The values of magnitude (Ml) have been calculated from the maximum amplitude horizontal components of signal recorded by seismometers (1 sec., 3 ch.) used in the dial_up stations (Mark Product L-4C-3D o Lennartz Electronic LE-3-D) and turned into those recorded by a Wood-Anderson sensor.
Since the program calculates the magnitude of a single event for each station that has recorded it and taking into account possible local amplitude effects which make different the magnitude estimation for the same event from one station to another, we decided to consider as event magnitude the average of all magnitudes calculated for each single station.
We also linked an error to the mean magnitude, to give a preliminary evaluation of quality of obtained values.The magnitude error is calculated according to the following formula:.
Since for about 7000 events of weak magnitude we did not have information about the amplitude of sensors of the dial_up stations but only the length estimated by the interpreters for the stations of the INGV national network, we suggested an indicative value of magnitude calculated from recording length, available in the INGV bulletin, according to the following formula: Md = -1,80 + 2,41 Log10T (F. Brega, A. Tento, G. Zonno, G. Monachesi e S. Raccichini, 1982).
The graph on the right shows the comparison between Ml and Md values: in some cases there are significant differences but mostly the difference falls within the error proposed for the Ml (on average = ±0,34).
For about 1900 events with a very weak estimated energy, we not have any value of magnitude because these events were not recorded by the dial_up station and no information is available about the length for the stations of the RSNC.