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Last update: May 2016 
ISIDe is properly referenced as: 'ISIDe working group (2016) version 1.0, DOI: 10.13127/ISIDe'. 

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Magnitude

Most of the earthquakes published in ISIDe since April 16 2005 have a local ML magnitude. The large number of three component instruments installed in the last 6 years in Italy ensures the computation of local magnitudes even for very little earthquakes. The advantage of this dense network became evident in April 2005, when a new tools for the analysis of the entire set of seismic data became available and routinely used to publish the Italian Seismic Bulletin. The completeness of the catalog reached ML=1.6 in the last nine months of 2005, with an improvement of two tenths of magnitude, if compared to the seismicity recorded in the last nine month of the year 2004 Bulletin.

It is well known that the local magnitude requires a correction for those stations that are closer or further than 100 km from the hypocenter. A correction function suitable for the whole Italian country has not been computed yet, although some works partly covered some local areas in northern Italy and other investigations are in progress. At the moment we are using the Hutton and Boore relation, valid for California:

ML = log10(amp) + 1.110 log10(hd) + 0.00189 hd+ 3.591 (attenuation term by Hutton-Boore)

where “amp” is the peak-peak maximum amplitude in meters divided by 2, and “hd” is the hypocenter-station distance in kilometers. The above relation that was computed for the southern California region, mostly adheres to actual italian data for station-hypocenter distances greater than 100 kilometers, whereas it overestimates the local magnitude at closer stations (M. Di Bona, personal communication). As a result, the magnitudes of “large” earthquakes, recorded by many distant stations, are still reliable, while the magnitudes of little events could be biased by the prevalence of close stations and will require further future investigations. This fact suggests the hypothesis that, after a more precise calibration of an italian distance-magnitude relation, the actual completeness of the catalog might result to be lower than ML=1.6.

We compute an exact station local magnitude (from horizontal components) at each three component broad-band station and an event-magnitude as a trimmed (Huber) mean of the station magnitudes with a cut-off threshold of 0.3 . When an ML magnitude is not available, because the event was recorded by short period instruments only, we compute Md station magnitudes from the relation:

Md = 2 *log10(dur + ed* 0.082) - 0.87

for epicenter-station distances within 600 kilometers, where “dur” is the duration of the seismic record at the station and “ed” is the epicenter-station distance.