geosciences
Article Assessment of Seismic Bedrock in Deep Alluvial Plains. Case Studies from the Emilia-Romagna Plain
Luca Martelli
Geological, Seismic and Soil Survey, Emilia-Romagna Region, 40127 Bologna, Italy; [email protected]
Abstract: The estimation of seismic shaking is essential for a realistic assessment of the local seismic hazard and the implementation of effective strategies for prevention and mitigation of the seismic risk. One of the most important aspects in the analysis of the site seismic assessment is the recognition of the seismic bedrock and its depth. Unfortunately, these data are not always easy to evaluate, especially in areas where the thickness of loose or poorly consolidated sediments is high. This article illustrates data and case studies from the Emilia-Romagna sector of the Po Plain, in order to provide examples and suggestions for the recognition of the seismic bedrock in alluvial and coastal areas characterised by significant thicknesses of unconsolidated sediments, using available data and not expensive geophysical surveys. The application of the proposed method indicates that the study area can be divided into four domains characterized by different depths of the seismic bedrock: the marginal or pede-Apennine belt, the high structural zones, the syncline/minor anticline zones, and the Po delta-coast zone. Citation: Martelli, L. Assessment of Seismic Bedrock in Deep Alluvial Keywords: seismic bedrock; geological substratum; ambient vibrations; Vs propagation; Po Plain Plains. Case Studies from the stratigraphy Emilia-Romagna Plain. Geosciences 2021, 11, 297. https://doi.org/ 10.3390/geosciences11070297 1. Introduction Academic Editors: Jesus Martinez-Frias, Hans-Balder It is known that loose and poorly consolidated sediments, such as recent alluvial Havenith, Giuseppe Cavuoto, Stefano deposits or slope debris, can change the amplitude, frequency, and duration of the seismic Catalano, Vincenzo Di Fiore and motion (e.g., [1,2]). Roberto De Franco Therefore, it is very important for the evaluation of the expected seismic motion to know the thickness of loose sediments, poorly consolidated deposits and weathered or Received: 12 April 2021 fractured rocks, i.e., to identify the depth below which the lithotypes have rigid behaviour. Accepted: 29 June 2021 The recognition of the rigid substratum, or seismic bedrock, can be complex in the Published: 18 July 2021 case of non-rigid sediments with a thickness greater than 50 m, considering that the economic resources usually available only allow the use of current geophysical techniques Publisher’s Note: MDPI stays neutral (down-hole, MASW, ReMi), which rarely allow investigation beyond this depth. with regard to jurisdictional claims in This problem arises both in seismic microzonation studies for urban planning and published maps and institutional affil- in the estimates of expected seismic motion for design, and therefore has significant iations. implications in the prevention and reduction of seismic risk. Mascandola et al. (2019) [3] already discussed this topic and proposed a map of the seismic bedrock in the Po Plain, based on environmental noise measurements, from pre- existing and new data. The article is undoubtedly a fundamental reference on the subject, Copyright: © 2021 by the author. but the scale of the map does not allow its direct application, neither for urban planning, Licensee MDPI, Basel, Switzerland. which operates at a municipal or inhabited centre scale, nor at the site scale for the design. This article is an open access article Furthermore, some in-depth investigations carried out for seismic microzonation studies, distributed under the terms and have shown that the surface mapped by Mascandola et al. (2019) [3] does not always conditions of the Creative Commons correspond to the roof of the engineering seismic bedrock, i.e., the surface below which the Attribution (CC BY) license (https:// shear wave velocity (Vs) reaches and exceeds 800 m/s. Finally, the study by Mascandola creativecommons.org/licenses/by/ et al. (2019) [3] has not been extended to the southern plain, near the Apennines hills. 4.0/).
Geosciences 2021, 11, 297. https://doi.org/10.3390/geosciences11070297 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 297 2 of 22
The aim of this article is to provide useful information for the assessment of the seismic bedrock depth in wide and deep alluvial basins, such as the Po Plain, using data available in the literature and/or acquired through relatively inexpensive surveys. For this purpose, case studies in significant sites of the Emilia-Romagna plain will be illustrated. The proposed procedure is based on the comparison between Vs profiles, stratigraphic data and values of the resonance frequency of the ground (Fn) acquired with recordings of environmental vibrations [4]. It is commonly accepted in the literature (e.g., [5,6]) that Fn depends on the thickness of the unconsolidated sediments (H) and the average speed of the shear waves in the thickness range H (VS ) according to the relationship:
V F = S (1) n 4H It is therefore clear how, by having Fn values, Vs profiles and information on the subsoil lithostratigraphy, it is possible to estimate with a certain reliability the thickness H, i.e., the depth of the main resonant surface comparable to the roof of the bedrock. Seismic microzonation studies carried out in the Emilia-Romagna plain (https:// geo.regione.emilia-romagna.it/schede/pnsrs/, accessed on 1 July 2021) indicate that the interest range of the ground frequency Fn is between 0.2 Hz and 20 Hz and that, in general terms, thickness ranges of unconsolidated sediments can be associated with Fn values (Table1). The overlaps of the thickness ranges in Table1 depend on the Vs variability of the unconsolidated sediments.
Table 1. Comparison between ground frequency and unconsolidated sediments thickness in the Emilia-Romagna plain (modified from [7]).
Ground Frequency Unconsolidated Sediments Thickness Fn ≤ 0.6 Hz H ≥ 130 m 0.6 < Fn ≤ 1.2 Hz 170 ≤ H < 50 m 1.2 < Fn ≤ 2 Hz 80 ≤ H < 20 m 2 < Fn ≤ 5 Hz 30 ≤ H < 8 m 5 The use of ambient vibrations for the seismic-stratigraphic characterization of the subsoil is not new (e.g., [5]), but even today, the application of this procedure is not always optimal as the interpretation is often not supported by the comparison with stratigraphic data, even though these are readily available. The stratigraphic data are of great importance especially in the case of tectonically active basins, such as the Po Plain. In this kind of basins, the sedimentation is affected not only by glacio-eustatic variations, but also by the tectonic phases with the result that the stratigraphic succession is characterized by sedimentary cycles of various hierarchical order to which correspond stratigraphic units bounded by discontinuity surfaces. The level of deformation decreases from the oldest to the most recent stratigraphic units. This tectono-stratigraphic evolution confers a different degree of compaction and stiffness of the sediments, which decreases towards the top, and the surfaces of stratigraphic discontinuity that separate the various units are also surfaces of discontinuity of stiffness. Definitions of Cover Sediments, Geological Substratum, Seismic Bedrock Geophysical investigations, conducted both in mountain areas and in the plains, show that the lithotypes characterised by rigid behaviour do not always coincide with the geological substratum of the cover sediments. To understand this difference, it is important to define “cover sediments”, “geological substratum”, and “seismic bedrock”. Geosciences 2021, 11, x FOR PEER REVIEW 3 of 22 Geosciences 2021, 11, 297To understand this difference, it is important to define “cover sediments”, “geologi- 3 of 22 cal substratum”, and “seismic bedrock”. Cover sediments: stratigraphic succession consisting of loose and/or poorly consoli- dated sediments, suchCover as slope sediments: debris, stratigraphicrecent alluvial succession deposits consisting(e.g., younger of loose than and/or 1 Ma), poorly con- and/or weathered/fracturedsolidated sediments, rocks. Generally, such as slope these debris, sediments recent are alluvial characterised deposits by (e.g., Vs younger< than 400 m/s and assume1 Ma), importance and/or weathered/fractured for the site seismic rocks. assessment Generally, if the these thickness sediments is greater are characterised than 3 m. by Vs < 400 m/s and assume importance for the site seismic assessment if the thickness is Geological greatersubstratum: than 3stratigraphic m. succession generally made up of diagenised or consolidated rocks, atGeological the base of substratum: the cover se stratigraphicdiments. In the succession Po Plain, generally the geological made upsub- of diagenised stratum of the alluvialor consolidated sediments rocks, is made at the up base of consolidated of the cover sediments.marine sediments. In the Po Plain, the geological Seismic bedrock:substratum stratigraphi of the alluvialc succession sediments characterised is made up by of consolidatedrigid behaviour, marine that sediments. is Seismic bedrock: stratigraphic succession characterised by rigid behaviour, that is rocks characterised by Vs significantly higher than that of the sediments above (e.g., Vsbed- rocks characterised by Vs significantly higher than that of the sediments above (e.g., rock/Vscover > 2). In the design rules (e.g., [8,9]), the seismic bedrock is defined as rock or Vsbedrock/Vscover > 2). In the design rules (e.g., [8,9]), the seismic bedrock is defined as rock-like geologicalrock formation or rock-like characterised geological formation by Vs > 800 characterised m/s (ground by type Vs >A); 800 for m/s this (ground rea- type A); son, a stiff geologicalfor this unit reason, characterised a stiff geological by Vs ≥ unit 800 characterisedm/s is also indicated by Vs ≥ as800 “engineering m/s is also indicated as bedrock” (e.g., [3]).“engineering bedrock” (e.g., [3]). To better understandTo better the understandimportance theof the importance above definitions, of the above it is important definitions, to it con- is important to sider that the geologicalconsider substratum that the geological can act as substratum a seismic bedrock can act aseven a seismicin the case bedrock of Vs<800 even in the case m/s, if the seismicof impedance Vs < 800 m/s, contrast if the between seismic impedancesubstratum contrastand cover between is significant. substratum and cover is Figure 1 showssignificant. geophysical tests carried out on a landslide in the Northern Apen- nines [10]. In this site,Figure ambient1 shows noise geophysical measurements tests show carried a peak out on of a the landslide horizontal in the to Northernver- Apen- tical spectral ratiosnines (H/V) [10]. at In the this frequency site, ambient of 3.6–3.8 noise Hz. measurements To identify showthe depth a peak of the of the sur- horizontal to face responsible verticalfor this spectralpeak, a borehole ratios (H/V) was atdrilled, the frequency and a direct of 3.6–3.8 Vs measurement Hz. To identify (down- the depth of the hole) was carriedsurface out. Landslide responsible debris for this was peak, drilled a borehole up to a depth was drilled, of 25 m and and a then direct a Vssub- measurement stratum consisting(down-hole) of compact was grey carried marls out. wa Landslides drilled up debris to the was bottom drilled hole up (40 to am). depth The of 25 m and then a substratum consisting of compact grey marls was drilled up to the bottom hole grey marls can be correlated with the Montepiano Marls (upper Eocene–Lower Oligo- (40 m). The grey marls can be correlated with the Montepiano Marls (upper Eocene–Lower cene), extensively outcropping in the surrounding areas. The down-hole test recorded 300 Oligocene), extensively outcropping in the surrounding areas. The down-hole test recorded < Vs < 450 m/s in300 the Figure 1.FigureComparison 1. Comparison between between stratigraphic stratigraphic log, Vs profiles log, Vs from profiles down-hole from down-hole and ambient and noise ambient in an noise Apennines in site characterisedan Apennines by debris coversite characterised (landslide) on by marly debris geological cover (landslide) substratum on (modified marly geological from [10]). substratum (modi- fied from [10]). Similar cases are frequent in the Apennines. Similar cases are frequent in the Apennines. Geosciences 2021, 11, 297 4 of 22 Geosciences 2021, 11, x FOR PEER REVIEW 4 of 22 2. Useful Documents The first step in the recognition of the seismic bedrock is to reconstruct the geological model.2. Useful The Documents deep geological cross-sections are essential in order to identify the geological substratum.The first step in the recognition of the seismic bedrock is to reconstruct the geological model.The The sheets deep of geological the Geological cross-sections Map of Italyare essential 1: 50,000, in andorder related to identify sketches, the geological explanatory notes,substratum. and cross-sections, are available on the website of the Geological Survey of Italy: https://www.isprambiente.gov.it/Media/carg/The sheets of the Geological Map of Italy 1: 50,000, (accessed and related on 1 July sketches, 2021). explanatory Some sheets of plainnotes, areas and arecross-sections, accompanied are byavailable subsoil on maps, the website which represent of the Geologic the mainal Survey buried of geological Italy: elementshttps://www.isprambiente (roof of the sandy/gravelly.gov.it/Media/carg/ horizons, (accessed unconformities, on 1 July tectonic 2021). Some structures, sheets roof of of theplain geological areas are substratum,accompanied . .by . ).subsoil maps, which represent the main buried geological elementsAs regards (roof of thethe firstsandy/gravelly 500–600 m horizons, of subsoil unconformities, of the Po Plain tectonic valuable structures, information roof of is availablethe geological in the substratum, studies carried …). out by Emilia-Romagna Region and Lombardy Region, in collaborationAs regards with the ENI,first 500–600 for the m characterisation of subsoil of the ofPo thePlain water valuable resources information [11,12 is]. avail- In these publications,able in the studies maps carried of the isobathsout by Emilia-R and thicknessesomagna Region of the mainand Lombardy aquifers andRegion, cross-sections in col- uplaboration to the geological with ENI, substratum for the characterisation are available. of Athe similar water study,resources although [11,12]. with In these less detailedpub- mapslications, and maps geological of the isobaths cross-sections, and thicknesses was also of carriedthe main out aquifers by the and Piedmont cross-sections Region, up in collaborationto the geological with substratum the CNR and are theavailable. University A similar of Turin study, [13 ].although with less detailed mapsThe and study geological on the cross-sections, aquifers of the was Ferrara also carried Province out by is the a local Piedmont update Region, on the in Po col- Plain subsoillaboration [14 ].with the CNR and the University of Turin [13]. TheThe moststudy important on the aquifers geological of the cross-sections Ferrara Province (carried is a local out byupdate the regional on the Po Geological, Plain Seismicsubsoil [14]. and Soil Survey) and some geotechnical tests in the Emilia-Romagna plain are The most important geological cross-sections (carried out by the regional Geological, available on the Emilia-Romagna Region website https://geo.regione.emilia-romagna.it/ Seismic and Soil Survey) and some geotechnical tests in the Emilia-Romagna plain are cartografia_sgss/user/viewer.jsp?service=sezioni_geo (accessed on 1 July 2021). available on the Emilia-Romagna Region website https://geo.regione.emilia-roma- Finally, useful information for the geological, geotechnical, and geophysical charac- gna.it/cartografia_sgss/user/viewer.jsp?service=sezioni_geo (accessed on 1 July 2021). terisation of the subsoil can be found in seismic microzonation studies. Italian seismic Finally, useful information for the geological, geotechnical, and geophysical charac- microzonation studies, carried out with the funds of Law 77/2009, art. 11, are avail- terisation of the subsoil can be found in seismic microzonation studies. Italian seismic mi- able on the DPC website https://www.webms.it/ (accessed on 1 July 2021), managed crozonation studies, carried out with the funds of Law 77/2009, art. 11, are available on bythe CNR-IGAG. DPC website Seismic https://www.webms.it/ microzonation studies(accessed of on the 1 Emilia-RomagnaJuly 2021), managed Region by CNR- are also availableIGAG. Seismic on the microzonation website http://geo.regione.emilia-romagna.it/schede/pnsrs/ studies of the Emilia-Romagna Region are also available (accessed on onthe 1 website July 2021). http://geo.regione.emilia-romagna.it/schede/pnsrs/ (accessed on 1 July 2021). 3. Geological Framework 3. Geological Framework TheThe PoPo PlainPlain is the largest largest alluvial alluvial plain plain in in Italy Italy and and is islimited limited by bythe the Southern Southern Alps Alps toto thethe north, the the Western Western Alps Alps to tothe the west, west, the theNorthern Northern Apennines Apennines to the tosouth the and south the and theAdriatic Adriatic Sea Seato the to theeast east (Figure (Figure 2).2 ).The The Em Emilia-Romagnailia-Romagna plain plain constitutes constitutes the thecentral- central- southernsouthern andand eastern sectors. Figure 2. Geographical framework of the Po Plain. Figure 2. Geographical framework of the Po Plain. Geosciences 2021, 11, x FOR PEER REVIEW 5 of 22 Geosciences 2021, 11, x FOR PEER REVIEW 5 of 22 Geosciences 2021, 11, 297 5 of 22 The Po Plain represents the foredeep of two chains with opposite vergence. The NorthernThe Po Apennines, Plain represents south side, the are foredeep north-verg of twoing, andchains their with front opposite does not vergence. coincide withThe Northernthe morphological Apennines, transition south side, from are the north-verg hill to theing, plain and theirbut is front made does up notof buriedcoincide thrusts with the(Figure morphologicalThe 3) Po belonging Plain representstransition to the Emilia from the foredeep Foldsthe hill and to of Ferrara the two plain chains Folds but with[15]. is made The opposite Southernup of vergence. buried Alps, thrusts north The Northern(Figureside, have 3) Apennines,belonging vergence towards to south the Emilia the side, south Folds are north-verging,and and the Ferrara front isFolds also and [15].buried their The front by Southern the does alluvial not Alps, coincidedeposits. north withside,Therefore, have the morphological vergence the subsoil towards of transitionthe the Po southPlain from andis stru the thectured hill front to isinto the also plainanticlines buried but by isand the made synclines, alluvial up of deposits. and buried the thrustsTherefore,thickness (Figure ofthe alluvial 3subsoil) belonging sediments of the to Po the isPlain Emilia very is variable, Foldsstructured and even Ferrara into in anticlines the Folds distal [ 15 and areas:]. The synclines, from Southern over and Alps, 500– the norththickness600 m side, in theof have alluvialsynclines vergence sediments up to towards a few is verytens the southofvariable, meters and evenon the the frontin anticlines the is distal also (Figure buriedareas: from by4). the over alluvial 500– deposits.600 m in the Therefore, synclines the up subsoil to a few of thetens Po of Plainmeters is on structured the anticlines into anticlines (Figure 4). and synclines, and the thickness of alluvial sediments is very variable, even in the distal areas: from over 500–600 m in the synclines up to a few tens of meters on the anticlines (Figure4). Figure 3. Geological cross-section from the Bologna Apennines to the Po River [16]; trace in Figure Figure 3. Geological cross-section from the Bologna Apennines to the Po River [16]; trace in Figure4. Figure4. 3. Geological cross-section from the Bologna Apennines to the Po River [16]; trace in Figure 4. FigureFigure 4.4.Map Map of of the the basal basal unconformity unconformity of of the the alluvial alluvial deposits deposits in thein the Lombard Lombard and and Emilia-Romagna Emilia-Roma- Figure 4. Map of the basal unconformity of the alluvial deposits in the Lombard and Emilia-Roma- plaingna plain (isobaths (isobaths referred referred to the seato the level) sea (modifiedlevel) (mod fromified [11 from,12]); [11,12]); the black the dots black indicate dots indicate the main the cities. gnamain plain cities. (isobaths referred to the sea level) (modified from [11,12]); the black dots indicate the main cities. The alluvial sediments of the Emilia-Romagna plain are the result of the deposi- tionalThe activity alluvial of sediments the Po River of the and Emilia-Romagna its right tributaries plain are and the can result be dividedof the depositional into two The alluvial sediments of the Emilia-Romagna plain are the result of the depositional mainactivity stratigraphic of the Po River units and (Figure its right5): the tributaries lower Emilia-Romagna and can be divided Synthem into two (0.65–0.45 main strati- Ma) activity of the Po River and its right tributaries and can be divided into two main strati- and the Upper Emilia-Romagna Synthem (0.45 Ma–Present), both limited at the base by discontinuity surfaces [11,12]. The literature indicates these Synthems, respectively named “Sintema Emiliano-Romagnolo Inferiore” and “Sintema Emiliano-Romagnolo Superiore” in Italian, with the acronyms AEI and AES (see Geological Map of Italy: Geosciences 2021, 11, x FOR PEER REVIEW 6 of 22 graphic units (Figure 5): the lower Emilia-Romagna Synthem (0.65–0.45 Ma) and the Up- Geosciences 2021, 11, 297 per Emilia-Romagna Synthem (0.45 Ma–Present), both limited at the base by discontinuity6 of 22 surfaces [11,12]. The literature indicates these Synthems, respectively named “Sintema Emiliano-Romagnolo Inferiore” and “Sintema Emiliano-Romagnolo Superiore” in Italian, with the acronyms AEI and AES (see Geological Map of Italy: https://www.isprambi- https://www.isprambiente.gov.it/Media/carg/emilia.htmlente.gov.it/Media/carg/emilia.html, accessed on 1 July 2021), accessedor SERI and on 1 SERS July 2021) [16–18]. or SERIBoth andSynthems SERS can [16– be18 ].divided Both Synthems into subunits can ma bede divided up of alternations into subunits of madecoarse up (sands of alter- and nationsgravels) of and coarse fine (clays (sands and and silts) gravels) sediments. andfine The (clayscoarse andsediments silts) sediments. are obviously The prevalent coarse sedimentsin the marginal are obviously areas while prevalent the successions in the marginal of the areas distal while areas, the both successions in the synclines of the distal and areas,on the both anticlines, in the synclines are made and up onof prevailing the anticlines, fine are sediments made up (clays of prevailing and silts), fine with sediments interca- (clayslations and of sandy silts), horizons. with intercalations The most superficial of sandy horizons. deposits, Holocene The most in superficial age, are loose deposits, sedi- Holocenements; the in consolidation age, are loose sediments;degree of the the underl consolidationying sediments degree of increases the underlying with depth; sediments how- increasesever, the withalluvial depth; sediments however, of thethe alluvial Po Plain, sediments even those of the at Pogreater Plain, depth, even thoseare poorly at greater con- depth,solidated are and poorly susceptible consolidated to subsidence. and susceptible to subsidence. FigureFigure 5.5.Geological Geological cross-sectioncross-section throughthrough thethe ModenaModena plainplain andand thethe MantuaMantua Province Province south south of of the Pothe River Po River [19]; [19]; trace trace in Figure in Figure4. 4. TheThe substratumsubstratum is generally generally made made up up of of transitional transitional sands, sands, Middle Middle Pleistocene Pleistocene in age in age(Imola (Imola Sands, Sands, Yellow Yellow Sands, Sands, Costamezzana Costamezzana Synthem Synthem Auctt.Auctt.), and), and marine marine sediments sediments (al- (alternatingternating clay, clay, marl, marl, and and sandstone), sandstone), Pliocene–Lower Pliocene–Lower Pleistocene Pleistocene in age in (known age (known as Argille as ArgilleAzzurre Azzurre Auctt.),Auctt extensively.), extensively outcropping outcropping in the Apennines in the Apennines hills [20,11]. hills [ 11,20]. ApenninesApennines andand AlpsAlps areare still-formingstill-forming chains,chains, asas evidencedevidenced byby thethe frequentfrequent seismicityseismicity recordedrecorded onon the front front of of the the two two chains chains (e.g., (e.g., [18,21]). [18,21 Therefore,]). Therefore, the tectonic the tectonic activity activity influ- influencedenced the thesedimentation sedimentation of ofall allgeological geological units, units, whose whose basal basal discontinuity discontinuity surfacessurfaces areare characterisedcharacterised by by angularangular unconformity,unconformity, more more evident evident at at the the basin basin margins margins and and in in the the high high structuralstructural areasareas (Figure(Figure5 ).5). 4. Case Studies 4. Case Studies Based on the variability of the thickness of the alluvial cover and the distribution of Based on the variability of the thickness of the alluvial cover and the distribution of lithotypes, large alluvial basins can be divided into: lithotypes, large alluvial basins can be divided into: (1) Marginal areas; (2)1) Marginal Distal areas, areas; which, if tectonically articulated, can in turn be divided into high structural2) Distal areas areas, (top ofwhich, the anticlines) if tectonically and other articulated, areas (limbs can in and turn synclines). be divided into high structuralThe Emilia-Romagna areas (top of the plain anticlines) includes and all other areas indicatedareas (limbs above. and Thankssynclines). to the numerous studiesThe of Emilia-Romagna the site seismic plain assessment includes carried all area outs indicated in the last above. 10 years, Thanks it isto possiblethe numer- to illustrateous studies examples of the site of seismic seismic bedrock assessment determination carried out in in each the oflast these 10 years, areas. it is possible to illustrateIn particular, examples the of examples seismic considerbedrock datadetermination and results in from each studies of these carried areas. out for the char- acterisation of the environmental effects observed during the Emilia 2012 earthquake [3,22–26] and the seismic microzonation for the reconstruction [19] and for the urban planning (see https://geo.regione.emilia-romagna.it/schede/pnsrs/, accessed on 1 July 2021), as well as important studies for the evaluation of stability in seismic conditions of major works [27–31]. Geosciences 2021, 11, x FOR PEER REVIEW 7 of 22 In particular, the examples consider data and results from studies carried out for the characterisation of the environmental effects observed during the Emilia 2012 earthquake [3,22–26] and the seismic microzonation for the reconstruction [19] and for the urban plan- ning (see https: //geo.regione.emilia-romagna.it/schede/pnsrs/, accessed on 1 July 2021), Geosciences 2021, 11, 297 as well as important studies for the evaluation of stability in seismic conditions of 7major of 22 works [27–31]. Figure 6 shows the locations of the case studies illustrated in this paper. The most significant examples are illustrated below. Other examples can be found in the SupplementaryFigure6 shows theMaterials. locations of the case studies illustrated in this paper. FigureFigure 6.6. Location of the case case studies studies (white (white dots). dots). Larger Larger symbols symbols and and blue blue toponyms toponyms indicate indicate sites with geophysical/geotechnical tests and geological cross-sections; smaller symbols and black sites with geophysical/geotechnical tests and geological cross-sections; smaller symbols and black toponyms indicate sites with only geophysical tests. toponyms indicate sites with only geophysical tests. 4.1. MarginalThe most Area significant examples are illustrated below. Other examples can be found in the SupplementaryThe marginal Materials.area of the Emilia-Romagna plain is the southern zone, close to the Northern Apennines. The southern coast, between Rimini and the Gabicce promontory 4.1.(Marche Marginal region), Area is also part of this area. TheThis marginal sector is characterised area of the Emilia-Romagna by prevalently coarse plain issequences, the southern made zone, up of close plurimetric to the Northerngravelly horizons, Apennines. generally The southern amalgamated coast, between towards Riminithe mountain, and the where Gabicce the promontory thicknesses (Marcheof gravels region), can also is alsobe decametric, part of this and area. interbedded with sandy and silty horizons towards the plain.This sector The geological is characterised substratum by prevalently is usually coarse madesequences, up of transitional made up sands of plurimetric of Middle gravellyPleistocene horizons, age and/or generally marine amalgamated clays and si towardslts of Pliocene–Lower the mountain, wherePleistocene the thicknesses age. of gravelsGeophysical can also be surveys decametric, indicate and that, interbedded generally, with in gravelly sandy andhorizons, silty horizons Vs is greater towards than the400 plain. m/s while The geologicalthe Vs in the substratum first meters is of usually the geological made up substratum of transitional is about sands 300–400 of Middle m/s. PleistoceneFigure 7 shows age and/oran example marine of stratigraphic clays and silts log of and Pliocene–Lower Vs profile (from Pleistocene down-hole age. test) in an alluvialGeophysical fan of the surveys Apennines-plain indicate that, margin. generally, in gravelly horizons, Vs is greater than 400 m/s while the Vs in the first meters of the geological substratum is about 300–400 m/s. Figure7 shows an example of stratigraphic log and Vs profile (from down-hole test) in an alluvial fan of the Apennines-plain margin. GeosciencesGeosciences 2021 2021, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 of of 22 22 Geosciences 2021, 11, 297 8 of 22 Figure 7. Example of stratigraphic log and Vs-Vp profiles (down-hole) in the foothills area (Santa FigureFigure 7. 7. Example Example of of stratigraphic stratigraphic log log and and Vs-Vp Vs-Vp profiles profiles (down-hole) (down-hole) in in the the foothills foothills area area (Santa (Santa Monica site, Misano Adriatico Municipality, Rimini Province). MonicaMonica site, site, Misano Misano Adriatico Adriatico Municipality, Municipality, Rimini Rimini Province). Province). Figure8 shows a Vs profile (from a cross-hole test) in the southern area of the city FigureFigure 8 8 shows shows a a Vs Vs profile profile (from (from a a cross-hole cross-hole test) test) in in the the southern southern area area of of the the city city of of of Forlì (from http://itaca.mi.ingv.it/ItacaNet_30/#/station/IT/FOR, accessed on 1 July ForlìForlì (from (from http://itaca.mi.ingv.it/ItacaNet_30/#/station/IT/FOR http://itaca.mi.ingv.it/ItacaNet_30/#/station/IT/FOR, ,accessed accessed on on 1 1 July July 2021), 2021), 2021), where the subsoil is characterised by alluvial fan deposits (Montone, Rabbi and wherewhere the the subsoil subsoil is is characterised characterised by by alluvi alluvialal fan fan deposits deposits (Montone, (Montone, Rabbi Rabbi and and Bidente Bidente Bidente Rivers) and the geological substratum is deeper than 200 m. The figure shows that Rivers)Rivers) and and the the geological geological substratum substratum is is deeper deeper than than 200 200 m. m. The The figure figure shows shows that that in in the the firstin the gravelly first gravelly horizon, horizon, between between 12 and 12 15 and m, 15Vs m, = 400 Vs =m/s 400 and m/s in and the insecond the second gravelly gravelly hori- firsthorizon, gravelly below horizon, the depth between of 42–43 12 and m, 15 Vs m,≥ Vs800 = 400 m/s. m/s The and ambient in the second noise gravelly measurement hori- zon, below the depth of 42–43 m, Vs ≥ 800 m/s. The ambient noise measurement shows a zon,shows below a peak the ofdepth the H/Vof 42–43 spectral m, Vs ratio ≥ 800 around m/s. The 1.7–1.8 ambient Hz, consistent noise measurement with the Vs shows profile. a peakpeak of of the the H/V H/V spectral spectral ratio ratio around around 1.7–1.8 1.7–1.8 Hz, Hz, consistent consistent with with the the Vs Vs profile. profile. FigureFigure 8. 8. Stratigraphic StratigraphicStratigraphic log, log, log, Vs-Vp Vs-Vp Vs-Vp profiles profiles profiles (cross-hole, (cross-hole, (cross-hole, from from from http://itaca.mi.ingv.it/ItacaNet_30/# http://itaca.mi.ingv.it/ItacaNet_30/#/station/IT/FOR/station/IT/FORhttp://itaca.mi.ingv.it/ItacaNet_30/#/station/IT/FOR, with modifications, accessed on, ,with with 1 July modifications, modifications, 2021) and ambient ac accessedcessed noise on on 1 1 measurementJuly July 2021) 2021) and ambient noise measurement (from seismic microzonation, Forlì Municipality) in the Forlì sta- and(from ambient seismic noise microzonation, measurement Forl (fromì Municipality) seismic microzonation, in the Forlì station Forlì Municipality) of the National in Accelerometricthe Forlì sta- tion of the National Accelerometric Network; geological cross-section from CARG 1:50,000, sheet tionNetwork; of the National geological Accelerometric cross-section fromNetwork; CARG ge 1:50,000,ological cross-section sheet n. 240 “Ravenna”. from CARG 1:50,000, sheet n.n. 240 240 “Ravenna”. “Ravenna”. Geosciences 2021, 11, x FOR PEER REVIEW 9 of 22 The comparison between geophysical and stratigraphic data shows that the main Vs Geosciences 2021, 11, 297 discontinuity corresponds to the stratigraphic discontinuity that separates the sub-syn-9 of 22 thems AES7 and AES6. Similar information derives from the cross-hole test carried out in Viserba (Rimini Municipality), in a 96 meters deep borehole (Figure 9) in the alluvial fan of the Marecchia RiverThe [32,33], comparison see also betweenFigure S1 geophysical in the Supplementary and stratigraphic Materials. data shows that the main Vs discontinuityHigh Vs values corresponds were also tomeasured the stratigraphic in the gravels discontinuity of the alluvial that separatesfans of the the Parma, sub- synthemsEnza, Secchia AES7 and and Panaro AES6. Rivers [27,29]. Down-hole tests provided Vs > 400 m/s already in theSimilar first gravelly information horizons, derives at fromdepths the lower cross-hole than test20 m, carried and higher out in ViserbaVs values, (Rimini often Municipality),greater than 600 in m/s, a 96 metersin gravels deep at boreholedepths of (Figure 30–40 m9) (Figures in the alluvial 9 and fan10). of the Marecchia River [32,33], see also Figure S1 in the Supplementary Materials. FigureFigure 9.9. ((a)a) StratigraphicStratigraphic logslogs andand Vs-VpVs-Vp profilesprofiles ([([27],27], withwith modifications)modifications) andand (b)) ambientambient noise measurementsmeasurements (from (from seismic seismic microzonations, microzonations, Parma Parm Municipalitya Municipality and and Montecchio Montecchio Emilia Emilia Municipal- Munici- ity)pality) in the in alluvialthe alluvial fan areasfan area ofs the of Parmathe Parma and and Enza Enza Rivers. Rivers. High Vs values were also measured in the gravels of the alluvial fans of the Parma, Enza, Secchia and Panaro Rivers [27,29]. Down-hole tests provided Vs > 400 m/s already in the first gravelly horizons, at depths lower than 20 m, and higher Vs values, often greater than 600 m/s, in gravels at depths of 30–40 m (Figures9 and 10). Geosciences 2021, 11, x FOR PEER REVIEW 10 of 22 Geosciences 2021, 11, x FOR PEER REVIEW 10 of 22 Geosciences 2021, 11, 297 10 of 22 Figure 10. (a) Stratigraphic logs and Vs-Vp profiles ([27], with modifications) and (b) ambient FigureFigure 10.10.noise ((a)a) measurements StratigraphicStratigraphic logslogs (from andand seismic Vs-VpVs-Vp microzonations, profilesprofiles ([([27],27], withwith Parma modifications) modifications) Municipality, and and Campogalliano ( b(b)) ambient ambient noise Munici- measurementsnoise measurementspality, (fromand Modena seismic(from seismic Municipality) microzonations, microzonations, in Parmathe alluvial Municipality,Parma fan Municipality, areas Campogalliano of the Parma, Campogalliano Secchia Municipality, and Munici- Panaro and Riv- Modenapality, anders. Municipality) Modena Municipality) in the alluvial in fanthe areasalluvial of fan the areas Parma, of Secchiathe Parma, and Secchia Panaro Rivers.and Panaro Riv- ers. The down-holeThe down-hole tests carried tests outcarried in the out Parma in the and Parma Enza and Rivers Enza are Rivers both located are both on located the on top ofThe athe buried down-hole top of structural a buried tests high structuralcarried (Monticelli out high in the Terme)(Monticell Parma where iand Terme) the Enza thickness where Rivers the of are alluvialthickness both located sediments of alluvial on sed- variesthe top fromiments of a aboutburied varies 60–70 structural from m toabout 100–110high 60–70 (Monticell m m (Figures to 100–110i Terme) 11 and mwhere (Figures12), whereasthe thickness11 and the 12), down-hole of whereasalluvial tests sed-the down- relatingiments holevaries to the tests from Secchia relating about and to60–70 Panarothe Secchiam to Rivers 100–110 and are Panaro m in (Figures areas Rivers with 11 are highand in 12),areas thicknesses whereas with high ofthe alluvial thicknessesdown- of sediments,hole testsalluvial relating (about sediments, 300to the m, Secchia Figures (about and13300 and m,Panaro 14Figures). Rivers 13 and are 14).in areas with high thicknesses of alluvial sediments, (about 300 m, Figures 13 and 14). Figure 11. GeologicalFigure cross-section 11. Geological across the cross-section southern side across of the Parma southern River side alluvial of the fan Parma ([27], River with alluvial modifications). fan ([27 ], Figure 11. Geological cross-sectionwith modifications). across the southern side of the Parma River alluvial fan ([27], with modifications). Geosciences 2021Geosciences, 11, 297 2021, 11, x FOR PEER REVIEW 11 of 22 11 of 22 Geosciences 2021, 11, x FOR PEER REVIEW 11 of 22 Figure 12. Geological cross-section across the Enza River alluvial fan (from https://geo.regione.emilia-romagna.it/carto- Figure 12. Geological cross-section across the Enza River alluvial fan (from https://geo.regione. grafia_sgss/user/viewer.jsp?service=sezioni_geo, accessed on 1 July 2021, with modifications). Figure 12. Geologicalemilia-romagna.it/cartografia_sgss/user/viewer.jsp?service=sezioni_geo cross-section across the Enza River alluvial fan (from https://geo.regione.emilia-romagna.it/carto-, accessed on 1 July 2021, grafia_sgss/user/viewer.jsp?service=sezioni_geowith modifications). , accessed on 1 July 2021, with modifications). Figure 13. Geological cross-section across the Secchia River alluvial fan (from CARG 1:50,000, sheet n. 201 “Modena”, with modifications). Figure 13.FigureGeological 13. Geological cross-section cross-section across the across Secchia the River Secchia alluvial River fan alluvial (from CARGfan (from 1:50,000, CARG sheet 1:50,000, sheet n. 201 “Modena”, with modifications). n. 201 “Modena”, with modifications). The ambient noise measurements show a single high frequency peak (Fn > 10 Hz) where the succession is almost exclusively made up of gravels, indicating that the main Vs contrast surface is the roof of the first gravel horizon (Figures9, 11 and 12). Where the succession consists of gravelly and silty-clayey horizons (Figures 10, 13 and 14) the ambient noise measurements show two peaks, the main one at lower frequencies and the secondary one at high frequencies (Fn > 5 Hz, sometimes Fn > 10 Hz), indicating that, in this case as well, the roof of the first gravelly horizon is an important impedance contrast surface, but the most important contrast surface of Vs is deeper. Geosciences 2021, 11, 297 12 of 22 Geosciences 2021, 11, x FOR PEER REVIEW 12 of 22 Figure 14. GeologicalGeological cross-section across the Panaro River alluvial fan (modified(modified from [[11]).11]). 4.2. DistalThe ambient Areas noise measurements show a single high frequency peak (Fn > 10 Hz) whereThe the geological succession substratum is almost exclusively of alluvial sedimentsmade up of in gravel the Pos, Plain indicating is structured that the in main anti- Vsclines contrast and synclines surface is due the toroof the of activity the first of gr blindavel thrusts.horizon This(Figures determines 9, 11 and a high12). Where variability the successionin the thickness consists of alluvialof gravelly sediments: and silty-clayey at the top horizons of the higher (Figures anticlines, 10, 13 and it is 14) reduced the ambi- to a entfew noise tens ofmeasurements meters (San Colombano show two peaks, al Lambro, the main Monticelli one at Terme,lower frequencies Mirandola) and while the in sec- the ondarysynclines one it exceedsat high frequencies 600 m (south-eastern (Fn > 5 Hz, Lombardy sometimes plain) Fn > (Figure10 Hz),4 indicating). that, in this case asMeasurement well, the roof of of ambient the first gravelly noise conducted horizon is along an important the PoRiver impedance [28,30 ]contrast and in sur- the face,epicentral but the areas most of important the Emilia contrast 2012 seismic surface sequence of Vs is[ 19deeper.,22,23 ] highlighted two resonance frequencies of the ground: one at 0.8–0.9 Hz and the other at 0.25–0.3 Hz, with the exception 4.2.of the Distal buried Areas structural highs (Mirandola and Casaglia) where a single ground frequency was detected, ranging from 0.8 Hz to 1.4 Hz, with a peak of the H/V spectral ratio usually The geological substratum of alluvial sediments in the Po Plain is structured in anti- greater than 3 (see also [34]). clines and synclines due to the activity of blind thrusts. This determines a high variability Based on these observations, the description of the case studies in the Emilia-Romagna in the thickness of alluvial sediments: at the top of the higher anticlines, it is reduced to a plain will be distinguished in high structural zones (top of the higher anticlines) and other few tens of meters (San Colombano al Lambro, Monticelli Terme, Mirandola) while in the areas (synclines, limbs, and lower anticlines). synclines it exceeds 600 m (south-eastern Lombardy plain) (Figure 4). 4.2.1.Measurement High Structural of ambient Zones noise conducted along the Po River [28,30] and in the epi- central areas of the Emilia 2012 seismic sequence [19,22,23] highlighted two resonance In the Mirandola and Casaglia areas, located on the southern and northern buried frequencies of the ground: one at 0.8–0.9 Hz and the other at 0.25–0.3 Hz, with the excep- ridges of the Ferrara Folds [16], boreholes were drilled up to the geological substratum of tion of the buried structural highs (Mirandola and Casaglia) where a single ground fre- the alluvial deposits (Figure 15), made up of pre-Quaternary marine sediments (marly clays quencyof Pliocene was agedetected, in the ranging Mirandola from site, 0.8 marlsHz to of1.4 Tortonian–Messinian Hz, with a peak of the age H/V in spectral the Casaglia ratio usuallysite). In greater both bore-holes than 3 (see the also Vs was[34]). measured with cross-hole tests and the results indicate thatBased in the on geological these observations, substratum the the description Vs is significantly of the case higher studies than in that the ofEmilia-Roma- the alluvial gnasediments plain will and be exceeds distinguished 800 m/s in [19 high,22]. structural zones (top of the higher anticlines) and other areas (synclines, limbs, and lower anticlines). 4.2.1. High Structural Zones In the Mirandola and Casaglia areas, located on the southern and northern buried ridges of the Ferrara Folds [16], boreholes were drilled up to the geological substratum of the alluvial deposits (Figure 15), made up of pre-Quaternary marine sediments (marly clays of Pliocene age in the Mirandola site, marls of Tortonian–Messinian age in the Casa- glia site). In both bore-holes the Vs was measured with cross-hole tests and the results indicate that in the geological substratum the Vs is significantly higher than that of the alluvial sediments and exceeds 800 m/s [19,22]. Geosciences 2021, 11, 297 13 of 22 GeosciencesGeosciences 2021 2021, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 1313 of of 22 22 Figure 15. Stratigraphic logs and Vs profiles in Mirandola and Casaglia [19,22]. FigureFigure 15. 15. StratigraphicStratigraphic logs logs and and Vs Vs profiles profiles in in Mirandola Mirandola and and Casaglia Casaglia [19,22]. [19,22]. ResultsResults ofof ambient ambientambient noise noisenoise measurements measurementsmeasurements carried carriecarrie outdd nextoutout next tonext the toto boreholes thethe boreholesboreholes(Figures (Figures(Figures 16 and 171616) andandshow, 17)17) in show,show, both cases,inin bothboth a singlecases,cases, a peaka singlesingle of thepeakpeak H/V ofof thethe spectral H/VH/V spectral ratiospectral at aratioratio frequency atat aa frequencyfrequency of 0.9 Hz ofof with 0.90.9 HzHzhigh withwith amplitude highhigh amplitudeamplitude [19,22–24 [19,22–24,34,35].[19,22–24,34,35].,34,35]. Geosciences 2021, 11, x FOR PEER REVIEW 14 of 22 Figure 16. (a) Geological cross-section across the Mirandola anticline, (b) H/V spectral ratio and Vs Figureprofile 16. from (a) ambientGeological vibration cross-section measurement across the (array) Mirandola in the Mirandolaanticline, (b urban) H/V area spectral [19]. ratio and Vs profile from ambient vibration measurement (array) in the Mirandola urban area [19]. Figure 17. (a) Geological cross-section across the Casaglia anticline [14] and Vs profile from cross- hole in the Casaglia bore-hole [19,22], (b) H/V spectral ratio in the Casaglia site [23,28]. Geosciences 2021, 11, x FOR PEER REVIEW 14 of 22 Geosciences 2021, 11, 297 14 of 22 Figure 16. (a) Geological cross-section across the Mirandola anticline, (b) H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Mirandola urban area [19]. FigureFigure 17. (a) (a) Geological cross-section acrossacross the the Casaglia Casaglia anticline anticline [14 [14]] and and Vs Vs profile profile from from cross-hole cross- holein the in Casaglia the Casaglia bore-hole bore-hole [19,22 [19,22],], (b) H/V (b) H/V spectral spectral ratio ratio in the in Casagliathe Casaglia site [site23,28 [23,28].]. Moreover, ambient vibrations in array mode were recorded for the seismic microzona- tion aimed at supporting the Emilia 2012 post earthquake reconstruction. The tests carried out along the Mirandola buried ridge gave results consistent with the cross-hole test: the Vs profile from the Mirandola array shows an important Vs contrast at depth of about 110 m, below which Vs > 800 m/s (compares Figures 15 and 16; see also Figures S2–S5 in the Supplementary Materials). 4.2.2. Other Areas In the Emilia-Romagna plain, direct measurements of Vs at depths greater than 50 m are almost non-existent (except for the high structural zones). Therefore, data for the identification of the seismic bedrock mainly derive from ambient noise measurements and comparisons with the geological cross-sections. In the syncline and on the limbs, different tests for the estimation of Vs at high depths were carried out near Mirabello and in Cavezzo. Minarelli et al. (2016) [36] performed Vs measurement with a down-hole test in a 300 m deep borehole, just south of Mirabello. The alluvial sediment base was recognised at a depth of about 290 m, but the Vs measurement was carried out up to a depth of 265 m; Geosciences 2021, 11, x FOR PEER REVIEW 15 of 22 Moreover, ambient vibrations in array mode were recorded for the seismic microzo- nation aimed at supporting the Emilia 2012 post earthquake reconstruction. The tests car- ried out along the Mirandola buried ridge gave results consistent with the cross-hole test: the Vs profile from the Mirandola array shows an important Vs contrast at depth of about 110 m, below which Vs > 800 m/s (compares Figures 15 and 16; see also Figures S2–S5 in the Supplementary Materials). 4.2.2. Other Areas In the Emilia-Romagna plain, direct measurements of Vs at depths greater than 50 m are almost non-existent (except for the high structural zones). Therefore, data for the iden- tification of the seismic bedrock mainly derive from ambient noise measurements and comparisons with the geological cross-sections. In the syncline and on the limbs, different tests for the estimation of Vs at high depths were carried out near Mirabello and in Cavezzo. Minarelli et al. (2016) [36] performed Vs measurement with a down-hole test in a 300 m deep borehole, just south of Mirabello. The alluvial sediment base was recognised at a depth of about 290 m, but the Vs measurement was carried out up to a depth of 265 m; therefore, direct Vs measurements in the geological substratum are not available. In any case, this down-hole provides important information on Vs in alluvial sediments. The rec- ords did not show important Vs discontinuities (Figure 18a): Vs gradually increases for the first 100 m, up to the AES6 base, and in the lower part of AES it is generally about 400 m/s. A Vs increase is observed at about 160 m, passing from AES to the underlying sedi- ments (AEI): below this depth, the Vs is generally between 400 and 500 m/s; the maximum value, Vs=525 m/s, was measured at about 260 m. Geosciences 2021, 11, 297 Ambient noise measurements (array mode), carried out just south and north15 of of the 22 borehole, are substantially consistent with the down-hole data (Figure 18b). These tests show peaks of the H/V spectral ratios at frequencies of 0.2–0.3 Hz and 0.8–1 Hz; the re- constructed Vs profiles indicate contrasts at depths between 180 and 230 m in Sant’Ago- therefore, direct Vs measurements in the geological substratum are not available. In any stino and between about 100 and 120 m in San Carlo and Vigarano Mainarda. In these last case, this down-hole provides important information on Vs in alluvial sediments. The two sites, Vs contrasts between 400 and 450 m were also detected, with Vs > 800 m/s at records did not show important Vs discontinuities (Figure 18a): Vs gradually increases depths greater than 400 m. Minor Vs contrasts are present at depths of a few tens of meters for the first 100 m, up to the AES6 base, and in the lower part of AES it is generally about and about 60–80 m. 400 m/s. A Vs increase is observed at about 160 m, passing from AES to the underlying Comparing these results on a geological cross-section (Figure 18), we can see that the sediments (AEI): below this depth, the Vs is generally between 400 and 500 m/s; the Vs contrasts can be associated with the basal unconformities of the main stratigraphic maximum value, Vs=525 m/s, was measured at about 260 m. units (see also [37]). Geosciences 2021, 11, x FOR PEER REVIEW 16 of 22 FigureFigure 18. 18. (a) (a) GeologicalGeological cross-section cross-section from from Sant’Agostino to Vigarano Mainarda [[19]19] andand VsVs profilepro- filefrom from Mirabello Mirabello down-hole down-hole [36 [36],], (b ()b H/V) H/V spectralspectral ratiosratios and Vs Vs pr profilesofiles from from ambient ambient vibration vibration measurementsmeasurements (array) (array) [19]. [19]. InAmbient the town noise of Cavezzo measurements geophysical (array surv mode),eys carriedwere carried out just out south by andvarious north authors of the (Figureborehole, 19); are first substantially for studies consistentin support withof post-earthquake the down-hole reconstruction data (Figure 18 [19],b). These then for tests a seismicshow peaks microzonation of the H/V pilot spectral study, ratios which at frequencies was part of of the 0.2–0.3 LiquefAct Hz and project 0.8–1 [26]. Hz; theThe recon- first measurementstructed Vs profiles consists indicate of ambient contrasts noise at record depthsing between (array 180mode) and in 230 the m south-east in Sant’Agostino urban area.and betweenThe result about indicates 100 and two 120 peaks m in of San the CarloH/V spectral and Vigarano ratio, at Mainarda. frequencies In theseof 0.2 lastHz and two 0.8sites, Hz; Vs the contrasts reconstructed between Vs 400 profile and 450shows m were two alsocontrasts, detected, the withfirst at Vs a >depth 800 m/s of about at depths 80– 90greater m and than the 400 second m. Minor at a depth Vs contrasts of about are 240–260 present m. at depthsVs reaches of a few800 tensm/s below of meters 250 andm. Followingabout 60–80 investigations m. consist of new ambient noise recordings (array mode), made by INGV in the west side of the urban area, and a high-resolution reflection seismic line, made by OGS just southwest of the INGV array. The Vs profiles reconstructed by OGS and INGV, in collaboration with EUCENTRE, show similar Vs values up to a depth of 170 m and indicate that up to this depth Vs ≤ 500 m/s. Below this depth, the Vs profile pro- posed by INGV-EUCENTRE indicates Vs = 800 m/s while the Vs profile reconstructed by OGS indicates Vs = 500 m/s up to a depth of about 245 m, below which Vs > 700 m/s (Figure 19). The OGS profile is therefore consistent with the Vs profile resulting from the first investigation. Geosciences 2021, 11, x FOR PEER REVIEW 16 of 22 Geosciences 2021, 11, 297 16 of 22 Figure 18. (a) Geological cross-section from Sant’Agostino to Vigarano Mainarda [19] and Vs pro- file fromComparing Mirabello these down-hole results [36], on a(b geological) H/V spectral cross-section ratios and Vs (Figure profiles 18 from), we ambient can see vibration that the Vsmeasurements contrasts can (array) be associated [19]. with the basal unconformities of the main stratigraphic units (see also [37]). InIn thethe town town of of Cavezzo Cavezzo geophysical geophysical surveys surveys werewere carried carried out out by by various various authors authors (Figure(Figure 19 19);); first first for for studies studies in in support support of of post-earthquake post-earthquake reconstruction reconstruction [ 19[19],], then then for for a a seismicseismic microzonation microzonation pilot pilot study, study, which which was was part part of of the the LiquefAct LiquefAct project project [ 26[26].]. The The first first measurementmeasurement consistsconsists ofof ambientambient noisenoise recording recording (array (array mode) mode) in in the the south-east south-east urban urban area.area. TheThe result indicates two two peaks peaks of of the the H/V H/V spectral spectral ratio, ratio, at frequencies at frequencies of 0.2 of 0.2Hz Hzand and0.8 Hz; 0.8 Hz;the thereconstructed reconstructed Vs profile Vs profile shows shows two two contrasts, contrasts, the thefirst first at a at depth a depth of about of about 80– 80–9090 m mand and the the second second at at a adepth depth of of about about 240–260 240–260 m. Vs reachesreaches 800800 m/sm/s belowbelow 250250 m.m. FollowingFollowing investigationsinvestigations consist of new new ambient ambient noise noise recordings recordings (array (array mode), mode), made made by byINGV INGV in inthe the west west side side of of the the urban urban area, area, an andd a high-resolutionhigh-resolution reflectionreflection seismicseismic line,line, mademade byby OGS OGS just just southwest southwest of of the the INGV INGV array. array. The The Vs Vs profiles profiles reconstructed reconstructed by by OGS OGS andand INGV,INGV, inin collaborationcollaboration with EUCENTRE, EUCENTRE, show show similar similar Vs Vs values values up up to toa depth a depth of 170 of 170 m and indicate that up to this depth Vs ≤ 500 m/s. Below this depth, the Vs profile m and indicate that up to this depth Vs ≤ 500 m/s. Below this depth, the Vs profile pro- proposed by INGV-EUCENTRE indicates Vs = 800 m/s while the Vs profile reconstructed posed by INGV-EUCENTRE indicates Vs = 800 m/s while the Vs profile reconstructed by by OGS indicates Vs = 500 m/s up to a depth of about 245 m, below which Vs > 700 m/s OGS indicates Vs = 500 m/s up to a depth of about 245 m, below which Vs > 700 m/s (Figure (Figure 19). The OGS profile is therefore consistent with the Vs profile resulting from the 19). The OGS profile is therefore consistent with the Vs profile resulting from the first first investigation. investigation. Figure 19. Geological cross-section from Cavezzo to Mirandola, H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Cavezzo urban area [19] and Vs profiles from Cavezzo seismic microzonation [26]. Other examples are available in Supplementary Materials (Figures S6–S10). A specific discussion should be reserved for the coast north of Rimini, from Cesenatico to the Po delta. This sector is characterised everywhere by poorly consolidated sediments for at least 300 m and the AEI base is, almost throughout the whole sector, at depths greater than 450 m (Figures5 and 20; see also Figures S11–S14 in Supplementary Materials). Geosciences 2021, 11, x FOR PEER REVIEW 17 of 22 Figure 19. Geological cross-section from Cavezzo to Mirandola, H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Cavezzo urban area [19] and Vs profiles from Cavezzo seismic microzonation [26]. Other examples are available in Supplementary Materials (Figures S6–S10). A specific discussion should be reserved for the coast north of Rimini, from Cese- natico to the Po delta. This sector is characterised everywhere by poorly consolidated sed- iments for at least 300 m and the AEI base is, almost throughout the whole sector, at depths greater than 450 m (Figures 5 and 20; see also Figures S11–S14 in Supplementary Materi- als). Geosciences 2021, 11, 297 Ambient noise measurements, both in single station and in array mode, do not17 show of 22 peaks of the H/V spectral ratio in the frequency range between 0.2 and 10 Hz (Figure 20) and, therefore, important Vs discontinuities were not detected in the first 300 m. FigureFigure 20.20. GeologicalGeological cross-sectioncross-section alongalong the the coast coast from from Cesenatico Cesenatico to to Viserba Viserba (from (from Geological Geological Map ofMap Italy, of sheetItaly, no.sheet 256 no. “Rimini”), 256 “Rimini”), H/V spectralH/V spectr ratioal andratio Vs and profile Vs profile from ambientfrom ambient vibration vibration measure- measurement (array) in the Cesenatico urban area [38]. ment (array) in the Cesenatico urban area [38]. 5. ResultsAmbient noise measurements, both in single station and in array mode, do not show peaksFor of thethe H/Vdetermination spectral ratio of the in theseismic frequency bedrock, range it is between also important 0.2 and to 10 consider Hz (Figure recent 20) and,tectonics therefore, and how important this affected Vs discontinuities the stratigraphic were evolution not detected of the in thebasin. first 300 m. The Po Plain is an active foredeep basin; so, sedimentation and post-sedimentary 5.evolution Results of the alluvial units have been affected by the tectonic deformation. The main stratigraphicFor the determination units, AEI and of AES, theseismic and related bedrock, subunits, it is also are importantin fact bounded to consider by disconti- recent tectonicsnuity surfaces, and how the thisoldest affected of which the stratigraphicalso show evidence evolution of angular of the basin. unconformity (Figures 5, 8, andThe 12; Po see Plain also is Figure an active S1 in foredeep the Supplementary basin; so, sedimentationMaterials). The and maps post-sedimentary of these surfaces evolution[11,16] highlight of the alluvialimportant units tectonic have beendeformation affected at by least the up tectonic to the deformation.AES6 (Bazzano The subsyn- main stratigraphicthem, 0.230–0.125 units, Ma). AEI This and AES,can explain and related the di subunits,fferences arein the in fact densification bounded byof disconti-the sedi- nuityments surfaces, and the impedance the oldest of contrasts which also observed show evidenceat the boundary of angular surfaces unconformity of the main(Figure alluvial5 , Figureunits. 8 , and Figure 12; see also Figure S1 in the Supplementary Materials). The maps of theseThe surfaces case studies [11,16 illustrated] highlight in important the previous tectonic chapters deformation suggest the at least following up to theconsider- AES6 (Bazzanoations. subsynthem, 0.230–0.125 Ma). This can explain the differences in the densification of theThe sediments subsoil andof the the foothills impedance is characterised contrasts observed by plurimetric at the boundaryand decametric surfaces horizons of the mainof coarse alluvial deposits units. (gravels, sandy and silty gravels) of alluvial fan (Figures 8–10; see also FigureThe S1 case in the studies Supplementary illustrated in Materials). the previous Th chapterse roof of suggestthe Upper the Pleistocene following considerations coarse hori- . zon Theoften subsoil constitutes of the a foothills Vs contrast is characterised surface and by the plurimetric underlying and gravels decametric often horizonshave Vs ofhigher coarse than deposits that of (gravels, the geological sandy substratum, and silty gravels) sometimes of alluvial reaching fan and (Figures exceeding8–10; see800 also m/s. FigureIn general, S1 in it theis difficult Supplementary to identify Materials). the seismic The bedrock roof ofin the foothill Upper Pleistoceneareas, but from coarse the horizon often constitutes a Vs contrast surface and the underlying gravels often have Vs higher than that of the geological substratum, sometimes reaching and exceeding 800 m/s. In general, it is difficult to identify the seismic bedrock in the foothill areas, but from the comparison between geophysical data and geological cross-sections, it emerges that, in the proximal areas, the seismic bedrock commonly consists of a coarse horizon belonging to the alluvial sequence, at depths of a few tens of meters (Figures8,9, 11 and 12), while in the external areas the seismic bedrock is located at depths greater than 100 m and can be associated with the lower horizons of AES or AEI (Figures 13 and 14). In these contexts, the seismic bedrock is not as deep as the geological substratum. In the distal areas, where the alluvial sequence consists of fine and poorly consolidated sediments (alternating sands, silts, and clays), direct measurements indicate that the Vs is never high (Vs < 500 m/s) and reaches 800 m/s only in correspondence of the geological substratum (Figures 15 and 18b). Geosciences 2021, 11, 297 18 of 22 At the top of the main buried structural highs, where the thickness of alluvial sedi- ments is less than 150 m, the ambient noise measurements indicate a single and clear Vs contrast. In these environments, the seismic bedrock is generally easily identifiable and coincides with the geological substratum (Figures 15–17). In the other areas of the Emilia-Romagna plain, where the thickness of alluvial sed- iments is greater than 150 m (regardless of the geometry of the geological substratum), the geophysical measurements usually show two Vs contrasts (Figures 18 and 19; see also Figures S6–S8 in the Supplementary Materials): the shallower one is always associated with a basal unconformity of an alluvial unit, while the second is often associated with the roof of the geological substratum or at greater depths. In these contexts, for the correct calculation of the expected shaking on the surface, the seismic bedrock can be ascribed to the geological substratum, but the presence of important Vs contrasts in the overlying succession must always be evaluated. In the Po delta and along the Adriatic coast, where the thickness of alluvial sediments is greater than 300 m everywhere, geophysical measurements indicate the absence of significant velocity contrasts, and the Vs seems to gradually increase with depth. Vs values tend to be low in the upper alluvial cycle (AES, see Figure 20; see also Figures S11–S13 in the Supplementary Materials). The seismic bedrock is at a depth of at least 300–350 m and coincides with the first alluvial cycle (AEI) or with the geological substratum. Table2 summarizes the main geological and geophysical characteristics recognized in the Emilia-Romagna plain and described above. Table 2. Main geological and geophysical characteristics of the Emilia-Romagna plain. Distal Areas Other Areas Main Characteristics of Marginal Area Top of the Main the Zones Synclines, Limbs, and Structural Highs Coast and Po Delta Minor Anticlines Alluvial sequence made up of plurimetric and Thickness of the alluvial Thickness of the alluvial Thickness of the alluvial Geological features decametric coarse sequence H ≤ 150 m sequence H > 150 m sequence H > 300 m horizons Fn = 0.2–0.3 Hz; Fn = No Fn in the 0.2–10 Hz Ground Frequencies Fn > 5 Hz Fn = 0.8–1.4 Hz 0.7–Hz range Top of the shallower Basal unconformities of Seismo-Stratigraphic plurimetric coarse horizon Top of the geological No seismo-stratigraphic the main alluvial units Discontinuities (usually Upper Pleistocene substratum discontinuities (e.g., AES6, AES, AEI) age) Plurimetric/decametric Seismic Bedrock coarse horizon (Middle Lower alluvial cycle (AEI) Lower alluvial cycle (AEI) (Vs /Vs > 2 or Pleistocene age), usually Geological substratum bedrock cover or geological substratum or geological substratum Vs ≥ 800 m/s) at depth between 40 m and 100 m The data presented are in good agreement with the ground frequencies map and the seismic bedrock map proposed by Mascandola et al. (2019) [3]. The comparison between the data presented and Figure9 by Mascandola et al. (2019) highlights that the depth indicated by these authors as the roof of the seismic bedrock certainly corresponds to the depth of the main impedance contrast observed but does not always correspond to the roof of rocks characterised by Vs ≥ 800 m/s (“engineering bedrock”). In any case, the map proposed by Mascandola et al. (2019) remains the best reference for the estimation of the main Vs discontinuity in the Po Plain. 6. Conclusions Where the thickness of unconsolidated sediments is high and it is not possible to measure the Vs with direct investigations up to the seismic bedrock, the least expensive way to identify the latter is by comparing the results of the ambient noise measurements Geosciences 2021, 11, 297 19 of 22 (preferably in array), the derived Vs profiles, and the available lithologic and stratigraphic data (borehole logs and geological cross-sections). This approach was tested in the Emilia-Romagna plain and produced the following results (see also Table2): • In the foothills, where the coarse deposits of alluvial fan are prevalent, the seismic bedrock is sometimes difficult to identify; the roof of the first coarse horizon from the surface often constitutes an important velocity discontinuity and Vs already exceeds 800 m/s in the gravel horizons at a depth of a few tens of meters; • In the distal areas, where the alluvial sequence is made up of fine sediments (alternat- ing sands, silts, and clays), the Vs of alluvial deposits is generally less than 500 m/s; in this context, three typical situations were recognised: 1. At the top of the higher buried ridges, where the thickness of alluvial sediments is less than 150 m, the seismic bedrock is generally well recognizable and tends to coincide with the geological substratum; 2. In the other areas of the Emilia-Romagna plain, the thickness of alluvial sed- iments is greater than 150 m and the geophysical data usually indicate two important Vs contrasts: the first generally corresponds to the basal unconformity of one of the alluvial units (AES6 or AES), while the second generally corre- sponds to the base of alluvial deposits (or the roof of the geological substratum); in these successions the seismic bedrock seems to correspond to the lower allu- vial cycle (AEI), but it cannot be excluded that it is deeper and coincides with the geological substratum; 3. Along the Adriatic Coast and in the Po delta the thickness of alluvial sediments is greater than 300 m everywhere and the geophysical data indicate the absence of significant velocity contrasts; Vs is generally low in the whole upper alluvial cycle and reaches 800 m/s at depth of at least 300–350 m; in this context as well, the seismic bedrock seems to correspond to the lower alluvial cycle (AEI) or to the geological substratum. Finally, based on the presented data, the map of the seismic bedrock depth proposed by Mascandola et al. (2019) does not always seem to indicate the roof of the “engineering bedrock” (Vs ≥ 800 m/s), but rather the depth of the main Vs contrast surface. In spite of that, the work of Mascandola et al. (2019) remains a fundamental reference for site seismic assessment in the Po Plain. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/geosciences11070297/s1, Figure S1: Marginal area: comparison between stratigraphic log, Vs-Vp profiles and ambient noise measurements in Viserba (Rimini Municipality) [32,33]. Figure S2: Distal areas, high structural zones: comparison between stratigraphic data [11], H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Novi di Modena urban area [19], on the westward continuation of the blind Mirandola anticline. Figure S3: Distal areas, high structural zones: comparison between stratigraphic data, H/V spectral ratio and Vs profile from ambient vibra- tion measurement (array) in the San Felice sul Panaro urban area [19], on the eastward continuation of the Mirandola anticline. Figure S4: Distal areas, high structural zones: H/V spectral ratios and Vs profiles from ambient vibration measurement (array) around the Mirandola anticline area [19]. Figure S5: Distal areas, high structural zones: H/V spectral ratios and Vs profiles from ambient vibration measurements (array) in Bondeno and Scortichino, on the intermediate structural high of the Ferrara Folds [19]. Figure S6: Distal areas, other areas: comparison between stratigraphic data (geological cross-section from Geological Map of Italy, sheet no. 202 “S. Giovanni in Persiceto”), H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Crevalcore urban area [19]. Figure S7: Distal areas, other areas: comparison between stratigraphic data, H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Cento urban area [19]. Figure S8: Distal areas, other areas: comparison between stratigraphic data (geological cross-section from Geological Map of Italy, sheet no. 203 “Poggio Renatico”), H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Poggio Renatico urban area [19]. Figure S9: Distal areas, other areas: H/V spectral ratios and Vs profiles from ambient vibration measurements Geosciences 2021, 11, 297 20 of 22 (array) in Reggiolo and Camposanto urban areas [19]. Figure S10: Distal areas, other areas: com- parison between stratigraphic data, H/V spectral ratios and Vs profiles from ambient vibration measurements in the Boretto area [28]. Figure S11: Distal areas, coast and Po delta: comparison between stratigraphic data (geological cross-section from Geological Map of Italy, sheet no. 187 “Codigoro”), H/V spectral ratios and Vs profiles from ambient vibration measurements (array) in the Codigoro and Goro urban areas [38]. Figure S12: Distal areas, coast and Po delta: comparison between stratigraphic data (geological cross-section from Geological Map of Italy, sheet no. 205 “Comacchio”), H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Comacchio urban area [38]. Figure S13: Distal areas, coast and Po delta: comparison between stratigraphic data [11], H/V spectral ratio and Vs profile from ambient vibration measurement (array) in the Casal Borsetti area [38]. Figure S14: Distal areas, coast and Po delta: H/V spectral ratios and Vs profiles from ambient vibration measurements (array) along the coast from Porto Garibaldi to Cervia [38]. Funding: This research received no external funding. Institutional Review Board Statement: “Not applicable” for studies not involving humans or animals. Informed Consent Statement: “Not applicable” for studies not involving humans. Data Availability Statement: The data presented in this study are available within the article or supplementary material. Conflicts of Interest: The authors declare no conflict of interest. References 1. Seed, H.B.; Idriss, I.M. Influence of soil conditions on ground motion during earthquakes. J. Soil Mech. Found. Div. ASCE 1969, 95, 99–137. [CrossRef] 2. 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