Nat Hazards Earth Syst Sci 14 731ndash737 2014wwwnat-hazards-earth-syst-scinet147312014doi105194nhess-14-731-2014copy Author(s) 2014 CC Attribution 30 License

Natural Hazards and Earth System

SciencesO

pen Access

Electrical resistivity tomography for studying liquefaction inducedby the May 2012 Emilia-Romagna earthquake (Mw= 61 northernItaly)

A Giocoli12 B Quadrio3 J Bellanova1 V Lapenna1 and S Piscitelli1

1CNR-IMAA Tito Potenza Italy2ENEA Rome Italy3CNR-IGAG Rome Italy

Correspondence toA Giocoli (alessandrogiocolieneait)

Received 16 July 2013 ndash Published in Nat Hazards Earth Syst Sci Discuss 16 October 2013Revised 30 January 2014 ndash Accepted 27 February 2014 ndash Published 1 April 2014

Abstract This work shows the result of an electrical re-sistivity tomography (ERT) survey carried out for imagingand characterizing the shallow subsurface affected by the co-seismic effects of the Mw= 61 Emilia-Romagna (northernItaly) earthquake that occurred on 20 May 2012 The mostcharacteristic coseismic effects were ground failure lateralspreading and liquefaction that occurred extensively alongthe paleo-Reno River in the urban areas of San Carlo andMirabello (southwestern portion of Ferrara Province) In to-tal six electrical resistivity tomographies were performedand calibrated with surface geological surveys exploratoryboreholes and aerial photo interpretations This was oneof first applications of the electrical resistivity tomographymethod in investigating coseismic liquefaction

1 Introduction

On 20 May 2012 a reverse-fault earthquake (Mw= 61 QR-CMT 2012) hit the Emilia-Romagna Region (northern Italy)The hypocenter of the event was 63 km depth and the epi-center was localized at 44889 N and 11228 E near thetown of Finale Emilia (Fig 1a) The coseismic effects as-sociated with this event were observed in the nearby villageslocated within 20ndash30 km from the epicenter The most im-portant coseismic effect was related to the occurrence of liq-uefaction and the subsequent formation of ground failuresThe most impressive cases were those observed along thepaleo-Reno River in the localities of San Carlo (Fig 1b) and

Mirabello (Fig 1c) Here liquefaction and hundred meter-long fractures affected agricultural fields buildings wallspipelines and roads causing severe damage (see the photoin Fig 1b) (Galli et al 2012) In the urban areas of SanCarlo and Mirabello we investigated by means of an electri-cal resistivity tomography (ERT) survey the subsurface sur-rounding some of the major superficial manifestations of liq-uefaction In all cases we used surface geological surveysexploratory boreholes and aerial photo interpretations to cali-brate the electrical resistivity models and to correlate resistiv-ity values directly with the lithostratigraphic characteristicsof the subsoil The main aim of the ERT investigation was toprovide rapid and valuable geological information (eg theshape thickness and depth of the different geological unitsdepth of the water table etc) on the uppermost part of thesubsoil affected by the coseismic liquefaction

2 Geological and geomorphological framework

The epicentral area of the 20 May 2012 Emilia-Romagnaearthquake is located south of the Po River in correspon-dence of the active front of the northern Apennines trust beltthat is composed of buried folds and trust faults verging to thenorth (Fig 1a) This area is a morphologically uniform sectorof the Po Plain with modest reliefs in correspondence withthe natural levees of the water courses the banks of the aban-doned river channels and the anthropogenic backfills Mostof the liquefaction effects occurred between San Carlo andMirabello along the paleo-Reno River (15thndash18th century)

Published by Copernicus Publications on behalf of the European Geosciences Union

732 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 1 (a)Simplified tectonic map showing the epicenter of the 20 May 2012 Emilia-Romagna earthquake (modified from CNR-PFG 1991)Sketch map of the surface deposits derived from digital elevation models field observations and boreholes(b) San Carlo Black triangle isthe location of photo(c) Mirabello

Here evidence of the past hydrography is still marked in thelocal topography In particular the bed (about 12 m asl)the lateral banks (about 165 m asl) and the floodplain(about 13 m asl) of the paleo-Reno River are still well pre-served Recent very detailed investigations at San Carlo andMirabello (Calabrese et al 2012) allowed us to reconstructthe shallow sub-surface stratigraphy within the first 15ndash20 m(the usual depth range for liquefaction occurrence Youd etal 2001) Essentially three main units were identified

The upper unit referred to as the Fluvial Channel Unit(FCU) consists prevalently of channel anthropogenic back-fill riverbank and flood deposits The FCU is distinguishedinto three sub-units (1) paleo-riverbank (raised area at theside of the paleo-riverbed) made up of fine sand alternatingwith sandy silt in the most proximal portion passing laterally

(distal riverbanks) to sandy silts and clay (2) paleo-riverbed(topographic low between the paleo-riverbanks) composedof gravel and sand with a lenticular shape and (3) anthro-pogenic backfill and reworked deposits From the groundlevel the FCU is extended to an average depth of 15 m vari-able between 8 and 20 m depending on the considered pointAt the bottom the FCU consists of lenses of medium to finegray sands (MFGS) which reach their maximum thickness(about 6 m) in the borehole S10 (grey horizon in boreholelogs Fig 2) These lenses of medium to fine gray sands areproved to have been responsible for the coseismic surface ef-fects (Calabrese et al 2012)

The intermediate unit referred to as Marshes Unit (MU)consists mainly of clay and silt with abundant organic portionand levels of peat at different stratigraphic depths This unit

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 733

Fig 2Five borehole logs used to calibrate the ERT (see Fig 1 for each borehole location)

testifies to the presence of extensive and persistent marshy ar-eas (ldquovalleysrdquo) into which rivers flow which have developedfrom the maximum Holocene transgression These environ-ments not disturbed by fluvial and anthropogenic activitiesremained until modern times The estimated average thick-ness of MU is between 5ndash10 m and 10ndash15 m depending onthe area

The lower stratigraphical unit is the Pleistocene AlluvialPlain Unit (PAPU) that consists of an alternation of sandysilts and silty sands to the roof of an extended and continuoushorizon of medium and fine sands The depth of PAPU isabout 25 m below the paleo-riverbanks and 15ndash20 m in theplain

3 Electrical resistivity tomography

Electrical resistivity tomography (ERT) is an active geo-physical method applied to obtain high-resolution imagesof the subsurface resistivity pattern In the last few yearsthe ERT has been increasingly applied to study active faults(Galli et al 2006 2014 Improta et al 2010 Giocoli et al2011) volcanic areas (Finizzola et al 2010 Siniscalchi etal 2010) landslides (Perrone et al 2004) geological andstructural setting of sedimentary basins (Giocoli et al 2008)local seismic response (Boncio et al 2011 Mucciarelli et al2011 Moscatelli et al 2012) etc Only in very few caseswas the ERT method applied in laboratory experiments in

order to study liquefaction phenomenon during and after theshaking (Jinguuji et al 2007)

In this paper we present one of first application of theERT method in investigating coseismic liquefaction SeveralERT were carried out in order to image and characterizethe shallow subsurface of the areas affected by the coseis-mic effects of the Emilia-Romagna earthquake occurred on20 May 2012 In particular we carried out four ERT in SanCarlo (Fig 1b) and two ERT in Mirabello (Fig 1c) along thepaleo-Reno River where the ground failure and liquefactionoccurred extensively

All the ERT were performed by means of a Syscal R2 (IrisInstruments) resistivity meter coupled with a multielectrodeacquisition system (48 electrodes) A constant spacing ldquoardquo(5 m) between adjacent electrodes was used Due to the lim-ited number of electrodes of the system in some cases weused the roll-along method to extend horizontally the ERTprofile The length of each ERT ranged from 235ndash355 m(roll-along profiles) Along each profile we applied differ-ent array configurations (Wenner WennerndashSchlumberger andDipolendashDipole) and different combinations of dipole length(1a 2a and 3a) and ldquonrdquo number of depth levels (n le 6) ob-taining investigation depths of about 35ndash40 m The WennerWennerndashSchlumberger and DipolendashDipole apparent resistiv-ity data were inverted using the RES2DINV software (Loke2001) to obtain the 2-D resistivity models of the subsurfaceFor each ERT we present the 2-D resistivity model obtainedfrom array configuration that allowed to acquire data with

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

734 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 3ERT1 and ERT2 carried out in the northeastern sector of San Carlo (see Fig 1b for the location of ERT)

the higher signal-to-noise (sn) ratio a larger investigationdepth and a better sensitivity pattern to both horizontal andvertical changes in the subsurface resistivity The WennerndashSchlumberger array provided the best result In all cases theroot mean square (RMS) error is less than 70 and the re-sistivity values range from 5 to more than 127m

Generally since the electrical resistivity of a rock is con-trolled by different factors (water content porosity clay con-tent etc) there are wide ranges in resistivity for any par-ticular rock type and accordingly resistivity values can-not be directly interpreted in terms of lithology In addi-tion the interpretation of resistivity data can be ambigu-ous due to well-known principles of equivalence and sup-pression (Kunetz 1966) For these reasons we used datagathered through geological surveys and exploratory bore-holes to calibrate the ERT and to directly correlate resistivityvalues with the lithostratigraphic characteristics Thus thehigher resistivity values (gt 15m) are associated with theFluvial Channel Unit (FCU) the medium resistivity values(13ndash40m) to the Pleistocene Alluvial Plain Unit (PAPU)

and the lower resistivity values (lt 20m) are related to theMarshes Unit (MU) In particular the FCU is characterizedby paleo-riverbank deposits (20ndash80m) paleo-riverbed de-posits (from 50 to more than 128m) anthropogenic back-fills and reworked material (20ndash50m) and at the bottomby lenses of medium to fine gray sands (MFGS) (15ndash25m)

31 San Carlo

ERT1 and ERT2 were carried out in the northeast sector ofSan Carlo (Fig 1b) The first one was performed across thepaleo-bed of the Reno River whereas the second one wascarried out along the paleo-bank of the Reno River The twoprofiles cross each other at 90 At the intersection both elec-trical resistivity models (ERT1 and ERT2) show the same re-sistivity distribution (Fig 3) This is evidence of the goodquality of data as ERT1 and ERT2 were acquired and pro-cessed independently

ERT3 and ERT4 were carried out across the paleo-bed ofthe Reno River in the western part of San Carlo and nearthe cemetery respectively (Fig 1b) In particular the south-

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 735

Fig 4ERT3 and ERT4 carried out in the western part of San Carlo and near the cemetery respectively (see Fig 1b for the location of ERT)

ern part of the ERT3 profile crossed the ldquoRed Zonerdquo between125 and 295 m where extensive liquefaction and fractures af-fected buildings pipeline and roads causing severe damage(Fig 4)

Generally all electrical resistivity models put into evi-dence three plane-parallel layers with different thickness andresistivity values In agreement with borehole data (Fig 2)and geological observation (Fig 1b) the surficial layer(thicknesslt 20 m) of higher resistivity values (gt 15m)can be attributed to the FCU the intermediate layer (averagethickness about 15 m) of lower resistivity values (lt 20m)can be associated prevalently with the MU and the lowerlayer of medium resistivity values (13ndash40m) can be re-lated to the PAPU Based on the comparison between theexploratory borehole data and the ERT we noted an over-lap of the resistivity ranges The resistivity values includedbetween 15 and 20m are common to the MFGS (bottom ofthe FCU) and MU whereas the resistivity values between 13and 20m can be associated with both MU and PAPU Forthis reason we delineated sectors T1 (diagonal line pattern)and T2 (dotted pattern) to represent the uncertainty of theFCU-MU and MU-PAPU boundary locations respectively(Figs 3ndash5) In particular since sector T1 is related only tothe MGFS and MU we can speculate that T1 also highlightsthe sector where it is possible to find the MGFS

32 Mirabello

ERT5 runs NWndashSE across the paleo-bed of the Reno River(Fig 1c) It crossed between 90 and 235 m a soccer field anda car park ERT6 can be considered the southeastward con-tinuation of ERT5 (see Fig 1c) The ERT5 and ERT6 pro-files were not combined in a single long profile by applying

a roll-along acquisition method due to logistical conditions(presence of a main road) In this case there are no bore-hole data to constrain the interpretation of electrical modelsHowever it was possible to interpret both resistivity modelson the basis of the data coming from superficial geologicalobservation

Considering both resistivity models (Fig 5) from 65 to300 m between about 5 and 17 m asl it is possible to ob-serve a high-resistivity sector (gt 15m) that could be asso-ciated with the FCU Between about 5 and 0 m asl both re-sistivity models show a low-resistivity layer (lt 20m) thatcould be associated with the MU At the bottom (below about0 m asl) a medium-resistivity sector (13ndash40m) could berelated to the PAPU Also in this case we indicated the un-certainty of the FCU-MU and MU-PAPU boundaries usingsectors T1 and T2 respectively

4 Conclusions

A few days after the main shock of the 20 May 2012 Emilia-Romagna (northern Italy) earthquake (Mw= 61) we per-formed an ERT survey to investigate the subsurface sur-rounding some of the major superficial manifestations ofcoseismic liquefaction in the urban areas of San Carlo andMirabello The ERT were used as a reconnaissance method todetect the litostratigraphic characteristics in areas with lim-ited or nonexistent subsoil data

In particular this paper documents one of thefirst applications of the ERT method in investigatingcoseismic liquefaction

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

736 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 5ERT5 and ERT6 carried out in Mirabello (see Fig 1c for the location of ERT)

In all cases we used surface geological surveys ex-ploratory boreholes and aerial photo interpretations to cali-brate the electrical resistivity models and to directly corre-late resistivity values with the lithostratigraphic characteris-tics We have also taken into account the results obtained byAbu Zeid et al (2012) who also investigated coseismic liq-uefaction only in San Carlo a few weeks after the main seis-mic event of 20 May 2012 By comparing and matching allthese data we succeed in imaging the lithostratigraphic set-ting across the superficial manifestations of coseismic lique-faction Summarizing our investigations put into evidence

ndash a surficial layer with higher resistivity values(gt 15m) related to FCU (thicknesslt 20 m)

ndash an intermediate layer with lower resistivity values(lt 20m) related to MU (average thickness 10ndash15 m)

ndash a lower layer with lowndashmedium resistivity values (13ndash40m) related to PAPU

Furthermore we highlight sector T1 where it is possible tofind the MFGS responsible for the coseismic surface effects

In conclusion taking into account all the above inferencesthe ERT has proved to be an effective technique for obtain-ing rapid and valuable geological information on the upper-most part of the subsoil affected by the coseismic liquefac-tion (eg the lithostratigraphic setting the sectors in whichit is possible to find the MFGS and the possible associatedliquefaction phenomena etc) Thus especially in areas withlimited or nonexistent subsoil data we think that the ERTcan provide valuable data for designing further investigations(eg complementary geophysical surveys drill holes etc) in

order to better understanding the liquefaction phenomenonand assessing the associated hazard

AcknowledgementsThis work was funded by the Departmentof Civil Protection Rome We also thank all the working groupinvolved in the study of liquefaction phenomena induced by theMay 2012 Emilia-Romagna earthquake for the discussions duringthe advancement of the work

Edited by O KatzReviewed by three anonymous referees

References

Abu Zeid N Bignardi S Caputo R Santarato G and StefaniM Electrical resistivity tomography investigation of coseismicliquefaction and fracturing at San Carlo Ferrara Province ItalyAnn Geophys-Italy 55 713ndash716 2012

Boncio P Pizzi A Cavuoto G Mancini M Piacentini TMiccadei E Cavinato GP Piscitelli S Giocoli A FerrettiG De Ferrari R Gallipoli R Mucciarelli M Di Fiore VNaso G Giaccio B Moscatelli M Spadoni M Romano GFranceschini A Spallarossa D Pasta M Pavan M ScafidiD Barani S Eva C Pergalani F Compagnoni M CampeaT Di Berardino GR Mancini T Marino A MontefalconeR and Mosca F Geological and geophysical characterizationof the Paganica ndash San Gregorio area after the April 6 2009LrsquoAquila earthquake (Mw 63 central Italy) implications for siteresponse B Geofis Teor Appl 52 491ndash512 2011

Calabrese L Martelli L and Severi P Stratigrafia dellrsquoareainteressata dai fenomeni di liquefazione durante il terremotodellrsquoEmilia (Maggio 2012) 31th GNGTS Trieste Italy 20-22November 2012 119-126 available athttpwww2ogstriesteitgngts(last access October 2013) 2012

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

732 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 1 (a)Simplified tectonic map showing the epicenter of the 20 May 2012 Emilia-Romagna earthquake (modified from CNR-PFG 1991)Sketch map of the surface deposits derived from digital elevation models field observations and boreholes(b) San Carlo Black triangle isthe location of photo(c) Mirabello

Here evidence of the past hydrography is still marked in thelocal topography In particular the bed (about 12 m asl)the lateral banks (about 165 m asl) and the floodplain(about 13 m asl) of the paleo-Reno River are still well pre-served Recent very detailed investigations at San Carlo andMirabello (Calabrese et al 2012) allowed us to reconstructthe shallow sub-surface stratigraphy within the first 15ndash20 m(the usual depth range for liquefaction occurrence Youd etal 2001) Essentially three main units were identified

The upper unit referred to as the Fluvial Channel Unit(FCU) consists prevalently of channel anthropogenic back-fill riverbank and flood deposits The FCU is distinguishedinto three sub-units (1) paleo-riverbank (raised area at theside of the paleo-riverbed) made up of fine sand alternatingwith sandy silt in the most proximal portion passing laterally

(distal riverbanks) to sandy silts and clay (2) paleo-riverbed(topographic low between the paleo-riverbanks) composedof gravel and sand with a lenticular shape and (3) anthro-pogenic backfill and reworked deposits From the groundlevel the FCU is extended to an average depth of 15 m vari-able between 8 and 20 m depending on the considered pointAt the bottom the FCU consists of lenses of medium to finegray sands (MFGS) which reach their maximum thickness(about 6 m) in the borehole S10 (grey horizon in boreholelogs Fig 2) These lenses of medium to fine gray sands areproved to have been responsible for the coseismic surface ef-fects (Calabrese et al 2012)

The intermediate unit referred to as Marshes Unit (MU)consists mainly of clay and silt with abundant organic portionand levels of peat at different stratigraphic depths This unit

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 733

Fig 2Five borehole logs used to calibrate the ERT (see Fig 1 for each borehole location)

testifies to the presence of extensive and persistent marshy ar-eas (ldquovalleysrdquo) into which rivers flow which have developedfrom the maximum Holocene transgression These environ-ments not disturbed by fluvial and anthropogenic activitiesremained until modern times The estimated average thick-ness of MU is between 5ndash10 m and 10ndash15 m depending onthe area

The lower stratigraphical unit is the Pleistocene AlluvialPlain Unit (PAPU) that consists of an alternation of sandysilts and silty sands to the roof of an extended and continuoushorizon of medium and fine sands The depth of PAPU isabout 25 m below the paleo-riverbanks and 15ndash20 m in theplain

3 Electrical resistivity tomography

Electrical resistivity tomography (ERT) is an active geo-physical method applied to obtain high-resolution imagesof the subsurface resistivity pattern In the last few yearsthe ERT has been increasingly applied to study active faults(Galli et al 2006 2014 Improta et al 2010 Giocoli et al2011) volcanic areas (Finizzola et al 2010 Siniscalchi etal 2010) landslides (Perrone et al 2004) geological andstructural setting of sedimentary basins (Giocoli et al 2008)local seismic response (Boncio et al 2011 Mucciarelli et al2011 Moscatelli et al 2012) etc Only in very few caseswas the ERT method applied in laboratory experiments in

order to study liquefaction phenomenon during and after theshaking (Jinguuji et al 2007)

In this paper we present one of first application of theERT method in investigating coseismic liquefaction SeveralERT were carried out in order to image and characterizethe shallow subsurface of the areas affected by the coseis-mic effects of the Emilia-Romagna earthquake occurred on20 May 2012 In particular we carried out four ERT in SanCarlo (Fig 1b) and two ERT in Mirabello (Fig 1c) along thepaleo-Reno River where the ground failure and liquefactionoccurred extensively

All the ERT were performed by means of a Syscal R2 (IrisInstruments) resistivity meter coupled with a multielectrodeacquisition system (48 electrodes) A constant spacing ldquoardquo(5 m) between adjacent electrodes was used Due to the lim-ited number of electrodes of the system in some cases weused the roll-along method to extend horizontally the ERTprofile The length of each ERT ranged from 235ndash355 m(roll-along profiles) Along each profile we applied differ-ent array configurations (Wenner WennerndashSchlumberger andDipolendashDipole) and different combinations of dipole length(1a 2a and 3a) and ldquonrdquo number of depth levels (n le 6) ob-taining investigation depths of about 35ndash40 m The WennerWennerndashSchlumberger and DipolendashDipole apparent resistiv-ity data were inverted using the RES2DINV software (Loke2001) to obtain the 2-D resistivity models of the subsurfaceFor each ERT we present the 2-D resistivity model obtainedfrom array configuration that allowed to acquire data with

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

734 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 3ERT1 and ERT2 carried out in the northeastern sector of San Carlo (see Fig 1b for the location of ERT)

the higher signal-to-noise (sn) ratio a larger investigationdepth and a better sensitivity pattern to both horizontal andvertical changes in the subsurface resistivity The WennerndashSchlumberger array provided the best result In all cases theroot mean square (RMS) error is less than 70 and the re-sistivity values range from 5 to more than 127m

Generally since the electrical resistivity of a rock is con-trolled by different factors (water content porosity clay con-tent etc) there are wide ranges in resistivity for any par-ticular rock type and accordingly resistivity values can-not be directly interpreted in terms of lithology In addi-tion the interpretation of resistivity data can be ambigu-ous due to well-known principles of equivalence and sup-pression (Kunetz 1966) For these reasons we used datagathered through geological surveys and exploratory bore-holes to calibrate the ERT and to directly correlate resistivityvalues with the lithostratigraphic characteristics Thus thehigher resistivity values (gt 15m) are associated with theFluvial Channel Unit (FCU) the medium resistivity values(13ndash40m) to the Pleistocene Alluvial Plain Unit (PAPU)

and the lower resistivity values (lt 20m) are related to theMarshes Unit (MU) In particular the FCU is characterizedby paleo-riverbank deposits (20ndash80m) paleo-riverbed de-posits (from 50 to more than 128m) anthropogenic back-fills and reworked material (20ndash50m) and at the bottomby lenses of medium to fine gray sands (MFGS) (15ndash25m)

31 San Carlo

ERT1 and ERT2 were carried out in the northeast sector ofSan Carlo (Fig 1b) The first one was performed across thepaleo-bed of the Reno River whereas the second one wascarried out along the paleo-bank of the Reno River The twoprofiles cross each other at 90 At the intersection both elec-trical resistivity models (ERT1 and ERT2) show the same re-sistivity distribution (Fig 3) This is evidence of the goodquality of data as ERT1 and ERT2 were acquired and pro-cessed independently

ERT3 and ERT4 were carried out across the paleo-bed ofthe Reno River in the western part of San Carlo and nearthe cemetery respectively (Fig 1b) In particular the south-

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 735

Fig 4ERT3 and ERT4 carried out in the western part of San Carlo and near the cemetery respectively (see Fig 1b for the location of ERT)

ern part of the ERT3 profile crossed the ldquoRed Zonerdquo between125 and 295 m where extensive liquefaction and fractures af-fected buildings pipeline and roads causing severe damage(Fig 4)

Generally all electrical resistivity models put into evi-dence three plane-parallel layers with different thickness andresistivity values In agreement with borehole data (Fig 2)and geological observation (Fig 1b) the surficial layer(thicknesslt 20 m) of higher resistivity values (gt 15m)can be attributed to the FCU the intermediate layer (averagethickness about 15 m) of lower resistivity values (lt 20m)can be associated prevalently with the MU and the lowerlayer of medium resistivity values (13ndash40m) can be re-lated to the PAPU Based on the comparison between theexploratory borehole data and the ERT we noted an over-lap of the resistivity ranges The resistivity values includedbetween 15 and 20m are common to the MFGS (bottom ofthe FCU) and MU whereas the resistivity values between 13and 20m can be associated with both MU and PAPU Forthis reason we delineated sectors T1 (diagonal line pattern)and T2 (dotted pattern) to represent the uncertainty of theFCU-MU and MU-PAPU boundary locations respectively(Figs 3ndash5) In particular since sector T1 is related only tothe MGFS and MU we can speculate that T1 also highlightsthe sector where it is possible to find the MGFS

32 Mirabello

ERT5 runs NWndashSE across the paleo-bed of the Reno River(Fig 1c) It crossed between 90 and 235 m a soccer field anda car park ERT6 can be considered the southeastward con-tinuation of ERT5 (see Fig 1c) The ERT5 and ERT6 pro-files were not combined in a single long profile by applying

a roll-along acquisition method due to logistical conditions(presence of a main road) In this case there are no bore-hole data to constrain the interpretation of electrical modelsHowever it was possible to interpret both resistivity modelson the basis of the data coming from superficial geologicalobservation

Considering both resistivity models (Fig 5) from 65 to300 m between about 5 and 17 m asl it is possible to ob-serve a high-resistivity sector (gt 15m) that could be asso-ciated with the FCU Between about 5 and 0 m asl both re-sistivity models show a low-resistivity layer (lt 20m) thatcould be associated with the MU At the bottom (below about0 m asl) a medium-resistivity sector (13ndash40m) could berelated to the PAPU Also in this case we indicated the un-certainty of the FCU-MU and MU-PAPU boundaries usingsectors T1 and T2 respectively

4 Conclusions

A few days after the main shock of the 20 May 2012 Emilia-Romagna (northern Italy) earthquake (Mw= 61) we per-formed an ERT survey to investigate the subsurface sur-rounding some of the major superficial manifestations ofcoseismic liquefaction in the urban areas of San Carlo andMirabello The ERT were used as a reconnaissance method todetect the litostratigraphic characteristics in areas with lim-ited or nonexistent subsoil data

In particular this paper documents one of thefirst applications of the ERT method in investigatingcoseismic liquefaction

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

736 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 5ERT5 and ERT6 carried out in Mirabello (see Fig 1c for the location of ERT)

In all cases we used surface geological surveys ex-ploratory boreholes and aerial photo interpretations to cali-brate the electrical resistivity models and to directly corre-late resistivity values with the lithostratigraphic characteris-tics We have also taken into account the results obtained byAbu Zeid et al (2012) who also investigated coseismic liq-uefaction only in San Carlo a few weeks after the main seis-mic event of 20 May 2012 By comparing and matching allthese data we succeed in imaging the lithostratigraphic set-ting across the superficial manifestations of coseismic lique-faction Summarizing our investigations put into evidence

ndash a surficial layer with higher resistivity values(gt 15m) related to FCU (thicknesslt 20 m)

ndash an intermediate layer with lower resistivity values(lt 20m) related to MU (average thickness 10ndash15 m)

ndash a lower layer with lowndashmedium resistivity values (13ndash40m) related to PAPU

Furthermore we highlight sector T1 where it is possible tofind the MFGS responsible for the coseismic surface effects

In conclusion taking into account all the above inferencesthe ERT has proved to be an effective technique for obtain-ing rapid and valuable geological information on the upper-most part of the subsoil affected by the coseismic liquefac-tion (eg the lithostratigraphic setting the sectors in whichit is possible to find the MFGS and the possible associatedliquefaction phenomena etc) Thus especially in areas withlimited or nonexistent subsoil data we think that the ERTcan provide valuable data for designing further investigations(eg complementary geophysical surveys drill holes etc) in

order to better understanding the liquefaction phenomenonand assessing the associated hazard

AcknowledgementsThis work was funded by the Departmentof Civil Protection Rome We also thank all the working groupinvolved in the study of liquefaction phenomena induced by theMay 2012 Emilia-Romagna earthquake for the discussions duringthe advancement of the work

Edited by O KatzReviewed by three anonymous referees

References

Abu Zeid N Bignardi S Caputo R Santarato G and StefaniM Electrical resistivity tomography investigation of coseismicliquefaction and fracturing at San Carlo Ferrara Province ItalyAnn Geophys-Italy 55 713ndash716 2012

Boncio P Pizzi A Cavuoto G Mancini M Piacentini TMiccadei E Cavinato GP Piscitelli S Giocoli A FerrettiG De Ferrari R Gallipoli R Mucciarelli M Di Fiore VNaso G Giaccio B Moscatelli M Spadoni M Romano GFranceschini A Spallarossa D Pasta M Pavan M ScafidiD Barani S Eva C Pergalani F Compagnoni M CampeaT Di Berardino GR Mancini T Marino A MontefalconeR and Mosca F Geological and geophysical characterizationof the Paganica ndash San Gregorio area after the April 6 2009LrsquoAquila earthquake (Mw 63 central Italy) implications for siteresponse B Geofis Teor Appl 52 491ndash512 2011

Calabrese L Martelli L and Severi P Stratigrafia dellrsquoareainteressata dai fenomeni di liquefazione durante il terremotodellrsquoEmilia (Maggio 2012) 31th GNGTS Trieste Italy 20-22November 2012 119-126 available athttpwww2ogstriesteitgngts(last access October 2013) 2012

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 733

Fig 2Five borehole logs used to calibrate the ERT (see Fig 1 for each borehole location)

testifies to the presence of extensive and persistent marshy ar-eas (ldquovalleysrdquo) into which rivers flow which have developedfrom the maximum Holocene transgression These environ-ments not disturbed by fluvial and anthropogenic activitiesremained until modern times The estimated average thick-ness of MU is between 5ndash10 m and 10ndash15 m depending onthe area

The lower stratigraphical unit is the Pleistocene AlluvialPlain Unit (PAPU) that consists of an alternation of sandysilts and silty sands to the roof of an extended and continuoushorizon of medium and fine sands The depth of PAPU isabout 25 m below the paleo-riverbanks and 15ndash20 m in theplain

3 Electrical resistivity tomography

Electrical resistivity tomography (ERT) is an active geo-physical method applied to obtain high-resolution imagesof the subsurface resistivity pattern In the last few yearsthe ERT has been increasingly applied to study active faults(Galli et al 2006 2014 Improta et al 2010 Giocoli et al2011) volcanic areas (Finizzola et al 2010 Siniscalchi etal 2010) landslides (Perrone et al 2004) geological andstructural setting of sedimentary basins (Giocoli et al 2008)local seismic response (Boncio et al 2011 Mucciarelli et al2011 Moscatelli et al 2012) etc Only in very few caseswas the ERT method applied in laboratory experiments in

order to study liquefaction phenomenon during and after theshaking (Jinguuji et al 2007)

In this paper we present one of first application of theERT method in investigating coseismic liquefaction SeveralERT were carried out in order to image and characterizethe shallow subsurface of the areas affected by the coseis-mic effects of the Emilia-Romagna earthquake occurred on20 May 2012 In particular we carried out four ERT in SanCarlo (Fig 1b) and two ERT in Mirabello (Fig 1c) along thepaleo-Reno River where the ground failure and liquefactionoccurred extensively

All the ERT were performed by means of a Syscal R2 (IrisInstruments) resistivity meter coupled with a multielectrodeacquisition system (48 electrodes) A constant spacing ldquoardquo(5 m) between adjacent electrodes was used Due to the lim-ited number of electrodes of the system in some cases weused the roll-along method to extend horizontally the ERTprofile The length of each ERT ranged from 235ndash355 m(roll-along profiles) Along each profile we applied differ-ent array configurations (Wenner WennerndashSchlumberger andDipolendashDipole) and different combinations of dipole length(1a 2a and 3a) and ldquonrdquo number of depth levels (n le 6) ob-taining investigation depths of about 35ndash40 m The WennerWennerndashSchlumberger and DipolendashDipole apparent resistiv-ity data were inverted using the RES2DINV software (Loke2001) to obtain the 2-D resistivity models of the subsurfaceFor each ERT we present the 2-D resistivity model obtainedfrom array configuration that allowed to acquire data with

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

734 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 3ERT1 and ERT2 carried out in the northeastern sector of San Carlo (see Fig 1b for the location of ERT)

the higher signal-to-noise (sn) ratio a larger investigationdepth and a better sensitivity pattern to both horizontal andvertical changes in the subsurface resistivity The WennerndashSchlumberger array provided the best result In all cases theroot mean square (RMS) error is less than 70 and the re-sistivity values range from 5 to more than 127m

Generally since the electrical resistivity of a rock is con-trolled by different factors (water content porosity clay con-tent etc) there are wide ranges in resistivity for any par-ticular rock type and accordingly resistivity values can-not be directly interpreted in terms of lithology In addi-tion the interpretation of resistivity data can be ambigu-ous due to well-known principles of equivalence and sup-pression (Kunetz 1966) For these reasons we used datagathered through geological surveys and exploratory bore-holes to calibrate the ERT and to directly correlate resistivityvalues with the lithostratigraphic characteristics Thus thehigher resistivity values (gt 15m) are associated with theFluvial Channel Unit (FCU) the medium resistivity values(13ndash40m) to the Pleistocene Alluvial Plain Unit (PAPU)

and the lower resistivity values (lt 20m) are related to theMarshes Unit (MU) In particular the FCU is characterizedby paleo-riverbank deposits (20ndash80m) paleo-riverbed de-posits (from 50 to more than 128m) anthropogenic back-fills and reworked material (20ndash50m) and at the bottomby lenses of medium to fine gray sands (MFGS) (15ndash25m)

31 San Carlo

ERT1 and ERT2 were carried out in the northeast sector ofSan Carlo (Fig 1b) The first one was performed across thepaleo-bed of the Reno River whereas the second one wascarried out along the paleo-bank of the Reno River The twoprofiles cross each other at 90 At the intersection both elec-trical resistivity models (ERT1 and ERT2) show the same re-sistivity distribution (Fig 3) This is evidence of the goodquality of data as ERT1 and ERT2 were acquired and pro-cessed independently

ERT3 and ERT4 were carried out across the paleo-bed ofthe Reno River in the western part of San Carlo and nearthe cemetery respectively (Fig 1b) In particular the south-

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 735

Fig 4ERT3 and ERT4 carried out in the western part of San Carlo and near the cemetery respectively (see Fig 1b for the location of ERT)

ern part of the ERT3 profile crossed the ldquoRed Zonerdquo between125 and 295 m where extensive liquefaction and fractures af-fected buildings pipeline and roads causing severe damage(Fig 4)

Generally all electrical resistivity models put into evi-dence three plane-parallel layers with different thickness andresistivity values In agreement with borehole data (Fig 2)and geological observation (Fig 1b) the surficial layer(thicknesslt 20 m) of higher resistivity values (gt 15m)can be attributed to the FCU the intermediate layer (averagethickness about 15 m) of lower resistivity values (lt 20m)can be associated prevalently with the MU and the lowerlayer of medium resistivity values (13ndash40m) can be re-lated to the PAPU Based on the comparison between theexploratory borehole data and the ERT we noted an over-lap of the resistivity ranges The resistivity values includedbetween 15 and 20m are common to the MFGS (bottom ofthe FCU) and MU whereas the resistivity values between 13and 20m can be associated with both MU and PAPU Forthis reason we delineated sectors T1 (diagonal line pattern)and T2 (dotted pattern) to represent the uncertainty of theFCU-MU and MU-PAPU boundary locations respectively(Figs 3ndash5) In particular since sector T1 is related only tothe MGFS and MU we can speculate that T1 also highlightsthe sector where it is possible to find the MGFS

32 Mirabello

ERT5 runs NWndashSE across the paleo-bed of the Reno River(Fig 1c) It crossed between 90 and 235 m a soccer field anda car park ERT6 can be considered the southeastward con-tinuation of ERT5 (see Fig 1c) The ERT5 and ERT6 pro-files were not combined in a single long profile by applying

a roll-along acquisition method due to logistical conditions(presence of a main road) In this case there are no bore-hole data to constrain the interpretation of electrical modelsHowever it was possible to interpret both resistivity modelson the basis of the data coming from superficial geologicalobservation

Considering both resistivity models (Fig 5) from 65 to300 m between about 5 and 17 m asl it is possible to ob-serve a high-resistivity sector (gt 15m) that could be asso-ciated with the FCU Between about 5 and 0 m asl both re-sistivity models show a low-resistivity layer (lt 20m) thatcould be associated with the MU At the bottom (below about0 m asl) a medium-resistivity sector (13ndash40m) could berelated to the PAPU Also in this case we indicated the un-certainty of the FCU-MU and MU-PAPU boundaries usingsectors T1 and T2 respectively

4 Conclusions

A few days after the main shock of the 20 May 2012 Emilia-Romagna (northern Italy) earthquake (Mw= 61) we per-formed an ERT survey to investigate the subsurface sur-rounding some of the major superficial manifestations ofcoseismic liquefaction in the urban areas of San Carlo andMirabello The ERT were used as a reconnaissance method todetect the litostratigraphic characteristics in areas with lim-ited or nonexistent subsoil data

In particular this paper documents one of thefirst applications of the ERT method in investigatingcoseismic liquefaction

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

736 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 5ERT5 and ERT6 carried out in Mirabello (see Fig 1c for the location of ERT)

In all cases we used surface geological surveys ex-ploratory boreholes and aerial photo interpretations to cali-brate the electrical resistivity models and to directly corre-late resistivity values with the lithostratigraphic characteris-tics We have also taken into account the results obtained byAbu Zeid et al (2012) who also investigated coseismic liq-uefaction only in San Carlo a few weeks after the main seis-mic event of 20 May 2012 By comparing and matching allthese data we succeed in imaging the lithostratigraphic set-ting across the superficial manifestations of coseismic lique-faction Summarizing our investigations put into evidence

ndash a surficial layer with higher resistivity values(gt 15m) related to FCU (thicknesslt 20 m)

ndash an intermediate layer with lower resistivity values(lt 20m) related to MU (average thickness 10ndash15 m)

ndash a lower layer with lowndashmedium resistivity values (13ndash40m) related to PAPU

Furthermore we highlight sector T1 where it is possible tofind the MFGS responsible for the coseismic surface effects

In conclusion taking into account all the above inferencesthe ERT has proved to be an effective technique for obtain-ing rapid and valuable geological information on the upper-most part of the subsoil affected by the coseismic liquefac-tion (eg the lithostratigraphic setting the sectors in whichit is possible to find the MFGS and the possible associatedliquefaction phenomena etc) Thus especially in areas withlimited or nonexistent subsoil data we think that the ERTcan provide valuable data for designing further investigations(eg complementary geophysical surveys drill holes etc) in

order to better understanding the liquefaction phenomenonand assessing the associated hazard

AcknowledgementsThis work was funded by the Departmentof Civil Protection Rome We also thank all the working groupinvolved in the study of liquefaction phenomena induced by theMay 2012 Emilia-Romagna earthquake for the discussions duringthe advancement of the work

Edited by O KatzReviewed by three anonymous referees

References

Abu Zeid N Bignardi S Caputo R Santarato G and StefaniM Electrical resistivity tomography investigation of coseismicliquefaction and fracturing at San Carlo Ferrara Province ItalyAnn Geophys-Italy 55 713ndash716 2012

Boncio P Pizzi A Cavuoto G Mancini M Piacentini TMiccadei E Cavinato GP Piscitelli S Giocoli A FerrettiG De Ferrari R Gallipoli R Mucciarelli M Di Fiore VNaso G Giaccio B Moscatelli M Spadoni M Romano GFranceschini A Spallarossa D Pasta M Pavan M ScafidiD Barani S Eva C Pergalani F Compagnoni M CampeaT Di Berardino GR Mancini T Marino A MontefalconeR and Mosca F Geological and geophysical characterizationof the Paganica ndash San Gregorio area after the April 6 2009LrsquoAquila earthquake (Mw 63 central Italy) implications for siteresponse B Geofis Teor Appl 52 491ndash512 2011

Calabrese L Martelli L and Severi P Stratigrafia dellrsquoareainteressata dai fenomeni di liquefazione durante il terremotodellrsquoEmilia (Maggio 2012) 31th GNGTS Trieste Italy 20-22November 2012 119-126 available athttpwww2ogstriesteitgngts(last access October 2013) 2012

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

734 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 3ERT1 and ERT2 carried out in the northeastern sector of San Carlo (see Fig 1b for the location of ERT)

the higher signal-to-noise (sn) ratio a larger investigationdepth and a better sensitivity pattern to both horizontal andvertical changes in the subsurface resistivity The WennerndashSchlumberger array provided the best result In all cases theroot mean square (RMS) error is less than 70 and the re-sistivity values range from 5 to more than 127m

Generally since the electrical resistivity of a rock is con-trolled by different factors (water content porosity clay con-tent etc) there are wide ranges in resistivity for any par-ticular rock type and accordingly resistivity values can-not be directly interpreted in terms of lithology In addi-tion the interpretation of resistivity data can be ambigu-ous due to well-known principles of equivalence and sup-pression (Kunetz 1966) For these reasons we used datagathered through geological surveys and exploratory bore-holes to calibrate the ERT and to directly correlate resistivityvalues with the lithostratigraphic characteristics Thus thehigher resistivity values (gt 15m) are associated with theFluvial Channel Unit (FCU) the medium resistivity values(13ndash40m) to the Pleistocene Alluvial Plain Unit (PAPU)

and the lower resistivity values (lt 20m) are related to theMarshes Unit (MU) In particular the FCU is characterizedby paleo-riverbank deposits (20ndash80m) paleo-riverbed de-posits (from 50 to more than 128m) anthropogenic back-fills and reworked material (20ndash50m) and at the bottomby lenses of medium to fine gray sands (MFGS) (15ndash25m)

31 San Carlo

ERT1 and ERT2 were carried out in the northeast sector ofSan Carlo (Fig 1b) The first one was performed across thepaleo-bed of the Reno River whereas the second one wascarried out along the paleo-bank of the Reno River The twoprofiles cross each other at 90 At the intersection both elec-trical resistivity models (ERT1 and ERT2) show the same re-sistivity distribution (Fig 3) This is evidence of the goodquality of data as ERT1 and ERT2 were acquired and pro-cessed independently

ERT3 and ERT4 were carried out across the paleo-bed ofthe Reno River in the western part of San Carlo and nearthe cemetery respectively (Fig 1b) In particular the south-

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 735

Fig 4ERT3 and ERT4 carried out in the western part of San Carlo and near the cemetery respectively (see Fig 1b for the location of ERT)

ern part of the ERT3 profile crossed the ldquoRed Zonerdquo between125 and 295 m where extensive liquefaction and fractures af-fected buildings pipeline and roads causing severe damage(Fig 4)

Generally all electrical resistivity models put into evi-dence three plane-parallel layers with different thickness andresistivity values In agreement with borehole data (Fig 2)and geological observation (Fig 1b) the surficial layer(thicknesslt 20 m) of higher resistivity values (gt 15m)can be attributed to the FCU the intermediate layer (averagethickness about 15 m) of lower resistivity values (lt 20m)can be associated prevalently with the MU and the lowerlayer of medium resistivity values (13ndash40m) can be re-lated to the PAPU Based on the comparison between theexploratory borehole data and the ERT we noted an over-lap of the resistivity ranges The resistivity values includedbetween 15 and 20m are common to the MFGS (bottom ofthe FCU) and MU whereas the resistivity values between 13and 20m can be associated with both MU and PAPU Forthis reason we delineated sectors T1 (diagonal line pattern)and T2 (dotted pattern) to represent the uncertainty of theFCU-MU and MU-PAPU boundary locations respectively(Figs 3ndash5) In particular since sector T1 is related only tothe MGFS and MU we can speculate that T1 also highlightsthe sector where it is possible to find the MGFS

32 Mirabello

ERT5 runs NWndashSE across the paleo-bed of the Reno River(Fig 1c) It crossed between 90 and 235 m a soccer field anda car park ERT6 can be considered the southeastward con-tinuation of ERT5 (see Fig 1c) The ERT5 and ERT6 pro-files were not combined in a single long profile by applying

a roll-along acquisition method due to logistical conditions(presence of a main road) In this case there are no bore-hole data to constrain the interpretation of electrical modelsHowever it was possible to interpret both resistivity modelson the basis of the data coming from superficial geologicalobservation

Considering both resistivity models (Fig 5) from 65 to300 m between about 5 and 17 m asl it is possible to ob-serve a high-resistivity sector (gt 15m) that could be asso-ciated with the FCU Between about 5 and 0 m asl both re-sistivity models show a low-resistivity layer (lt 20m) thatcould be associated with the MU At the bottom (below about0 m asl) a medium-resistivity sector (13ndash40m) could berelated to the PAPU Also in this case we indicated the un-certainty of the FCU-MU and MU-PAPU boundaries usingsectors T1 and T2 respectively

4 Conclusions

A few days after the main shock of the 20 May 2012 Emilia-Romagna (northern Italy) earthquake (Mw= 61) we per-formed an ERT survey to investigate the subsurface sur-rounding some of the major superficial manifestations ofcoseismic liquefaction in the urban areas of San Carlo andMirabello The ERT were used as a reconnaissance method todetect the litostratigraphic characteristics in areas with lim-ited or nonexistent subsoil data

In particular this paper documents one of thefirst applications of the ERT method in investigatingcoseismic liquefaction

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

736 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 5ERT5 and ERT6 carried out in Mirabello (see Fig 1c for the location of ERT)

In all cases we used surface geological surveys ex-ploratory boreholes and aerial photo interpretations to cali-brate the electrical resistivity models and to directly corre-late resistivity values with the lithostratigraphic characteris-tics We have also taken into account the results obtained byAbu Zeid et al (2012) who also investigated coseismic liq-uefaction only in San Carlo a few weeks after the main seis-mic event of 20 May 2012 By comparing and matching allthese data we succeed in imaging the lithostratigraphic set-ting across the superficial manifestations of coseismic lique-faction Summarizing our investigations put into evidence

ndash a surficial layer with higher resistivity values(gt 15m) related to FCU (thicknesslt 20 m)

ndash an intermediate layer with lower resistivity values(lt 20m) related to MU (average thickness 10ndash15 m)

ndash a lower layer with lowndashmedium resistivity values (13ndash40m) related to PAPU

Furthermore we highlight sector T1 where it is possible tofind the MFGS responsible for the coseismic surface effects

In conclusion taking into account all the above inferencesthe ERT has proved to be an effective technique for obtain-ing rapid and valuable geological information on the upper-most part of the subsoil affected by the coseismic liquefac-tion (eg the lithostratigraphic setting the sectors in whichit is possible to find the MFGS and the possible associatedliquefaction phenomena etc) Thus especially in areas withlimited or nonexistent subsoil data we think that the ERTcan provide valuable data for designing further investigations(eg complementary geophysical surveys drill holes etc) in

order to better understanding the liquefaction phenomenonand assessing the associated hazard

AcknowledgementsThis work was funded by the Departmentof Civil Protection Rome We also thank all the working groupinvolved in the study of liquefaction phenomena induced by theMay 2012 Emilia-Romagna earthquake for the discussions duringthe advancement of the work

Edited by O KatzReviewed by three anonymous referees

References

Abu Zeid N Bignardi S Caputo R Santarato G and StefaniM Electrical resistivity tomography investigation of coseismicliquefaction and fracturing at San Carlo Ferrara Province ItalyAnn Geophys-Italy 55 713ndash716 2012

Boncio P Pizzi A Cavuoto G Mancini M Piacentini TMiccadei E Cavinato GP Piscitelli S Giocoli A FerrettiG De Ferrari R Gallipoli R Mucciarelli M Di Fiore VNaso G Giaccio B Moscatelli M Spadoni M Romano GFranceschini A Spallarossa D Pasta M Pavan M ScafidiD Barani S Eva C Pergalani F Compagnoni M CampeaT Di Berardino GR Mancini T Marino A MontefalconeR and Mosca F Geological and geophysical characterizationof the Paganica ndash San Gregorio area after the April 6 2009LrsquoAquila earthquake (Mw 63 central Italy) implications for siteresponse B Geofis Teor Appl 52 491ndash512 2011

Calabrese L Martelli L and Severi P Stratigrafia dellrsquoareainteressata dai fenomeni di liquefazione durante il terremotodellrsquoEmilia (Maggio 2012) 31th GNGTS Trieste Italy 20-22November 2012 119-126 available athttpwww2ogstriesteitgngts(last access October 2013) 2012

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 735

Fig 4ERT3 and ERT4 carried out in the western part of San Carlo and near the cemetery respectively (see Fig 1b for the location of ERT)

ern part of the ERT3 profile crossed the ldquoRed Zonerdquo between125 and 295 m where extensive liquefaction and fractures af-fected buildings pipeline and roads causing severe damage(Fig 4)

Generally all electrical resistivity models put into evi-dence three plane-parallel layers with different thickness andresistivity values In agreement with borehole data (Fig 2)and geological observation (Fig 1b) the surficial layer(thicknesslt 20 m) of higher resistivity values (gt 15m)can be attributed to the FCU the intermediate layer (averagethickness about 15 m) of lower resistivity values (lt 20m)can be associated prevalently with the MU and the lowerlayer of medium resistivity values (13ndash40m) can be re-lated to the PAPU Based on the comparison between theexploratory borehole data and the ERT we noted an over-lap of the resistivity ranges The resistivity values includedbetween 15 and 20m are common to the MFGS (bottom ofthe FCU) and MU whereas the resistivity values between 13and 20m can be associated with both MU and PAPU Forthis reason we delineated sectors T1 (diagonal line pattern)and T2 (dotted pattern) to represent the uncertainty of theFCU-MU and MU-PAPU boundary locations respectively(Figs 3ndash5) In particular since sector T1 is related only tothe MGFS and MU we can speculate that T1 also highlightsthe sector where it is possible to find the MGFS

32 Mirabello

ERT5 runs NWndashSE across the paleo-bed of the Reno River(Fig 1c) It crossed between 90 and 235 m a soccer field anda car park ERT6 can be considered the southeastward con-tinuation of ERT5 (see Fig 1c) The ERT5 and ERT6 pro-files were not combined in a single long profile by applying

a roll-along acquisition method due to logistical conditions(presence of a main road) In this case there are no bore-hole data to constrain the interpretation of electrical modelsHowever it was possible to interpret both resistivity modelson the basis of the data coming from superficial geologicalobservation

Considering both resistivity models (Fig 5) from 65 to300 m between about 5 and 17 m asl it is possible to ob-serve a high-resistivity sector (gt 15m) that could be asso-ciated with the FCU Between about 5 and 0 m asl both re-sistivity models show a low-resistivity layer (lt 20m) thatcould be associated with the MU At the bottom (below about0 m asl) a medium-resistivity sector (13ndash40m) could berelated to the PAPU Also in this case we indicated the un-certainty of the FCU-MU and MU-PAPU boundaries usingsectors T1 and T2 respectively

4 Conclusions

A few days after the main shock of the 20 May 2012 Emilia-Romagna (northern Italy) earthquake (Mw= 61) we per-formed an ERT survey to investigate the subsurface sur-rounding some of the major superficial manifestations ofcoseismic liquefaction in the urban areas of San Carlo andMirabello The ERT were used as a reconnaissance method todetect the litostratigraphic characteristics in areas with lim-ited or nonexistent subsoil data

In particular this paper documents one of thefirst applications of the ERT method in investigatingcoseismic liquefaction

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014

736 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 5ERT5 and ERT6 carried out in Mirabello (see Fig 1c for the location of ERT)

In all cases we used surface geological surveys ex-ploratory boreholes and aerial photo interpretations to cali-brate the electrical resistivity models and to directly corre-late resistivity values with the lithostratigraphic characteris-tics We have also taken into account the results obtained byAbu Zeid et al (2012) who also investigated coseismic liq-uefaction only in San Carlo a few weeks after the main seis-mic event of 20 May 2012 By comparing and matching allthese data we succeed in imaging the lithostratigraphic set-ting across the superficial manifestations of coseismic lique-faction Summarizing our investigations put into evidence

ndash a surficial layer with higher resistivity values(gt 15m) related to FCU (thicknesslt 20 m)

ndash an intermediate layer with lower resistivity values(lt 20m) related to MU (average thickness 10ndash15 m)

ndash a lower layer with lowndashmedium resistivity values (13ndash40m) related to PAPU

Furthermore we highlight sector T1 where it is possible tofind the MFGS responsible for the coseismic surface effects

In conclusion taking into account all the above inferencesthe ERT has proved to be an effective technique for obtain-ing rapid and valuable geological information on the upper-most part of the subsoil affected by the coseismic liquefac-tion (eg the lithostratigraphic setting the sectors in whichit is possible to find the MFGS and the possible associatedliquefaction phenomena etc) Thus especially in areas withlimited or nonexistent subsoil data we think that the ERTcan provide valuable data for designing further investigations(eg complementary geophysical surveys drill holes etc) in

order to better understanding the liquefaction phenomenonand assessing the associated hazard

AcknowledgementsThis work was funded by the Departmentof Civil Protection Rome We also thank all the working groupinvolved in the study of liquefaction phenomena induced by theMay 2012 Emilia-Romagna earthquake for the discussions duringthe advancement of the work

Edited by O KatzReviewed by three anonymous referees

References

Abu Zeid N Bignardi S Caputo R Santarato G and StefaniM Electrical resistivity tomography investigation of coseismicliquefaction and fracturing at San Carlo Ferrara Province ItalyAnn Geophys-Italy 55 713ndash716 2012

Boncio P Pizzi A Cavuoto G Mancini M Piacentini TMiccadei E Cavinato GP Piscitelli S Giocoli A FerrettiG De Ferrari R Gallipoli R Mucciarelli M Di Fiore VNaso G Giaccio B Moscatelli M Spadoni M Romano GFranceschini A Spallarossa D Pasta M Pavan M ScafidiD Barani S Eva C Pergalani F Compagnoni M CampeaT Di Berardino GR Mancini T Marino A MontefalconeR and Mosca F Geological and geophysical characterizationof the Paganica ndash San Gregorio area after the April 6 2009LrsquoAquila earthquake (Mw 63 central Italy) implications for siteresponse B Geofis Teor Appl 52 491ndash512 2011

Calabrese L Martelli L and Severi P Stratigrafia dellrsquoareainteressata dai fenomeni di liquefazione durante il terremotodellrsquoEmilia (Maggio 2012) 31th GNGTS Trieste Italy 20-22November 2012 119-126 available athttpwww2ogstriesteitgngts(last access October 2013) 2012

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

736 A Giocoli et al Electrical resistivity tomography for studying liquefaction

Fig 5ERT5 and ERT6 carried out in Mirabello (see Fig 1c for the location of ERT)

In all cases we used surface geological surveys ex-ploratory boreholes and aerial photo interpretations to cali-brate the electrical resistivity models and to directly corre-late resistivity values with the lithostratigraphic characteris-tics We have also taken into account the results obtained byAbu Zeid et al (2012) who also investigated coseismic liq-uefaction only in San Carlo a few weeks after the main seis-mic event of 20 May 2012 By comparing and matching allthese data we succeed in imaging the lithostratigraphic set-ting across the superficial manifestations of coseismic lique-faction Summarizing our investigations put into evidence

ndash a surficial layer with higher resistivity values(gt 15m) related to FCU (thicknesslt 20 m)

ndash an intermediate layer with lower resistivity values(lt 20m) related to MU (average thickness 10ndash15 m)

ndash a lower layer with lowndashmedium resistivity values (13ndash40m) related to PAPU

Furthermore we highlight sector T1 where it is possible tofind the MFGS responsible for the coseismic surface effects

In conclusion taking into account all the above inferencesthe ERT has proved to be an effective technique for obtain-ing rapid and valuable geological information on the upper-most part of the subsoil affected by the coseismic liquefac-tion (eg the lithostratigraphic setting the sectors in whichit is possible to find the MFGS and the possible associatedliquefaction phenomena etc) Thus especially in areas withlimited or nonexistent subsoil data we think that the ERTcan provide valuable data for designing further investigations(eg complementary geophysical surveys drill holes etc) in

order to better understanding the liquefaction phenomenonand assessing the associated hazard

AcknowledgementsThis work was funded by the Departmentof Civil Protection Rome We also thank all the working groupinvolved in the study of liquefaction phenomena induced by theMay 2012 Emilia-Romagna earthquake for the discussions duringthe advancement of the work

Edited by O KatzReviewed by three anonymous referees

References

Abu Zeid N Bignardi S Caputo R Santarato G and StefaniM Electrical resistivity tomography investigation of coseismicliquefaction and fracturing at San Carlo Ferrara Province ItalyAnn Geophys-Italy 55 713ndash716 2012

Boncio P Pizzi A Cavuoto G Mancini M Piacentini TMiccadei E Cavinato GP Piscitelli S Giocoli A FerrettiG De Ferrari R Gallipoli R Mucciarelli M Di Fiore VNaso G Giaccio B Moscatelli M Spadoni M Romano GFranceschini A Spallarossa D Pasta M Pavan M ScafidiD Barani S Eva C Pergalani F Compagnoni M CampeaT Di Berardino GR Mancini T Marino A MontefalconeR and Mosca F Geological and geophysical characterizationof the Paganica ndash San Gregorio area after the April 6 2009LrsquoAquila earthquake (Mw 63 central Italy) implications for siteresponse B Geofis Teor Appl 52 491ndash512 2011

Calabrese L Martelli L and Severi P Stratigrafia dellrsquoareainteressata dai fenomeni di liquefazione durante il terremotodellrsquoEmilia (Maggio 2012) 31th GNGTS Trieste Italy 20-22November 2012 119-126 available athttpwww2ogstriesteitgngts(last access October 2013) 2012

Nat Hazards Earth Syst Sci 14 731ndash737 2014 wwwnat-hazards-earth-syst-scinet147312014

A Giocoli et al Electrical resistivity tomography for studying liquefaction 737

CNR-PFG (Progetto Finalizzato Geodinamica) Syntheticstructural-kinematic map of Italy in Structural Model ofItaly and Gravity Map Quaderni de la Ricerca ScientificaSELCA Firenze Sheet n 5 1991

Finizola A Ricci T Deiana R Barde Cabusson S RossiM Fraticelli N Giocoli A Romano G Delcher E SuskiB Revil A Menny P Di Gangi F Letort J Peltier AVillasante-Marcos V Douillet G Avard G and Lelli MAdventive hydrothermal circulation on Stromboli volcano (Ae-olian Islands Italy) revealed by geophysical and geochemicalapproaches implications for general fluid flow models on vol-canoes J Volcanol Geoth Res 196 111ndash119 2010

Galli P Bosi V Piscitelli S Giocoli A and Scionti V LateHolocene earthquakes in southern Apennine s paleoseismologyof the Caggiano fault Int J Earth Sci 95 855ndash870 2006

Galli P Castenetto S and Peronace E The MCS macroseismicsurvey of the Emilia 2012 earthquakes Ann Geophys-Italy 55663ndash672 2012

Galli PAC Giocoli A Peronace E Piscitelli S Quadrio Band Bellanova J Integrated near surface geophysics across theactive Mount Marzano Fault System (southern Italy) seismo-genic hints Int J Earth Sci 103 315ndash325 2014

Giocoli A Magrigrave C Piscitelli S Rizzo E Siniscalchi A Bur-rato P Vannoli P Basso C and Di Nocera S Electrical Re-sistivity Tomography Investigations in the Ufita Valley (SouthernItaly) Ann Geophys-Italy 51 213ndash223 2008

Giocoli A Galli P Giaccio B Lapenna V Messina P Per-onace E Piscitelli S and Romano G Electrical Resistiv-ity Tomography across the Paganica-San Demetrio fault sys-tem (April 2009 LrsquoAquila earthquake Mw 63) B Geofis TeorAppl 52 457ndash469 2011

Improta L Ferranti L De Martini PM Piscitelli S Bruno PPBurrato P Civico R Giocoli A Iorio M Drsquoaddezio G andMaschio L Detecting young slow-slipping active faults by ge-ologic and multidisciplinary high-resolution geophysical inves-tigations A case study from the Apennine seismic belt Italy JGeophys Res 115 B11307 doi1010292010JB000871 2010

Jinguuji M Toprak S and Kunimatsu S Visualization techniquefor liquefaction process in chamber experiments by using electri-cal resistivity monitoring Soil Dyn Earthq Eng 27 191ndash1992007

Kunetz G Principles of Direct Current Resistivity ProspectingGebruder Borntraeger Berlin-Nikolassee 103 pp 1966

Loke M H Tutorial 2-D and 3-D electrical imaging surveysavailable at httpwwwgeoelectricalcom(last access June2013) 2001

Moscatelli M Pagliaroli A Mancini M Stigliano F CavuotoG Simionato M Peronace E Quadrio B Tommasi P Cav-inato GP Di Fiore V Angelino A Lanzo G Piro S Za-muner D Di Luzio E Piscitelli S Giocoli A Perrone ARizzo E Romano G Naso G Castenetto S Corazza AMarcucci S Cecchi R and Petrangeli P Integrated subsoilmodel for seismic microzonation in the Central ArchaeologicalArea of Rome (Italy) Disaster Advances 5 109ndash124 2012

Mucciarelli M Bianca M Ditommaso R Vona M GallipoliMR Giocoli A Piscitelli S Rizzo E and Picozzi M Pe-culiar earthquake damage on a reinforced concrete building inSan Gregorio (LrsquoAquila Italy) site effects or building defectsB Earthq Eng 9 825ndash840 2011

Perrone A Iannuzzi A Lapenna V Lorenzo P Piscitelli SRizzo E and Sdao F High-resolution electrical imaging of theVarco drsquoIzzo earthflow (southern Italy) J Appl Geophys 5617ndash29 2004

QRCMT Quick Regional Centroid Moment Tensor INGV-Bologna available athttpautorcmtboingvitquickshtml|(last access June 2013) 2012

Siniscalchi A Tripaldi S Neri M Giammanco S PiscitelliS Balasco M Behncke B Magrigrave C Naudet V and RizzoE Insights into fluid circulation across the Pernicana Fault (MtEtna Italy) and implications for flank instability J VolcanolGeoth Res 193 137ndash142 2010

Youd T L Idriss I M Andrus R D Arango I Castro GChristian J T Dobry R Liam Finn W D Harder L FHynes M E Ishihara K Koester J P Liao S S Marcu-son W F Martin G R Mitchell J K Moriwaki Y PowerM S Robertson P K Seed R B and Stoke K H Liquefac-tion resistance of Soils summary report from the 1996 NCEERand 1998 NCEERNSF workshops on evaluation of liquefactionresistance of soils J Geotech Geoenviron 127 817ndash833 2001

wwwnat-hazards-earth-syst-scinet147312014 Nat Hazards Earth Syst Sci 14 731ndash737 2014