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

In the framework of a joint project of the Italian Gruppo Nazionale per la Difesa dai Terremoti (GNDT) and the Istituto Nazionale di Geofisica (ING), devoted to the reorganization of the Italian earthquake instrumental database from 1981 to 1996, we approached the problem of the homogeneous determination of local magnitude in Italy. As a first step, we built a database of real Wood-Anderson amplitudes deduced from the available data of the two instruments that were in operation in Italy up to 1989 as well as of simulated Wood Anderson amplitudes computed by Very Broad Band recordings, available since 1990. The simultaneous computation of the attenuation function together with the magnitudes and the station residuals allows us to verify that, for the Italian area, this function does not significantly differ from the original one given by Richter. In the second step, the so obtained Ml magnitudes are used as a reference data set to estimate a new empirical relation for the duration magnitude Md using the unified phases database of the Italian National Seismic Network (RSNC) of ING and of the most important local seismic networks operating in Italy. The same data set was also employed to calibrate magnitude Ma based on amplitudes coming from short period vertical seismograms recorded by the RSNC automatic acquisition system. At last a set of reasonable criteria to choose the most reliable among Ml, Md and Ma has been formulated. The resulting set of magnitudes proved to be definitely better calibrated than the one obtained by procedures in use at RSNC.