Estimating the Magnitude of Historical Earthquakes from Macroseismic Intensity Data: New Relationships for the Volcanic Region of Mount Etna (Italy)

Abstract

The historical activity of Mt. Etna is well documented by a large amount of sources that have reported the seismic and volcanic phenomena occurring on the volcano since the late 1600s (Azzaro et al. 2000; Branca and Del Carlo 2005). Such a large dataset of historical information is not common and is comparable, in Italy, to that of Vesuvius (Giudicepietro et al. 2010). As for long-term seismicity known through macroseismic data, the first release of the historical catalog of Mt. Etna earthquakes from 1832 to 1998 was published ten years ago (hereinafter CMTE catalog) and since then has been regularly updated (CMTE Working Group 2008). With 1,790 earthquakes listed, the CMTE catalog provides an overall picture of the space-time evolution of the major seismicity and possible relationships with past eruptive activity ( e.g. , Azzaro and Barbano 1996; Azzaro et al. 2001). On the other hand, it was only in 1967 that the first seismograph station was installed at Etna. A complete seismograph network began to operate in the early 1980s with nine short-period analog stations, evolving in the last decade into a broadband digital seismograph network consisting of some 30 stations (Patane et al. 2004). For a long time, this network has operated without uniform coverage of the volcano, since it was largely aimed at monitoring eruptive activity in the middle-upper parts of Etna; such a situation has meant favoring the macroseismic approach in studying the severely damaging earthquakes that struck the populated slopes of the volcano. As a consequence, this typology of data has been used for defining the seismotectonic features (Azzaro 2004) and seismic hazard of this area (Azzaro et al. 2008). In the seismic catalogs the estimation of magnitude for earthquakes occurring in the pre-instrumental period is obtained through empirical relationships using the value …