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fRoM thE SEISMIC MICRozonAtIon to thE REConStRUCtIon PRojECt:thE CASE of SAnt’AGoStIno MUnICIPALIty, ItALy A. Mantovani1, L. Martelli2, E. Melloni3, M. Romani2, R. Caputo1,4

1 Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Italy2 Servizio Geologico, Sismico e dei Suoli, Regione Emilia-Romagna, Bologna, Italy 3 Sportello Unico per l’Edilizia, Comune di Sant’Agostino (FE), Italy 4 Research and Teaching Centre for Earthquake Geology, Tyrnavos, Greece

Introduction. On May 2012, a damaging seismic sequence affected a wide area of the eastern sector of the Emilia-Romagna region, between the boundaries of the Modena, Ferrara and Bologna Provinces. The sequence has been characterized by two main shocks, with ML ranging from 5.9 and 5.8, respectively (Fig. 1). The first occurred on May 20 at 02:03 UTC, with epicenter between Finale Emilia and San Felice sul Panaro (44°51’50”N, 11°14’31”E, h = 6.3 km), while the second occurred on May 29 at 07:00 UTC, 15 km south-west of the first shock, near Mirandola (Massa et al., 2012). The causative faults of the two main shocks are segments of the Ferrara Arc thrust system, which is one of the three major arcs of blind, mainly north-vergent thrusts and folds representing the frontalmost portion of the Northern Apennines (Pieri and Groppi, 1981; Bigi et al., 1982; Boccaletti et al., 2004; Bonini et al., 2014). These events, and in particular the first one, were responsible for a series of secondary ground deformation effects (Fig. 2), like widespread liquefaction phenomena and soil fracturing along abandoned river channels, that have affected especially the western sector of the Ferrara Province, causing severe structural damage to non-reinforced masonry and precast industrial buildings (Caputo and Papathanassiou, 2012; Galli et al., 2012; GdL Liquefazione RER, 2012; Emergeo Woking Group, 2013; Papathanassiou et al., 2012). Following the seismic sequence of May 2012, with the Ordinance 70/2012 the President of the Regione Emilia-Romagna predisposed the realization of seismic microzonation studies and analysis of the emergency limit condition for the most damaged municipalities included in the isoseismal VI (MCS) of the macroseismic field (Galli et al., 2012). The purpose of the seismic microzonation studies is the characterization of a territory according to the local seismic response, i.e. the behaviour of the subsoil during a seismic event. In particular, these studies enable the identification and the delimitation of areas characterized by homogeneous behaviour distinguishing between i) stable zones, ii) areas susceptible of amplification of the ground motion and iii) areas subject to instability, such as landslides, liquefaction, and surface fractures. The distinction between areas characterized by different seismic hazard represents a necessary information for the local administrations in order to prevent and reduce the risk, as well as a fundamental support to the local urban planning so as to lead the choices to areas with a lower hazard and/or use of building constructions with a lower vulnerability to possible local effects. Among the municipalities concerned by the above mentioned Ordinance is that of Sant’Agostino, west Ferrara Province. Parallel to and complementary with the studies carried out by the Geological Survey of the Regione Emilia-Romagna limited to the urbanized areas, also the Earthquake Geology Team of the Ferrara University carried out aseismic microzonation study in the frame of a specific

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project financed by the local administration. Accordingly, all investigation activities have been carried out in a complementary and synergistic way in order to provide the local Administration a refined survey of the entire territory. The technical references for the realization of this study are described in the specific attachments of the Ordinance 70/2012 which derived mainly from the regional guidelines, “Indirizzi per gli studi di microzonazione sismica in Emilia-Romagna per la pianificazione territoriale e urbanistica” approved with the Deliberation no. 112/2007 of the Legislative Assembly of the Regione Emilia-Romagna (DAL 112/2007), and national ones, “Indirizzi e criteri per la microzonazione sismica” approved by the Civil Protection Department (Gruppo di lavoro MS, 2008) (Martelli and Romani, 2013).

Geological and geomorphological framework. The municipal area of Sant’Agostino is characterized by a largely flat morphology, with variable altitudes ranging from 8 m to 16 m a.s.l. The only relief is represented by the levees of an abandoned reach of the Reno River aligned in a NE-SW direction, and from that of the active Reno River delimiting the southern sector of the administrative boundary. The abandoned branch of the Reno River was active from the Middle Ages until the XVIII century (Cibin and Segadelli, 2009). In particular, following the disastrous flooding events of the 1451 and 1457 (Frizzi, 1848), the Reno River was artificially channeled running between Cento and Pieve and it soon started to rapidly prograding several kilometers in just few years. Therefore, the course was more or less artificially extended towards Sant’Agostino and Vigarano Mainarda, where it was left to freely flood the area southwest of Ferrara in order to infill this natural depression. This hydrographic setting persisted for about 300 years and geographical stability allowed the establishment of new settlements like San Carlo and Mirabello. Until the late XVIII century the Reno River was unable to reach the Adriatic Sea and also to directly flow into the Po River, so wide inland marshes and lakes were formed. The depositional evolution of the territory was marked by the ineffective construction

Fig. 1 – Emilia seismic sequence of the 2012. The red and orange stars are referred respectively to the main shocks of May 20 and 29. The black stars show the location of the major aftershocks (Mw ≥ 5.0), while the dots blacks show the events with Mw ≥ 1.5. The blue triangles show the observed liquefaction (Caputo and Papathanassiou, 2012; EMERGEO Working Group, 2012; WG Liquefaction RER, 2012]. Limits of the Sant’Agostino Municipality are indicated.

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of many embankment works and land reclamation, in order to impede further disastrous flooding events near Sant’Agostino (Franceschini, 1983), which led to the almost straight trend of the paleo Reno. Despite all the efforts, at the end of the XVIII century the river was definitely diverted southeastwards, in a preexisting channel of the Po River (Po Primaro) (Cremonini, 1988). The old course was thus gradually dried up, while still maintaining its morphological evidence, only locally obliterated by human activities. Therefore, the morphologically elevated areas corresponding to the levee bodies mostly accumulated during the Middle Ages (Caputo et al., 2012).

Since the beginning, these areas have shown obvious advantages in terms of urbanization and simultaneous development of the road network. Indeed, these sand bodies are characterized by excellent geotechnical properties (in static conditions) for the construction of buildings and are raised with respect to the surrounding flat countryside, thus avoiding flooding due to overflowing of the rivers or heavy rainfalls. On the other hand, as was evident in concomitance with the earthquake of May 20, saturated sand bodies are prone to liquefaction. Moreover, since the crests of the paleo levees are up to 5 m higher than the surrounding interfluvial areas, high topographic gradients exist and, in case of widespread liquefaction, this setting may play a key role in the lateral spreading phenomena.

Data collection and analysis. The first phase of this work consisted in the collection, georeferencing and archiving of all the geognostic and geophysical data that can provide

Fig. 2 – Examples of secondary effects due to liquefaction phenomena occurred in occasion of the May 20, 2012 earthquake. a) fractures with gravitational movements and ejection of sand; b) eruption of sand from a floor inside a garage built in correspondence of an old water well; c) broken pipes; d) differential settlement of foundations and consequent damage to the building.

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information (stratigraphic, lithological, geomechanical, hydrogeological) on the shallow subsoil, say up to some tens of meters depth. Starting from the preexisting information stored in the database managed by the Regione Emilia-Romagna, it was subsequently conducted a systematic research in the archives of several professional geologists which worked in the broader area. The most important added value of this effort was the reconstruction and the upgrade of the geotechnical and geophysical archive of the municipality, that had been almost completely destroyed due to the collapse of the Sant’Agostino town hall. Thanks to the new data collection, more than three hundred new survey sites and associated metadata have been added, thus enabling also the Regione Emilia-Romagna to upgrade its database. At present are available about 700 sites (including penetration tests, water wells and geophysical surveys), that is more than twice existing before this investigation.

The second phase of the work consisted in the digitalization of the available penetration tests, more than four hundred, in order to exploit their numerical content for microzonation purposes, i.e. for the analysis of the liquefaction hazard. Liquefaction occurs due to the shaking generatedby moderate to strong earthquakes (M > 5.5), especially in floodplain and coastal areas that areoften characterized by loose granular sediments and shallow aquifer. In the first 20 m depth, as aresult of seismic stresses, the resistance of a saturated granular sediment (sandy silts, sands andsandy gravels) reduces to a critical level for which the sediments liquefy. During this process,the pore pressure increases and the water rises through preexisting and new fractures or byman-made structures such as water wells, pipelines and foundations. Liquefaction phenomenais the result of the combined effect of the triggering factors (characteristics of the earthquake interms of magnitude and local acceleration) and the predisposing ones (subsoil conditions andgeomorphological framework): in the case of the Sant’Agostino territory, liquefaction has beenone of major causes of damage of buildings and facilities (ie. underground utilities) due to theconcomitance of these factors.

As a third step of the working flow, all available data have been analyzed in order to recognize and define the possible areas subject to liquefaction hazard. All the penetration tests have been preliminarly analyzed from a semi-quantitative point of view for detecting the presence of liquefiable horizons; in particular, the surveys have been classified on the basis of the thickness and depth of the liquefiable layer(s). Subsequently, we performed a quantitative assessment of the liquefaction hazard by calculating the Liquefaction Potential Index, LPI (Iwasaki et al., 1978). This index is a synthetic parameter that quantifies the possible effects of liquefaction at a given site, taking into account a) the severity of the seismic event, in terms of magnitude and acceleration b) the depth, the thickness and the safety factor that characterizes the liquefiable layers. In the scientific literature several methods have been described for the estimation of the liquefaction hazard based on penetration tests, but based on a comparative analysis of different methods (Facciorusso et al., 2013) that proposed by Idriss and Boulanger (2008) provides LPI values consistent with the observed effects. In order to calculate this index, we used the CLiq software (1.7.5.27). The input parameters for the calculation are a) the magnitude MWmax, deduced from the seismogenic zonation ZS9 (Meletti and Valensise, 2004), equal to 6.14 for the Municipality of Sant’Agostino, b) the acceleration agmax_s (PGA*FAPGA), computed by the Regione Emilia-Romagna following the regional guidelines (DAL 112/2007) c) the water’s depth, conventionally assumed equal to 1 m for interfluvial areas and 3 m in correspondence of the palaeo levees. The calculation has been performed for both electrical penetration tests and mechanical ones, despite the high degree of uncertainty in the measurement of the lateral friction characteristic of the latter type of investigation. This parameter is measured considering that there is a gap in the depth to which the tip resistance is measured compared to that of the lateral friction. Consequently, it is necessary to consider this altitude gap for both the evaluation of the friction that for the uncertainty degree due to the fact this value could be measured in different stratigraphic levels. Moreover, in the case of the Begemann tip, due to the junction at the bottom end of the sleeve, the friction is partly affected by a resistance provided by the material flowed

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back from the passage of the tip. For this reason, the lateral friction measured with the mechanical tip is always greater than that measured with the electric tip. This condition has negligible influence in the case of cohesive soils, while for sandy ones values up to 2 times higher than those measured with other tips or electric ones can be found (Lo Presti et al., 2009).

In order to better address the reconstruction following the May 2012 seismic sequence, and to favour a correct urban planning by the local administrations, microzonation studies are of utmost importance. Indeed the production of a series of maps able to identify the criticalities of the area in case of future seismic crisis represents a crucial step in this regard. On October 15, 2013, the Geological Survey of the Regione Emilia-Romagna had already provided a preliminary series of maps, through limited to the urbanized and potentially urbanized areas. Additionally, in the frame of a dedicated research project financed by the Municipality of Sant’Agostino, the Department of Physics and Earth Sciences of the University of Ferrara, has carried out an independent seismic microzonation study of the whole territory. However, it is noteworthy that the finally produced maps represent an agreed and shared product of interest for the local administration. In the maps are defined and highlighted the areas subject to possible instability phenomena, such as liquefaction and amplification of the seismic motion. The interest has been clearly manifested by the Municipality of Sant’Agostino by assuming the results of the microzonation study as a formal administrative document (“Quadro Conoscitivo”). Moreover, based on the produced maps, the Administration will further formally acknowledge the results of the study deliberating, in agreement with the designers responsible for the reconstruction project, new specific regulations for future planning and building within its territory.

Fig. 3 – Map of the amplification factors and the liquefaction hazard of the Sant’Agostino Municipality, realized in the frame of the seismic microzonation study. In the background, the pink and orange colors refer to the value of the amplification factor. The red and gray cross-hatch symbologies delimit respectively the areas characterized by the presence of liquefiable horizons in the first 10 to 20 m. The colored squares and dots refer to the value of the Liquefaction Potential Index calculated respectively by electrical and mechanical penetration tests; in particular, the white color means low liquefaction hazard, while red one an elevated hazard.

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As shown in Fig. 3, the villages of the Sant’Agostino Municipality are subjected to possible liquefaction phenomena and amplification of the seismic motion. Most of these urbanized area are characterized by the presence of liquefiable layers in the first 10 m below the ground surface and by Liquefaction Potential Index values greater than 2 and up to a maximum of 13, which therefore reflect a moderate to high hazard. In this situation, the new regulations will suggest, prior to any urban and building transformation, the analysis of the liquefaction susceptibility for better addressing the proper application of the Technical Standards for Constructions (NTC, 2008). For strategic and public interest constructions, mitigation interventions of the liquefaction hazard will be advised (for example, the consolidation of the soil foundation). This important application of the seismic microzonation study is the result of the close collaboration between the University of Ferrara Working Group, the Geological Survey of the Regione Emilia-Romagna and the Administration of the Sant’Agostino Municipality. Acknowledgements. We warmly thank the Administration of the Sant’Agostino Municipality for the farsightedness shown in deciding to adopt an important study such as the seismic microzonation prior to the Ordinance 70/2012. We are grateful to Ioannides J. (GeoLogismiki Geotechnical Software) for providing the Academic license of the CLiq software. We thank the Tecnicoop designers for their willingness and for their contribution in the drafting of the regulation for the seismic risk reduction. We are also thankful to the several professional geologists that have made available their archives.

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