Distretti sismici del Veneto

In this paper we gather and review seismological database and seismotectonic literature for one of the north-eastern Italian regions to which OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale) provides seismic monitoring and alarm system, the Veneto region. We subdivide the region into 9 seismic districts (Fig. 1), according to some common criteria about seismogenic processes, or data availability. This zonation has a basic operational purpose: it helps the personnel of the Centro Ricerche Sismologiche (CRS) department involved in the rapid availability service, in having an updated and technical overview of the earthquake space-time distribution, and a more detailed description about the main events of the past and about the available knowledge of potential seismogenic sources. This state- of-the-art is a quick manual set up for the local authorities too, as the Regional Civil Defence system is charged to intervene in case of potentially damaging earthquakes. In chapter 2 we describe the dataset available and the ones we used: the data are described separately by historical database (par. 2.1), instrumental archives (par. 2.2), seismogenic sources (par. 2.3), and seismic hazard and regulation (par. 2.4), taking into account the most updated and reliable sources available at the time of the analysis (mid 2011); a synthetic geological and structural framework of eastern Southern Alps and northern Apennines is given as well in par. 2.5, based on literature data only. In chapter 3 some methodological considerations about data quality are given, in order to compile a reference dataset and to represent/analyze the seismicity. Heterogeneous data completeness and magnitude estimates are the main problems pertaining the national and regional instrumental dataset; as no authoritative region is set up for different data providers in north-eastern Italy (from 1977 OGS, but then Istituto Nazionale di Geofisica e Vulcanologia - INGV and Provincia Autonoma di Trento - PAT, with their independent stations and/or data processing), a composition of different datasets is suggested in Table 3.1 for two districts (Giudicarie and Pianura Veneto Ovest) that fall in marginal position with respect to the bulk of OGS monitoring system. Nevertheless, some mislocations, omissions or questionable differences in location parameters have been detected; they have been tracked in the description of individual districts. Chapter 4 describes separately each district, following a common scheme (structural and geological context, historical and instrumental seismicity, neotectonic and active sources, damaging effects and seismic regulation). The Veneto region confirms to be a mid-to-high seismic region, like documented by the historical records. Instrumental data are available since 1977, but their completeness is strongly heterogeneous in time and space; the seismicity pictures of the last decades should take this fact into account. Similarly, many Veneto municipalities exhibit a seismic protection deficit, as they entered in seismic regulation in 2003 only. Earthquakes above the perception threshold occur along a NE-SW widestripe of mountain-to-plain transition in the Southern Alps, and along the last trait of the Po river. In the Venetian Prealps there are several geomorphological signatures of active deformation, but rates and geometries of potential sources are in many cases still doubtful, or controversial; similarly, the comprehension of mechanisms and activity rates of seismogenic sources buried above thick alluvial deposits, in the lower plain, is tricky. Even if the seismometric monitoring of the Veneto region performed by OGS since 1977 has not been constant in time and coverage, the regional OGS database of instrumental seismicity is by far the most homogeneous and reliable dataset available, for all but the two districts on the westward border of the Lombardia region, and Emilia Romagna region southwards; in these areas, a manual revision of earthquake location is needed by integrating original phases of different seismometric networks, and by common data processing in earthquake location and magnitude assignment. The new seismometric stations installed in the Veneto region during the last 5 years, in the frame of the agreements between Protezione Civile Regionale and OGS, and the uniform processing of data belonging to other networks (the PAT network, and some INGV stations) have increased the detection capability and location quality of small earthquakes. Lastly, some districts (namely Lessini-Schio, Pedemontana Sud and Pianura Veneto Ovest) have been affected in the second half of 2011, by some of the most important earthquakes (widely felt, till to sporadic damages) ever experienced during the modern instrumental period: preliminary comments on this activity have been added to the initial manuscript during its revision, but additional analyses are still ongoing and they will probably increase the level of knowledge about seismogenesis in these areas. The seismological information here gathered and reviewed has been set up into electronic archives in the institutional OGS-CRS web site devoted to real time seismology (http://rts.crs.inogs.it), for dynamic queries under development.