A Local Magnitude Scale for Crustal Earthquakes in Italy

Abstract

Abstract In this study, a new local magnitude ( M L ) scale is derived for the Italian region from the analysis of seismic signals recorded by a dense broadband network between 2003 and 2009. The M L computation is performed by measuring peak amplitudes of synthetic Wood–Anderson seismograms. Based on the original definition by Richter (1935), 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 a function of R (hypocentral distance) and log R by means of a two‐parameter linear form and is 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 residual of 0.18. No anomalous trend with distance is appreciable in the magnitude residuals for the Italian region as a whole or for smaller areas along the Italian peninsula. The only exception is represented by northeastern Italy, where increasingly negative residuals are observed for distances decreasing below 100 km. The spatial pattern of site corrections reveals contrasting attenuation properties of the crust between the Tyrrhenian side of the Italian peninsula (high attenuation west of the Apennines) and the Adriatic side (low attenuation east of the Apennines), as previously found in other studies. Finally, the magnitudes computed with the new M L scale are compared with global estimates of body‐wave magnitude ( m b ) and with local and regional estimates of moment magnitude ( M w ); they display a satisfactory agreement in most of the magnitude range considered (roughly from 3 to 5.5). Online Material: Tables of station parameters, maximum amplitudes, and magnitude.