake source parameters , J. Geophys. Res., 120, 4263–4277. Abercrombie, R.E., S. Bannister, J. Ristau, and D. Doser (2017). V ariability of earthquake stress drop in a subduction setting, the Hikurangi Margin, New Zealand , Geophys. J. Int. 208, 306–320. Aki, K., (1969). Analysis of the seismic coda of local earthquakes as scattered waves , J. Geophys. Res., 74, 615–631. Aki, K., and B. Chouet (1975). Origin of coda waves: source, attenuation and scattering effects , J. Geophys. Res. 80, 3322-3342. Aki, K. (1992). Scattering conversions P to S versus S to P , Bull. Seism. Soc. Am. 82, 1969-1972. Andrews, D.J. (1986). Objective determination of source parameters and similarity of earthquakes of different size, in Earthquake Source Mechanics, S. Das, J. Boatwright, and C. H. Scholz (Editors), American Geophysical Union, Washington, pp. 259–267. Bindi, D., D. Spallarossa, M. Picozzi, and P. Morasca (2020). Reliability of source parameters for small events in central Italy: insights from spectral decomposition analysis applied to both synthetic and real data , Bull. Seismol. Soc. Am. 110, 3139–3157, doi: 10.1785/0120200126. Bonilla, L.F., J. H. Steidl, G. T. Lindley, A. G. Tumarkin, and R.J. Archuleta (1997). Site amplification in the San Fernando Valley, California: Variability of site-effect estimation using the S-wave, coda, and HIV methods , Bull. Seismol. Soc. Am 87, 710-730.