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Authors: Caciagli, M.* 
Camassi, R.* 
Danesi, S.* 
Pondrelli, S.* 
Salimbeni, S.* 
Title: Can We Consider the 1951 Caviaga (Northern Italy) Earthquakes as Noninduced Events?
Issue Date: 12-Aug-2015
Series/Report no.: 5/86(2015)
DOI: 10.1785/0220150001
Keywords: Induced Seismicity
Seismic analysis
Subject Classification04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics 
Abstract: On the night between May 15 and 16, 1951, two moderate earthquakes with estimated magnitudes of M 5.4 and 4.5 occurred in northern Italy, about 40 km southeast of Milan, close to the small town of Caviaga. They were recorded by several observatories worldwide, as reported by the International Seismological Summary (ISS) Bulletin. Despite the moderate magnitudes, these two events caught the attention of seismologists and have been studied in detail, in particular by Caloi et al. (1956), because they were close to Caviaga, in an area that was assumed to be aseismic. Moreover, their shallow hypocenters (ca. 5 km in Caloi et al., 1956) indicated a possible anthropogenic source, related to wells for gas withdrawal. In the absence of any further discussion or revision of the original study by Caloi et al. (1956), the Caviaga earthquakes have been included in several compilations of induced seismicity, and they have been generally accepted as cases of anthropogenic events. After 60 years it is possible to revisit this interpretation using improved computational techniques, the available high-resolution data, enriched historical catalogs, and a deeper understanding of the regional seismotectonic and crustal structure. The focus of this study is the relocation of these two events with the use of modern hypocentral location methods, and the analysis of the historical seismicity of the area. A complete seismic source parameterization is out of the scope of this preliminary study. In the following we describe the regional geological setting and the gas reservoir characterization, introduce the context of historical seismicity, provide a description of the main shock relocation, discuss the uncertainties of the hypocentral parameters and estimate the variation of the stress field in proximity to wells. We consider this revision necessary to be able to discuss the possibility that these two events were not induced by human activity, as well as to improve the quality of the dataset for decision makers involved in risk evaluation.
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