Geological and Geophysical Investigations of “Site Effects” Due to Liquefaction in Mirabello Following the May 20Th, 2012 Emilia Earthquake

FoCus

THE PIANURA PADANA EMILIANA EARTHQUAKE

The industrial warehouse area of Mirabello – located 12 km west-southwest of Ferrara, in northern Italy – suffered dissimilar intensities of damage during the earthquake sequence started on May 20th, 2012. The observed site effects were mainly concentrated along the course of the paleo- river bed of the Rhine River, where other villages are situated: Sant’Agostino, San Carlo, Mirabello and Vigarano Mainarda. The extent of the damage has highlighted the need to improve seismic resistance countermeasures of residential and industrial buildings as well as to identify appropriate measures to address the risk of liquefaction and surface rupturing. Our study tries to consider the effect that geomorphology had on the observed damage at Mirabello. The analysis is based on both newly collected geological and geophysical data and the revision of historical data related to the geomorphological evolution of the area. The main outcome of this work regards the importance of detailed mapping of the local structure of the paleo-river bed and its spatial variation. The opportunity to use the non-invasive and indirect methods, such as the Electrical Resistivity Tomography and Induced Polarization (ERT/IP), in mapping subsurface fractures and, possibly, the spatial continuity of the liqueﬁ ed sand layer(s), has proved its efﬁ ciency. Such methods could be promoted to become an integral part to support other subsurface methods routinely employed for the reconstruction of the geological and hydrogeological models. Finally, the possibility to repeat the geoelectric measurements at low costs may also promote it as a tool for short-/long-term consolidation monitoring Geological and geophysical investigations of “site effects” due to liquefaction in Mirabello following the May 20th, 2012 Emilia earthquake

■ Nasser Abu Zeid, Elena Marrocchino, Carmela Vaccaro, Daniel Nieto, Marilena Martinucci

■ Nasser Abu Zeid, Elena Marrocchino, Carmela Vaccaro n May 20th, 2012 at 04:03:53 an earthquake (ML: University of Ferrara O5.9, hypocenter depth: 6,3 km, INGV) hit the ■ Daniel Nieto area near to Finale Emilia. The co-seismic effects as- National Institute of Oceanography and Experimental sociated with this event were observed in the nearby Geophysics – OGS, Trieste towns located within 20/25 km from the epicenter ■ Marilena Martinucci and belonging to four provinces (i.e., Ferrara, Mod- Edilgeo ena, Mantova and Rovigo).

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FIGURE 1 Sketch summarizing the details of Reno River deviation, started in 1604, as was proposed by Giovanni Domenico Cassini. The proﬁ le extends from Bologna (left) to Ferrara (right) Source: Museum of Rural Life of Mirabello, Ferrara3

The observed heterogeneous near-field damage dis- evidences that during large flood events, water had tribution indicated that the geologic and geomor- submerged churches and towers whilst, in low level pe- phologic characteristics of these territories had an riods, the water level was much higher than in Sanmar- important role in causing, and sometimes augment- tina Valley, located further to the northeast of Mirabello. ing, the observed site effects1. The most important ef- This confirms the artificial and hanging river typology fects are related to the occurrence of liquefaction and of the Reno palaeo-river channels (Fig. 1). subsequent formation of tension cracks. These effects were observed, mainly, along the levees of the paleo- river bed of the Reno River, where the towns of San Geology and geomorphology Carlo, Sant’Agostino and Mirabello are situated. The site effects, at Mirabello, had heavily encroached the The subsurface geology of the Reno River, in the factories where fractures have crossed them, leading study area, can be better understood through a in many cases to their demolition and consequent re- brief reconstruction of the historical evolution of the construction for safety reasons2. Padana Alluvial Plain. Briefly, we can say that the gen- This work concerns the presentation and discussion eral structure of the paleo-river bed is the result of of the peculiar geological and geomorphological many anthropogenic interventions following, most characteristics of this area which, according to the probably, major climatic changes and seismic events observed damage distribution, could aid in clarifying that likely led to deviating and rectifying the Reno’s the reason(s) of these co-seismic site effects. Moreo- natural course. ver, modern geophysical techniques (ERT/IP) were Documented information related to the earliest works employed in this work to retrieve rapid subsurface can be traced back to the Roman Age (i.e., second information about the depth extent of these fractures. century A.D.)4,5,6,7,. In that period, the Sant’Agostino To start with, we show an historic map depicting the territories were subjected to intense rain periods. hydrometric profile in the year 1760, where it reports This indirectly supports the acceleration of works a summary of the hydrologic regime of the Paleo-Re- concluded in that period in order to mitigate the no River with respect to ground elevation of the sur- flood risk 8,9. A summary of the main climatic peri- rounding territories of Bologna and Ferrara. The profile ods is reported in Table 1.

EAI Energia, Ambiente e Innovazione 4-5/2012 - Parte II 40 TABLE 1 occurred on24 in Emilia events alsoresulted ofmany inthedestruction cities City on29 include thetwo large events thatdestroyed Verona Emilia Plain, theeleventh during afterChrist, century earthquakes thatoccurred mostimportant The inthe quakes thatmay have produced damage toitsbanks. climatic changesofpastearth- inadditiontoaseries movements probably ofitsbedandfloods causedby RenoRiver byThe hasbeenaffected numerous Although theRenoRiver atthis was notconcerned ries. resultedThis ofvast marshes. intheformation the subsidencephenomenon intheEmiliaterrito- iment transport from theApennine, which augmented oftheleveesand thefortification didprevent thesed- the flow andensure navigability. waterThe drainage Quaderna, Gaiana andSavena inorder toincrease the following rivers: Santerno, Lamone, Senio, Sillaro, several projects ofdamsalong theconstruction for ing thisdraught period, thecityofBolognafinanced which intheyear 1152was nolonger navigable. Dur- Mirabello, duetothe reduced flow oftheRenoRiver, ofthemunicipality oftheterritory the structuring in climaticconditionshasbeenvery important arid reconstruct thepaleo-rivers routes documentationmakes tolack ofhistorical itdifficult

10 Climate evolutionoftheSant’Agostinovillageandnearbyterritories . InFerrara we remember theearthquakes th March, 1001 and 1 th June, 1119. lowThe urbanizationand st March, 1117. These 11,12 . of period This ing between CentoandPieve villages. abandoned thePanaro changingitscourseandpass- astrous event occurred in1459. Since1497, theReno Reno River, thearea that affected where themostdis- ofthe the stabilityofbanksandassociatedfloods have thehypothesis port thatsimilareffects affected regime. Furthermore, theevents sup- ofliquefaction the impactthatanearthquake hasonthehydrological to routes of6 alteration significant oftheRenoRiverThe was related near Bondeno(Ferrara). toFinaleEmiliaendinthePanarodrifted River the change ofthecourseRenoRiver, thatwas hydraulicfurther problems thatendedin1240with curred in1165and1222, created theconditions for following thetworitory earthquakes, historical oc- hydraulic manifested The vulnerability of theter- oftheoptimummedieval. period thelongdry during probably duetothereduction ofsedimenttransport bybeen affected thephenomena ofsubsidence, stage ofthereorganization ofitsbed, itsbasinhas the recent earthquake of2012 River. share significant The observed variations with hydraulic systemandtheoverflowing oftheReno changesSuch morphological didproduce afragile ley oftheReno, resulting indamagingtheriver banks. VIII grade intensity(Mercalli) themiddleval- struck 8,9 EAI th February, 1455, when anearthquake of

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4-5/2012 -ParteII 41 Focus In the northern part of Sant’Agostino Municipality, the in 1598, by Gian Battista Aleotti of Argenta to Pope river then free of protective banks had distributed Clement VIII asking for support for the diversion of sediments in the swamp waters until it reached the the Reno River. The diversion of the Reno River to- Po River after crossing the marsh areas. In 1522, the wards Sanmartina Valley took place in 1604. extreme climate that evolved towards the cold peri- In this period numerous solutions were adopted to ods of the little ice age, in addition to the absence of reduce the sediment transport in the Reno River canalization of the river-bed, had favored a disastrous and transfer it to overcome subsidence in marshes. flood of the Reno, with major effects on Mirabello’s A particular intervention aimed at reducing sedi- settlements and agricultural activity. These conditions ment transport during floods is evidenced by the led the government of Este to accept the intervention morphological feature of the central-northern part of canalization of the river, that resulted in the struc- of the Mirabello Municipal area, where the paleo- turing of the river-bed. The same river-bed was af- river bed becomes wide, and in the urban or- fected by liquefaction after the recent earthquake of ganization traces of panels crossing the river are 20th May, 2012. evident, whose function was to facilitate the sedi- During the late medieval cold climate, the high rain- mentation of soil transported by the flood waters fall and sediment transport created the conditions for (Fig. 2). structuring the paleo-river bed of the Reno, which A representative example of these hydraulic works is was channeled with an almost straight bank from the Reno Chiavica (II in Fig. 3), constructed in 1723 Sant’Agostino to Porotto (8 km NE of Mirabello). by Cardinal Aldrovandi, that served to mitigate the The unfavorable weather conditions of heavy rain burial risk of the Po river once converged to the Reno. and high levels of sediment transport made it hard This solution proved unfortunate as it favored the for the Reno water to enter the Po, causing its rapid occurrence of a number of disastrous routes (e.g., silting. This fact is well documented by a letter sent, Bisacca, III in Fig. 3), after which it was necessary

FIGURE 2 Reconstruction of the principal anthropogenic structures along the Reno River based on the consultation of the available historical maps Source: Museum of Rural Life of Mirabello, Ferrara

EAI Energia, Ambiente e Innovazione 4-5/2012 - Parte II 42 between thedifferent classesare mainly related to five homogenous classes. observed differencesThe has resulted into inthesubdivision oftheterritory rabello. Inthisvillage, theanalysis oftheseeffects crozonation areas ofalltheaffected includingMi- urge asystematicmi- co-seismicsiteeffects The for at Mirabello Detailed reconstruction ofthe Renopaleo-river co ofthe and usedintheconstruction “ left embankmentoftheRenoRiver were excavated tion, thatoncewere mostofthematerials ofthe part known “ floods, ofthepresently thatleadtotheconstruction interventions tookplacefollowing the1949and1951 of Mirabello) withatightangle. Consequentmajor tion oftheRenowas diverted towards Argenta (SW to thesouthofSant’Agostino, por- thesouthwestern present structure oftheabundant river-bed. Further to divert ofthe itscourseleadingtotheformation site e (shown inFigure 2), 1-4:areas withsimilar pogenic structures alongtheRenoRiver graphic features. I-VI:principalanthro- di tory. Thedi Micro seismiczoningoftheMirabelloterri- FIGURE3 ff erent geomorphologicalandlithostrati- ” embankments. ff ects Cavo Napoleonico ff erent colorsindicateareas with ”. Inrelation toitsconstruc- Cavo Napoleoni- the surface. although very modestquantitiesofsandhave reached is believed thatthislayer has undergone liquefaction encountered withitsbaselocatedat10.5-11mb.g.l. It nearby boreholes, where asandy siltlayer hasbeen in accord lithology withthesubsurface obtainedfrom sandsediments.of siltandfine interpretationThis is 40 Ohm.m)are tobeassociatedwiththeenrichment heterogeneities (withresistivity values between 20and sociated withclay andsiltsediments, while thepresent heterogeneities are present laterally. canbeas- These the latter(a2)shows low resistivity values althoughfew ized by resistivity values greater than25Ohm.m, while levels denotedasa1anda2. ischaracter- former The resistivityThe model(Fig. two 4a)identified resistivity terraced clay. tured inawaterproofing systemmainly composedof only embankment. theright embankmentisstruc- The sector:1a) Southern Inthisarea we have identified detail asfollows (Fig. 3): pogenic activities. classesareThese in summarized lithologies andpastanthro-both different subsurface EAI

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FIGURE 4 2D inverted resistivity (upper) and chargeability (lower) models. a1: resistive horizon and a2: conductive horizon. A: resistivity anomaly due to lithologic variation (silt and ﬁ ne sand). Dashed white line: possible paleo-soil surface representing the old right embankment of the Reno River. The two shaded areas indicate the extension of the industrial warehouse with respect to the proﬁ le. The left one is related to the extreme western portion of the building that was severely damaged and subsequently demolished. ?: further analyses are needed to reveal the nature of these IP anomalies

As regards the first level, the resistivity model sug- towards NE. Moreover, the resistivity anomaly located gests the presence of lateral heterogeneities at the at chainage 48 was associated with the expelling of following chainages: 12 m, 35-65 m, and around 96 m. modest quantity of fine sand that reached the surface. These are believed to be associated with subsurface It is interesting to note the pathway followed by the fractures whose traces were visible on the surface at sand following the liquefaction of the sand layer. the moment of data acquisition. The corresponding chargeability model (Fig. 4b) evi- One of these main fractures (chainage: 35 meters) has dences the presence of chargeability anomalies, indi- caused major damage to the industrial nearby build- cated by rectangles, which are caused by variations ing located some 25 meters off the resistivity profile in the sediment texture (i.e., presence of silt and fine

EAI Energia, Ambiente e Innovazione 4-5/2012 - Parte II 44 liquefaction; differences causedbyto theobserved siteeffects episodes. differencesThese couldhave contributed ence ofsandsedimentsaccumulated afterflooding ally towards theplainmorphology duetothepres- Conversely, embankmentevolves theright gradu- logical jumptowards plain. thealluvialflood the paleo-leftembankmentthere isastrong morpho- favored thepreservation morphology. ofthefluvial In river was abandoned, theexpansion ofthe urbanarea area on the border oftheRenoRiver, andwhen the Duchy; theurbanizationofMirabello occurred inthis ofBolognaandFerrarathe territories theEste during of “ limit ofthissectorlays ontheancientpaleo-river bed two embankmentsare stillpreserved. northern The 2) Town sectorofMirabello: Inthisarea, ofthe portions in thenearby marshes; itwas thefloods usedtodisperse sediments during functioned alsoasawaterperiods, millindry while ments inthenearby plains. hydraulicThis structure Reno River ofsedi- thedistribution embankmentfor drainagetion andopeningofamasonry canalinthe operations thelockconstruc- filling during artificial sand sedimentsare superficial These related tothe swamps. nated withsandonly levels inthesuperficial ofthe sector ismainly composedofclay sedimentsalter- oftheancientRenoRiver: part 1b) eastern The This the splay oftheRenoRiver occurred in1526; embankment,right which was following constructed area beneaththegeoelectricprofile belongstothe and available lithology subsurface indicatesthatthe Finally, theintegration ofbothgeoelectricmodels 75 and90m. ments canbeseenat7 tures indicatingpossible upward movements ofsedi- nearly vertical fractures canbeinferred. Similarfea- between chainages: 45–64m, where thetrace ofthe with theseismicevent. islocated mostsignificant The presence of “dykes” thatmay have beenassociated to thefractures, where someofthemindicatethe sand). locationoftheseanomaliesisvery nearThe Spron Mavezzi ”, thatmarked thelimitsbetween m depthbetween chainages lic structures belongingtosector3. supply ofsedimentsthatwere blocked by thehydrau- width andheight, mostprobably duetothereduced bello. Inthissector, thepaleo-river bedisofminor oftheurbanized4) area portion north The ofMira- paleo-river bedwhere events liquefaction are rarer; to thenorth, there isanarrowing inthewidthof that beingsaturated underwent liquefaction. Further hasledtotheaccumulationofsandsediments This which theprotection served for oftheembankments. byacterized agreater widthofthepaleo-river bed represents thearea ofrecent urbanization. Itischar- 3) area central-northern The ofMirabello: sector This ments characterised by lateral lithologicvariations. Fig. 3), evidenced thepresence of 10metersofsedi- warehouse oftheindustrial acterization area (ERTin nique, employed lithologicchar- inthesubsurface useoftheindirectThe andnon-invasive ERT/IPtech- sediments. in order toidentifythetexture ofthese distribution sive altitudeincrease. plainshouldbestudied This past anthropogenic activities hadresulted inprogres- ment from theeastern, mostdepressed lands, where south. roadThis separates thepaleo-river embank- street connecting Mirabello toSant’Agostino towards its mainflexure was by identified the “ while theleftonehasbeendestroyed. Towards east, shows thepresence ofonlyembankment theright warehouseing totheindustrial area ofMirabello, paleo-riverThe bedintheseismiczone 1a, belong- archives andgeomorphological maps. reconstructed thankstotheavailability ofhistorical river oftheRenotreated portions inthisstudy were fused marshesinthenearby territories. paleo- The on theotherithelpedinreclamation ofthedif- the repeated occurrence ofdisastrous floods, while ever, thesuspendedbedhasfavored, ontheoneside, embankments against flooding.tion ofartificial How- the paleoRenoRiver courseurged theconstruc- for depressed nearbyThe morphology oftheterritories Conclusion EAI

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4-5/2012 -ParteII Corso Italia Corso ” 45 Focus These variations are related to the transition from the presence of silty sand sediments that may underwent river bed sands to loamy soils of the right embank- liquefaction. ment. These overlay clay sediments rich in peat, that This interpretation is supported by the observed sur- extend and interdigitate with the nearby sediments face ruptures at chainage 92-94 m (Fig. 4a,b). East of of the lowland areas. Within these, clay intercalating these ruptures, the ERT/IP models suggested that liq- sand sediments are present. The combined use of re- uefaction has also interested the former swamp lands. sistivity and induced polarization methods allowed, In this case, liquefaction is due to the accumulation of also, for tracing the surface ruptures down to a depth sand deposited after the repeated floods. of 10 to 12 meters. This constitutes an important element for the vulner- However, the IP section (Fig. 4b) provided indications ability of historic buildings and should be considered about the probable presence of deep zones (chain- in any study dealing with the microzonation of these age: 60m, depth: 20 m), which may be related to the territories. ●

[1] L. Arcoraci, M. Berardi, F. Bernardini, B. Brizuela, C.H. Caracciolo, C. Castellano, V. Castelli, A. Cavaliere, S. Del Mese, E. Ercolani, L. Graziani, A. Maramai, A. Massucci, A. Rossi, M. Sbarra, A. Tertulliani, M. Vecchi, S. Vecchi(2012), Rapporto macrosismico sui terremoti del 20 (ml 5.9) e del 29 maggio 2012 (ml 5.8 e 5.3) nella pianura Padano-Emiliana, INGV-Roma and INGV-Bologna.

references [2] http://ambiente.regione.emilia-romagna.it/geologia/temi/sismica/liquefazione-gruppo-di-lavoro/rapporto_sancarlo.pdf. Primo rapporto sugli effetti della liquefazione osservati a S. Carlo, frazione di S. Agostino (Provincia di Ferrara) A cura del gruppo di lavoro per la valutazione degli effetti di liquefazione a seguito dei terremoti del 20 e 29 maggio 2012 (Regione Emilia-Romagna, PG.2012.0134978 del 31/5/2012). [3] http://ilmuseodimirabello.wordpress.com/le-mostre/ [4] F. Berti (1995), “Uno sguardo sul passato. Archeologia nel Ferrarese”, Firenze. [5] M. Calzolari (1988), “L’età romana nel territorio di Bondeno: ricerche topografi co – archeologiche”, in Bondeno e il suo territorio dalle origini al Rinascimento, Casalecchio di Reno (Bo), pp. 169-182. [6] A. Marchesini, S. Nepoti(1980), “L’evoluzione della pianura emiliana durante l’eta` del Bronzo, l’età romana e l’alto medioevo: geomorfologia ed insediamenti. Padusa 16 (1), 53–158. [7] M. Bondesan, R. Ferri, M. Stefani (1995), “Rapporti fra lo sviluppo urbano di Ferrara e l’evoluzione idrografi ca, sedimentaria e geomorfologica del territorio”. In: Visser Travagli, A.M. (Ed.), Ferrara nel Medioevo-Topografi a storica e archeologia urbana. Grafi s, Bologna, pp. 27– 42. [8[ H.F. Lamb, F. Gasse, A. Bekaddour, N. El Hamouti, S. van der Kaars, W.T. Perkins, N.J. Pearce and C.N. Roberts (1995) “Relation between century-scale Holocene arid intervals in temperate and tropical zones”. Nature, 373, 134-137. [9] A. Veggiani (1983),“Degrado ambientale e dissesti idrogeologici indotti dal deterioramento climatico nell’Alto medioevo in Italia: I casi riminesi. Studi Roma- gnoli, 34, 123-146. [10] http://meteoterremoti.altervista.org/blog/ terremoti/ terremoti-storici-in-emilia-romagna/) studio micro rilievo. [11] C. Ravazzi, (1989), “Analisi polliniche del riempimento del fossato. Dati paleoecologici sulle variazioni ambientali”. In: Bernabo`Brea, M., Cremaschi, M. (Eds.), La terramara di Poviglio. Coopsette, Reggio Emilia, pp. 31-36. [12] M. Cremaschi, M. Marchetti, M. (1995), “Changes in fl uvial dynamics in the Central Po Plain (Italy) between Lateglacial and Early Holocene”. In: Frenzel, B. (Ed.), European River Activity and Climatic Change During the Late glacial and Early Holocene. Paleo-climate Research/Paläoklimaforschung, vol. 14, pp. 173-190.

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