A local magnitude scale for crustal earthquakes in Italy

Abstract

In this study a new local magnitude (ML) scale is derived for the Italian region from the analysis of seismic signals recorded by a dense broad-band network between 2003 and 2009. The ML computation is performed by measuring peak amplitudes of synthetic Wood-Anderson seismograms. Based on Richter's original definition, these amplitudes are inverted for earthquake magnitudes, site-corrections and the distance dependent term that corrects amplitudes for geometrical spreading and scattering-anelastic attenuation. The latter is expressed as function of R (hypocentral distance) and log R by means of a two-parameter linear form, and constrained at the distance of 100 km in accordance with Richter's definition. Using the newly derived distance dependent term and site-corrections, magnitudes are consistently estimated for the stations recording each earthquake, within an overall root mean square (RMS) residual of 0.18. No anomalous trend with distance is appreciable in the magnitude residuals, for the Italian region as a whole and smaller areas along the Italian peninsula as well. The only exception is represented by northeastern Italy where increasingly negative residuals are observed for distances decreasing below 100 km. Spatial pattern of site corrections reveals contrasting attenuation properties of the crust between the Tyrrhenian side of the Italian peninsula (west of the Apennines, high attenuation) and the Adriatic side (east of the Apennines, low attenuation), as previously found in other studies. Finally, the magnitudes computed with the new ML scale are compared with global estimates of body-wave magnitude (mb) and local and regional estimates of moment magnitude (Mw), finding a satisfactory agreement in most of the magnitude range considered (roughly from 3 to 5.5).