Can We Consider the 1951 Caviaga (Northern Italy) Earthquakes as Noninduced Events

Online Material: Table of Mercalli–Cancani–Sieberg (MCS) intensities, onset times and associated phases, complete parameter sets for each 1951 location and computation of the isostatic stress variation due to unloading in the upper crust. On the night of 15–16 May 1951, two moderate earthquakes with estimated magnitudes of M w 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) On‐Line Bulletin (ISS, 1951; International Seismological Centre [ISC], 2011). 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 (∼5  km in Caloi et al. , 1956) indicated a possible anthropogenic source, related to wells for gas withdrawal (Fig. 1; see Data and Resources). Figure 1. (top) Maps of the area of interest, showing the tectonic setting and seismicity between 1981 and 2012 (Italian Seismological Instrumental and Parametric Data‐Base Working Group, see Data and Resources; Catalogue of Italian Seismicity, Castello et al. , 2006). Symbols in color indicate earthquakes (circles for M w>2.0 and squares for M w>4.5). Symbols are scaled with magnitude and colors accord to depth scale. Focal mechanisms are for seismicity with M w>4.5 from 1977 to present (Pondrelli et al. , 2006). Top left inset: The black box is the study area and the red dashed line marks the boundary of the Adria plate. The (red) tectonic structures and (light green lines) geological sections are extracted from Pieri and Groppi (1981) and Cassano et al. (1986). Top right map: Blue stars indicate the original locations of the events …