Local magnitude revaluation for recent Italian earthquakes (1981–1996)

In the framework of a joint project of theItalian Gruppo Nazionale per la Difesadai Terremoti (GNDT) and the IstitutoNazionale di Geofisica (ING), devoted tothe reorganization of the Italianearthquake instrumental database from 1981to 1996, we approached the problem of thehomogeneous determination of localmagnitude in Italy. As a first step, webuilt a database of real Wood-Andersonamplitudes deduced from the available dataof the two instruments that were inoperation in Italy up to 1989 as well as ofsimulated Wood Anderson amplitudes computedby Very Broad Band recordings, availablesince 1990. The simultaneous computation ofthe attenuation function together with themagnitudes and the station residuals allowsus to verify that, for the Italian area,this function does not significantly differfrom the original one given by Richter. Inthe second step, the so obtained Mlmagnitudes are used as a reference data setto estimate a new empirical relation forthe duration magnitude Md using the unifiedphases database of the Italian NationalSeismic Network (RSNC) of ING and of themost important local seismic networksoperating in Italy. The same data set wasalso employed to calibrate magnitude Mabased on amplitudes coming from shortperiod vertical seismograms recorded by theRSNC automatic acquisition system. At lasta set of reasonable criteria to choose themost reliable among Ml, Md and Ma has beenformulated. The resulting set of magnitudesproved to be definitely better calibratedthan the one obtained by procedures in useat RSNC.