GNGTS 2013 - Atti del 32° Convegno Nazionale

team from the 1970s to the 1990s to re-read the amplitudes of the WA waveforms. Apart from isolated cases, there was substantial agreement among the three specialists confirming a small but noticeable tendency to underestimate the obtained magnitude compared to the original one obtained wrongly by vectorial composition. This little exercise corroborated the opinion that the azimuthally operated recovery of the original WA amplitudes was satisfactory and that the past WA magnitude overestimation of the Trieste station was due to the incorrect method of its calculation (vectorial composition instead of arithmetic mean). Second period: December 17, 2002 – August 09, 2004. December 17, 2002 marks the beginning of the recordings of the WA digitized seismograph. In this first stage 202 events has been recorded, however, only the magnitude and the epicentral distance have been catalogued. We have considered three location databases to associate to each event their hypocentral coordinates: that of OGS (www, preferred choice), that of the European-Mediterranean Seismological Centre (EMSC; www, second choice), and that of the National Institute of Geophysics and Volcanology [INGV; ISIDe Working Group (2010), last choice]. Moreover, we reported in the working data set the local magnitude provided by INGV, when available. If the difference in magnitude between the computed MAW and the ML provided by INGV was larger than 1, or the epicentral distances between the locations of the same event presented large discrepancies (greater than 50 km), the records were singularly double- checked, in order to avoid wrong earthquake associations. A total amount of 74 localized events has been retrieved for the time interval December 17, 2002 to August 9, 2004 and are largely clustered in the central Adriatic Sea (c in Fig. 1a). Third period: October 22, 2004 to May 24, 2005 and March 6, 2010 to August 6, 2012 . From October 22, 2004, the WA is placed side by side (at a distance of a few decimeters) to a Guralp 40-T BB seismometer with a period extended to 60 s. The MAW list was integrated with the hypocentral coordinates taken from the EMSC and INGV catalogues with the same approach adopted for the previous period. A total amount of 709 earthquakes have been recorded with an interruption in the recordings motivated by the renovation of the building where it is located. The instrument was temporarily moved from its historical site, and the recordings of this time period were discarded because the quality of the data in the temporary location was poor due to the high noise level. The final catalogue. Putting together the events recorded in the three periods analyzed, a final catalogue of 1102 earthquakes, whose geographical distribution is shown in Fig. 1a, has been assembled. Three main clusters are clearly recognizable also in Fig. 1a and 1b, where the distribution of the earthquakes as a function of epicentral distance is plotted. The events of the first period and recovered from the historical paper bulletins are mostly local, with nearly half of the events in the range between 60 and 100 km, corresponding to the already mentioned area of Friuli (a in Fig. 1a). The cluster of events in central Adriatic Sea (c in Fig. 1a) refers to 80% of the localized events recorded in the second period (digitized WA). Most of the events recorded in the third period (50% in the range between 200 and 240 km) refer to the Emilia seismic sequence, started after the two strong earthquakes on May 20 and 29, 2012 (d in Fig. 1a). As regards the MAW distribution (Fig. 1c), the most represented bins are in the range 2.5 - 3.5. There are 14 events with magnitude greater than, or equal to 5, with the maximum value up to 5.7. Comparative analysis of MAW values. As we have already stated, since 2004, the WA seismometer is placed side by side to a BB seismometer just with the intention to verify the goodness of the WA simulation on it. The WA magnification factor on the BB is set equal to 2800. Orthogonal regression has been performed on the data (Fig. 2a). This kind of approach, allows us taking into account the uncertainties of both magnitude values and achieving more reliable results (Castellaro et al. , 2006). The fitting equation is: M LBB = (1.016 ± 0.004) MAW + (0.066 ± 0.013) R 2 = 0.9961 (3) 119 GNGTS 2013 S essione 1.1