The W Po Plain (Northern Italy) is commonly regarded as a region characterized by a low seismicity hazard, due to the lack of historical and instrumental record of strong earthquakes. Nevertheless, recent studies performed in the Monferrato hills provide evidence of active faulting and possible paleoseismicity occurred in Middle Pleistocene to Holocene times.Therefore, for verifying the seismic potential of this area, we firstly revised the available geological and geophysical information. We selected the area between Valenza and Alessandria (Eastern Monferrato Arc), which show the best structural, geomorphic, and stratigraphic setting for documenting recent tectonic deformation and faulting. Therein, we identified, for the first time, evidence for earthquake surface displacement in a Late Quaternary pedosedimentary sequence exposed at Pecetto di Valenza. The outcropping section has been logged and investigated according to an approach integrating sedimentological and micropedological data with structural analyses and radiocarbon dating. This allowed reconstructing the recent surface evolution and the paleoseismic history of the site.From the bottom, the pedosedimentary sequence consists of a Miocene marly bedrock, whose weathering started in Marine Isotopic Stage (MIS) 7 or 5 interglacials. A colluvial deposit follows, whose formation can be attributed to MIS 5e. The upper part of the sequence consists of two loess covers, showing different degrees of weathering, possibly occurred at ca. 30 ka BP for the deepest loess and in the Middle Holocene (ca. 4 ka BP) for the uppermost one. The loess accretion is therefore older and likely associated to the MIS 3 and MIS 2 glacial phases.Each deposit records the deformation induced by earthquake surface reverse faulting and warping, giving constraints to the sequence of events that characterized this site. In fact, the structural analyses and a 2D balanced retrodeformation of the section, integrating pedostratigraphic constraints, allowed identifying at least two different phases of deformation, and more than five fault scarp-forming events, which caused a total net displacement of ca. 4.8 m during the past ca. 40 ka. Our approach highlighted the interaction between the tectonically induced surface deformation and the aeolian deposition, allowing reconstructing the recent evolution of a small drainage basin.The results of this paleoseismic analysis reinforce the conclusion of previous Authors that the Monferrato Arc should be viewed as a seismic gap, characterized by strong earthquakes (Mmax similar to 6.5) with long recurrence interval (in the order of several thousands years). This evidence has relevant implications for seismic hazard assessment, which must be checked with further trench investigations along the mapped Quaternary thrust faults affecting the western part of the Po Plain. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.

45 late pleIstocene reverse surFace FaultIng at the pecetto dI valenza sIte (al northern Italy): prelImInary results F. Livio1, M.F. Ferrario1, C. Frigerio1, A.M. Michetti1, A. Zerboni2, L. Bonadeo1, G. Fioraso3, A. Irace3, F. Brunamonte1 1 Dipartimento di Scienza ed Alta Tecnologia, Università dell’Insubria, Como, Italy 2 Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Italy 3 Istituto di Geoscienze e Georisorse (IGG), CNR, Torino, Italy

pedostratigraphy and micromorphoLogy of soiL thin sections as a tooL in paLeoseismoLogy: deciphering past processes interacting with tectonics C. Frigerio1, A. Zerboni2, F. Livio1, M.F. Ferrario1, A.M. Michetti1, L. Bonadeo3, F. Brunamonte1 1 Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, Como, Italy 2 Dipartimento di Scienze della Terra A. Desio, Università degli Studi di Milano, Italy 0 INGV, Roma, Italy

We present some preliminary results on the mapping of coseismically-induced ground ruptures following the Aug. 24, 2016, Central Italy earthquake (Mw 6.0). The seismogenic source, as highlighted by InSAR and seismological data, ruptured across two adjacent structures: the Vettore and Laga faults. We collected field data on ground breaks along the whole deformed area and two different scenarios of on-fault coseismic displacement arise from these observations. To the north, along the Vettore fault, surface faulting can be mapped quite continuously along a well-defined fault strand while such features are almost absent to the south, along the Laga fault, where flysch-like marly units are present. A major lithological control, affects the surface expression of faulting, resulting in a complex deformation pattern.

The historical center of Como (Northern Italy) is prone to lake flooding and subsidence, due to the presence of unconsolidated silty sediments with poor mechanical properties. The sedimentary basin beneath the town contains over 180 m thickness of Late-Quaternary lacustrine, palustrine and alluvial deposits. The landscape evolution and the present-day environmental setting of the Como area have been reconstructed based on (i) more than 250 core logs and related geotechnical tests, (ii) detailed stratigraphic, sedimentological, paleobotanical and geotechnical analysis of several key boreholes, (iii) multi-year hydrogeological monitoring, (iv) estimation of subsidence rates and (v) integration of geomorphology, archeological findings and historical documents.Based on our environmental analysis, we derived an integrated geological and geomorphological model of the latest Pleistocene to Holocene local landscape evolution. This model was used to help design an engineering facility to Mitigate flood hazards in the Como urban area.In 2012, we carried out investigations during a re-evaluation of the design parameters for the flood mitigation project at the Como lake-shore. The new campaign included seven boreholes, many in situ and laboratory tests, and four C-14 dates. We found an organic silty unit, historical in age, with bad mechanical properties that was critical in the design of the flood mitigation project. We also obtained index properties for static and dynamic conditions, necessary for robust engineering planning. The results were used to update the project and better define future executive phases. Although the importance of acquiring independent experimental data is often overlooked, they can significantly improve the reliability of engineered systems, as demonstrated by the Como town case history. (C) 2015 Elsevier B.V. All rights reserved.

The city of Como lies on the shores of Lake Como and is built on a sedimentary basin made of at least 155 m of post-LGM (Last Glacial Maximum) lacustrine, palustrine and alluvial deposits. The area is threatened by different kinds of geological hazards, mainly related to lake flooding and subsidence; the city is also particularly vulnerable in case of possible strong ground shaking, because of the local high liquefaction potential and the likely occurrence of slope instabilities and amplification phenomena. We applied a multidisciplinary approach aimed at reconstructing the late Quaternary and Holocene evolution of the area, based on field surveys and the analyses of stratigraphic and geotechnical data, hydrogeological and subsidence monitoring. Our model has been tested during the design of the new defense system for the mitigation of the flood hazard. We anticipated that the worst geological problems for this facility would have occurred in the area where, according to our model, the Roman lake harbour was located. We realized at this site a specific campaign consisting in the drilling of 7 new boreholes, a number of 14C dating, and geotechnical and seismic surveys. A previously unrecognized organic silty unit, rich in archaeological remains and consistent with our hypothesis, has been found. This unit is the most critical for engineering planning due to its very bad mechanical properties. These results demonstrate that the model can be used as a predictive tool for hazard management and for a more efficient urban planning.

It is commonly believed that the Po Plain is an area of low seismic hazard. This conclusion is essentially a combination of two factors: (1) the historical record of earthquakes, which shows a relatively small number of events of moderate magnitude, and only two significant earthquakes, which occurred in the Middle Ages; and (2) the lack of ad-hoc research on the geology of earthquakes in this area, as although many studies have highlighted the local Quaternary tectonics, only a very few of them have discussed the observed evidence in terms of seismic hazard. In contrast, the data presented in the present study strongly suggest that the level of earthquake hazard in the Po Plain is comparable to that of the well-known seismic areas of the Apennine range, at least in terms of maximum magnitudes. Indeed, the high population density and the concentration of industrial facilities make the Po Plain today one of the more high-risk areas of the Italian territory. [...]

Earthquake hazard analyses in Italy are mainly based on historical seismic catalogs. However, the completeness of the Italian catalog, which spans more than 2000 years, progressively decreases going back in time. In the Fucino basin, historical research has not produced evidence of large earthquakes before the January 13, 1915, event (Ms = 7.0; 33,000 people killed). This event generated surface faulting along at least two parallel Quaternary normal faults bounding the eastern side of the basin (the Celano‐Gioia and Parasano‐Cerchio faults), over an end‐to‐end length of about 23 km, with a downthrow of 30–90 cm toward southwest. To improve geological aspects of the seismic history of this area, we trenched one of the coseismic scarps along the Celano‐Gioia fault. At a trench site, near the village of San Benedetto de' Marsi, the fault traverses the area formerly covered by the Fucino Lake, drained for farming in 1875 A.D. The evidence collected in the trenches suggests at least two large events in historical times before 1915. Radiocarbon dates constrain the age of event B between the 6th and the 9th centuries A.D., possibly coinciding with the event felt in Rome in 801 A.D. [Molin and Guidoboni, 1989]. Event A probably occurred after the 10th century A.D. and almost certainly before the 1349 A.D. earthquake, because historical studies show (1) that event A was not produced during the earthquake sequence that occurred in central Italy in 1349 A.D. [Molin and Guidoboni, 1989] and (2) that the post‐1349 seismic catalog can be regarded as complete for surface faulting earthquakes. Surface displacements determined for events A and B, compared to the 1915 surface faulting, indicate that M > 6.5 earthquakes were associated with each rupture. These observations suggest that the return period for large earthquakes in the Fucino basin is shorter than that indicated by the historical record alone. The Fucino example clearly shows that the exceptionally long historical record in Italy is still inadequate for a comprehensive seismic hazard characterization.