Electronic Supplement to
Empirical Calibration of Local Magnitude Data Sets Versus Moment Magnitude in Italy

by Paolo Gasperini, Barbara Lolli, and Gianfranco Vannucci

Section 1. Description of local magnitude datasets available for Italy.

Catalogo Strumentale dei Terremoti Italiani (CSTI) version 1.1 (CSTI working group, 2005, see Data and Resources Section)

CSTI 1.1 reports 46404 located earthquakes from 1981 to 1996 but only for 32903 of them, a local magnitude is available. Actually, the CSTI website provides the catalog in two forms: the first one (catalogo.dat), thought to be a continuation of the Progetto Finalizzato Geodinamica (PFG) catalog (Postpischl, 1985), shares the same format of PFG catalog and reports only one (preferred) local magnitude estimate (Mp) for each earthquake: 31571 of them are Md (code "D"), 763 ML (both WA and SWA, code "L"), 332 are Ma (code "A", computed from amplitudes of non standard seismometers) and 237 come from previous determinations of unspecified nature (code "I", only for the time interval from 1981 to 1984). The second file (Italia.sum) is provided in the standard Hypoellipse (Lahr, 1999) format and reports the preferred magnitude (in col. 37-38) and the corresponding code (col. 80) but also, if available, even the coda duration (col. 70-71) and amplitude (col. 72-72) magnitudes for each earthquake. In this paper we referred to this second more complete file.

In Fig. S1 we can note how the number of located and sized earthquakes with Mp≤2.5 strongly increased since 1984 (after the installation of the first automatic seismic acquisition system at the ING). Starting from this year, the annual numbers of events for Mp>2.0 are about constant whereas for smaller magnitudes the number increases again around 1987 and 1989 (after other known improvements of the station network and of the seismic acquisition system). Over the entire time interval from 1981 to 1996 the number of earthquakes are about constant only for Mp>3.0-3.5.

Figure S1

▲ Figure S1. Number of earthquakes per year within various magnitude thresholds for the CSTI 1.1 catalog.

Catalogo della Sismicità Italiana (CSI) version 1.1 (Castello et al., 2006, see Data and Resources Section)

CSI 1.1 includes 91797 located earthquakes from 1981 to 2002 but provides a local magnitude only for 39543 shallow earthquakes (32484 Md with code "MlREG", 6396 ML from SWA waveforms with code "MlMDN" and 663 taken from CSTI with code MCSTI). The Mw or the body wave magnitude (mb) is reported for 122 intermediate and deep events (with codes "MwHVD", "MwMDN" and "MbISC"). Due to different criteria for assessing the reliability of hypocentral locations and magnitudes, in the time interval from 1981 to 1996, which is in common with CSTI (and substantially based on the same dataset provided by CSTI), the CSI includes 48764 located earthquakes (about two thousands more than CSTI) but only 19551 local magnitudes (18527 Md, 361 ML and 663 taken from CSTI) for shallow events and 65 Mw or mb for intermediate or deep events. A new version (2.0) of the CSI catalogs, extended to 2007, has been prepared for research projects funded by the Italian Dipartimento della Protezione Civile (Civil Protection Department, DPC) and by the INGV but it was never published officially and its circulation was restricted until now within the scope of researchers involved in the DPC projects.

From Fig. S2 we can note the marked increase of the number of events in 1997 (owing to the long and productive sequence occurred in the Umbria-Marche area) and also the increase of located and sized earthquakes for Mp≤2.0 (by a factor of about 2) since the same year approximately (presumably owing to the improvement of the network). In Fig. S2 we can note an increase of the number of earthquakes (from about 7 to about 15 events per year) for 4.0<Mp≤4.5, since about 1998-2000, which reasonably might be attributed to some (unknown) change in the method for computing magnitudes. Also note, for the same magnitude range the unique event reported in 1989 (versus an average of about 8 events per year in the previous period and the 6 events reported by CSTI catalog in the same year). We might argue that the choice of CSI compilers to discard all magnitudes from amplitudes and durations computed by local and regional seismic networks may have brought to loose almost all the information available on the earthquakes in this magnitude range.

In general, in the common period from 1981 to 1996, about 13000 magnitudes reported by CSTI are missing in CSI: 332 of them are Ma (discarded by CSI compilers because they considered unreliable the amplitude calibration of INGV stations) while the other are probably Md coming from durations provided from local or regional networks.

Figure S2

▲ Figure S2. Number of earthquakes per year within various magnitude thresholds for the CSI 1.1 catalog.

Bollettino Sismico Italiano (BSI) (see Data and Resources Section).

In the period considered in the present work from 2003 to 15 April 2005, BSI reports 4846 located earthquakes. All events have a local magnitude (4753 Md with blank code and 93 ML from SWA waveforms with code "L"). The above website allows to download the same data for 2002 that were used in some computation of the present work.

Italian Seismic Instrumental and parametric Data-basE (ISIDE) (Amato et al, 2006, see Data and Resources Section).

From 16 April 2005 to 2010, ISIDE includes 62803 revised locations of earthquakes. For all events, a preferred local magnitude is reported (60582 ML from SWA waveforms with code "L" and 2221 Md with blank code). From the ISIDE website it is possible to extract the Md even for earthquakes for which an ML estimate is available and then Md is not the preferred magnitude. The ISIDE database also provides preliminary (automatic) locations of earthquakes occurred after 2010 that we did not consider in our analysis.

From Fig. S3 we can note the strong increase of the number of events in 2009 for all magnitude ranges, owing to the long sequence after the April 06 2009 L'Aquila (Mw=6.3) earthquake. We can note as well, for 2003 and 2004 covered by BSI bulletin, the clear decrease of the number of events in the range 1.5<Mp≤2.0 and the increase in the ranges 2.5<Mp≤3.0 and 3.0<Mp≤3.5 that are probably due to the use of the Md formula by Console et al. (1989) by ISIDE compilers.

▲ Figure S3. Number of earthquakes per year within various magnitude thresholds for BSI and ISIDE on-line bulletins. The last three years (from 2000 to 2002) of CSI 1.1 catalog are also reported for comparison.

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