Microsoft Word - 03_Galli_LM05.doc Available online http:/amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 25 (2), 2012, 105-123 MAY 2012 EMILIA EARTHQUAKES (MW 6, NORTHERN ITALY): MACROSEISMIC EFFECTS DISTRIBUTION AND SEISMOTECTONIC IMPLICATIONS Paolo Galli1, Sergio Castenetto1, Edoardo Peronace2 1 Dipartimento della Protezione Civile, Roma, (Italy) 2 CNR-IGAG, AR Montelibretti, Roma (Italy) Corresponding author: P. Galli ABSTRACT: We have carried out the macroseismic survey of both the May 20 (Mw 6.1) and May 29 (Mw 6.0) earthquakes in Emilia (Po Plain, northern Italy) by applying the Mercalli-Cancani-Sieberg scale, on 190 localities, mainly spread south of the Po River. Our data account for an Io 7 MCS evaluated for both earthquakes, with an Imax 7-8 assigned to the village of Rovereto after the mainshock of May 29. Damages were mainly focused on the historical buildings of the region, such as churches, castles, and towers, beside on the industrial warehouses, the collapse of which caused most of the victims of the two mainshocks. Churches and bell-towers were hit also in villages located far from the mesoseismic area, resulting often the only buildings affected by the earthquake. The seismic shaking in- duced also hundred of liquefaction phenomena that, in some places, have increased the damage level to buildings. By comparing macroseismic and instrumental data (i.e., hypocentres distribution, focal mechanisms, interferograms), to the deep geological structures of the region, it emerges that the May 20 mainshock was caused by a segment of the outer Ferrara thrusts front, whereas the May 29 mainshock by the Mirandola thrust. As these thrusts have been seismically silent in the past millennium, the current sequence fills par- tially a seismic gap along the seismogenetic structures of the buried outer Apenninic front. Keywords: Macroseismic survey, 2012 Emilia earthquake, Po Plain, Buried thrusts, Active tectonics. 1. INTRODUCTION At dawn of May 20, 2012, a moderate earthquake (Mw 6.1; QRCMT, 2012) hit a wide area of the eastern Po Plain, awakening most of the inhabitants of northern Italy. The mainshock was preceded in the night by a Mw 4.3 foreshock and it has been followed by dozens after- shocks with Ml>4 and at least 12 with MI≥4.5 (Tab. 1). In the morning of May 29, a second mainshock (Mw 6.0; QRCMT, 2012) struck roughly the same villages, caus- ing, if possible, more panic in the population, scaring and awakening people from Florence and Pisa in the Apen- nines, to Venice and Milan in northern Italy. Unfortunate- ly, due to this event, the death toll rose from 9 to 27, with most of the victims caused by the collapse of the indus- trial warehouses, as it was already happened during the first shock. Earthquakes are an unexpected event in this region, as testified by the lack of local epicentres in the seismic catalogues and by the consequent low level of the seismic classification (seismic zone 3: 10% probabil- ity of exceeding ag ≤0.15 in 50 years; ag, maximum horizontal acceleration; DPC, 2012). Apart from the warehouses and hundreds of old, crumbling farmsteads spread all over the open country, the heavy damages focused on historical buildings, such as churches, bell-towers, castles, towers and pal- aces that were severely hit also in villages located very far from the epicentre, outside the mesoseismic area (i.e., 6 MCS). On the other hand, residential buildings suffered only light and/or moderate damage, apart from some exceptional cases. The mesoseismic area is roughly WNW-ESE elon- gated, reflecting the geometry, kinematics and rupture dynamics of the seismogenetic structure, which is sup- posed to be a complex structural envelope of buried, ~N- verging, “Apenninic” thrusts, as testified by all the focal mechanisms of the sequence (QRCMT, 2012), and by the available oil-exploration seismic profiles (Pieri and Groppi, 1981; Cassano et al., 1986). Both mainshocks have been accompanied by an impressive amount of liquefaction phenomena (i.e., sand blows, sand volca- noes, surface breaks and lateral spread) which, in some case, caused also severe damage to buildings, pipelines and roads. In order to provide the Civil Protection Depart- ment of Italy (DPC) with an expeditious, scientific and homogeneous pattern of the damaged localities, the macroseismic survey started few hours after the May 20 mainshock, proceeding continuatively until June 15. As in all the previous Italian earthquakes, it has been car- ried out by using the Mercalli-Cancani-Sieberg scale (MCS; Sieberg, 1930) accordingly to the methodology proposed by Molin (2003; 2009). We succeeded in visit- ing 52 localities affected by the May 20 mainshock be- fore the second event of May 29, with a total of 190 vis- ited by the end of the survey, and dozens villages visit- ed two-three times, because of the progressive growth of damage during the sequence. In this paper we present the results of this survey, together with the intensity datapoints distribution of both the May 20 mainshock and the cumulated distribution of May 20 and 29 earthquakes, hypothesizing also their Galli P. et al. 106 Fig. 1 - Structural map of the area affected by the 2012 seismic sequence (yellow dotted lines, buried thrusts, from Bigi et al., 1992; dashed line, front of Ferrara thrusts: FFT. Bold dots, front of Cavone-Mirandola thrusts. Circles, 2012 earthquakes, proportional to Ml>2.0. White, 19-28 May sequence; red, 29 May-5 June sequence; orange, up to July 5. ISIDe, 2012). Focal mechanisms from QRCMT (2012) and TDMT (2012). Black and white arrows are GPS horizontal coseismic offsets measured after the May 20 event, ac- counting for ~4 cm of shortening between Sermide (northernmost arrow) and San Giovanni in Persiceto (southernmost arrow; Anderlini et al., 2012). In background the differential interferogram elaborated by TRE (2012) from descending Radarsat satellite data (see line of sight color-scale: LOS. May 12-June 6), which magnificently shows the two “anticlines” grown on May 20 (~14 cm) and 29 (~11 cm), respectively. Sections A-A’ and B-B’ are in Figs. 2-3, respectively. Tab. 1 - Events with Ml≥4.5 recorded between May 20 and June 15 within the epicentral area of the Emilia earthquake of May 2012 (ISIDe, 2012). The last column provides the epicentral intensity (MCS) assigned to the two mainshocks in this paper. May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 107 possible seismotectonic framework. Moreover, we pro- vide a brief description of the liquefaction phenomena and of their areal occurrence.  2. SEISMOTECTONIC FRAMEWORK OF THE MAY 2012 SEQUENCE The villages struck by the May earthquakes are all founded in the Holocene-historical alluvial plain de- posits of the Po River and of its dextral tributaries, main- ly the Secchia, Panaro and Reno rivers, at elevation ranging 10-30 m a.s.l. In this part of the Plain, the soft post-Roman deposits (4th-6th cent. AD to Present) have a thickness ranging 5-10 m, depending from the pres- ence of fluvial ridges or paleo-riverbeds, whereas the post Last Glacial Maximum-Holocene fluvial deposits reaches generally 20 m (“Subsintema di Ravenna”, post 15-17 kyr BP calibrated age; CGI, 2009). Since the Fifties, ENI-AGIP hydrocarbon explora- tion led to the discovery of a buried, imbricated fold and thrust system up to 40 km north of the outcropping Ap- enninic frontal margin (Pieri and Groppi, 1981), below the Quaternary deposits of the Po Valley) (see dotted and dashed yellow lines in Fig. 1). The thrust belt has developed during Neogene-Quaternary in the hang- ingwall of a westward subduction zone (Spakman, 1990), the eastward hinge-retreat of which probably caused the foreland flexure, and the consequent pro- gressive thrusts front migration toward the eastward- moving foredeep basins (Patacca et al., 1990). In the investigated area, the buried Apenninic chain front matches with the Emilian and Ferrara Folds and Thrusts, which are generally sealed by thick Pleisto- cene-Holocene flat layers (Figs. 2-3). According to Castellarin (2001, and reference Fig. 2 - Geological cross section across the Cavone-Mirandola and outer Ferrara folds and thrusts front (modified from Carminati et al., 2010), showing the hypocentral distribution of the May 19-28 seismic sequence (events within 10 km from the section trace A-A’ in Fig. 1. ISIDe, 2012). The Mw 6.1 mainshock and the successive sequence lay entirely at depth along the Ferrara thrust front. q, Quater- nary; mup, Middle-Upper Pliocene; lp, Lower Pliocene; ucm, Upper Cretaceous-Miocene; tlc, Triassic-Lower Cretaceous. Fig. 3 - Geological cross section across the Cavone-Mirandola and outer Ferrara folds and thrusts front (modified from Pieri and Groppi, 1981), showing the hypocentral distribution of the May 29-June 05 seismic sequence (events within 10 km away from the section trace B-B’ in Fig. 1. ISIDe, 2012). The Mw 6.0 mainshock and the successive sequence lay, in this case, along the Mirandola anticline frontal thrust and/or over the sole thrust. q, Quaternary; mup, Middle-Upper Pliocene; plio, Pliocene; um, Upper Miocene; mm, Middle Miocene; mio, Miocene; mz, Mesozoic. Galli P. et al. 108 therein), the structural growing at the front of both the buried thrusts and of the outcropping chain ceased with the Pleistocene, i.e. it was certainly active during the Late Pliocene-Early Pleistocene, with some structures growing also in middle to late Pleistocene times (e.g., Mirandola anticline; Scrocca et al., 2007). Indeed, ac- cording to Pierdominici and Heidbach (2012), in the ar- ea struck by the May 2012 sequence the present-day stress data records exhibit a constant NNE–SSW com- pression regime. This is also evidenced by the geodetic velocity solutions obtained from the analyses of GPS data (Serpelloni et al., 2005), accounting for ~0.8 mm/yr crustal shortening oriented along the same direction. The May 2012 earthquakes confirmed this tectonic framework, as both the mainshocks and all the major aftershocks have a reverse faulting focal mechanism, striking roughly WNW-ESE – i.e. paralleling the different thrust system of the area (Fig. 1) – and hypocentral depth within the upper 10 km of crust (Figs. 2-3). Short- ening due to May 20 event has been evaluated by GPS data analyses (Anderlini et al., 2012) at ~4 cm along a NNW-SSE direction, between the villages of Sermide (to the north) and San Giovanni in Persiceto (to the south; see bold arrows in Fig. 1). Moreover, the differential de- scending interferogram obtained by TRE (2012, Radar- sat satellite) for both mainshocks (interval May 12-June 6) vividly represents the snapshot of the coseismic growing anticlines at surface (Fig. 1). Here the May 20 anticline matches with the hangingwall of the outer Fer- rara thrusts (Fig. 2; see white epicentres in Fig. 1), and it is crossed just in the middle by the Panaro River where, according to the number of visible fringes (each one of 2.8 cm), it reaches the maximum uplift of ~14 cm. On the other hand, the May 29 anticline fits with the hang- ingwall of the Mirandola thrust (Fig. 3. See red epicen- tres in Fig. 1), with an uplift of ~11 cm, as inferable by the number of fringes. Really, according to Scrocca et al. (2007), the structural and stratigraphical setting of this thrust-related fold suggests a syn-sedimentary growth also during the Middle-Late Pleistocene, with a decreasing rate of tec- tonic uplift during the last 1.4 Ma from 0.53 mm/yr to 0.16 mm/yr in the Late Pleistocene, which could be taken as a conservative vertical slip rate also for the Present. As far as the historical seismicity of this region is concerned, according to all the available seismic compi- lations the area has not been affected by events with epicentral intensity Io>6 MCS (Mercalli-Cancani- Sieberg; Sieberg, 1930). This can be easily seen from Figure 4 which reports the Io>6 epicentres contained in the Italian seismic catalogue CPTI11 (Rovida et al., 2011), all falling at the eastern and western boundaries of the region hit in 2012. In detail, the strongest event occurred in the vi- cinity of the May 2012 epicentres (~35 km far away), and the only one relevant for the villages struck by the current sequence, is the Ferrara earthquake (November 17, 1570; Io 7-8 MCS, Mw 5.46). Its mesoseismic area Fig. 4 - Distribution of the historical epicentres (CPTI11) within the area hit by the 2012 sequence (stars, instrumental epicentres of May 20 and 29; orange, macroseismic epicentre of the cumulated effects of May 20 and 29 events). Dotted and dashed yellow lines are the buried front of the Cavone-Mirandola and Ferrara folds and thrusts (see Fig. 1). 1570 earthquake-induced liquefactions are from Galli (2000). Note the absence of significant historical epicentres in the investigated area. May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 109 is NW-SE elongated (Postpischl, 1985), paralleling the local buried front of the Ferrara thrusts which are, in turn, the eastern prolongation of those activated in the current sequence. Also in this case, the sequence was characterized by more mainshocks, and it lasted at least until 1572. Among the localities hit by the 2012 events, the 1570-earthquake historical sources account for damages in Bondeno at the roof of the hospital and of the oratory; in Corpo Reno at the roof of Saint Antony church (in Marzola, 1976), while in Cento and Finale Emilia they report just the fall of some chimney (ASM, 1570a; 1570b), suggesting – in the whole – effects of 6 MCS (the Italian macroseismic database DBM11=Locati et al., 2011 reports 7 MCS to Bondeno, 6-7 MCS to Corpo Reno, and 6 MCS to Cento and Finale Emilia). Similarly to what happened in the 2012 events, also in the 1570 earthquake many liquefaction phenomena oc- curred in different localities, mainly surrounding Ferrara, and up to Ficarolo, on the left bank of the Po River (Gal- li, 2000; see Fig. 4). Prior to 1570, a coeval chronicle (Giacomo da Marano, 14th cent.) suggests the existence of another strong earthquake in Ferrara in the year 1346 when, on February 22 “many houses fell down, palaces, towers… and, in the villages, tenements, barns… and other build- ings”. The event was felt also in Modena (even if Gio- vanni da Bazzano, 15th cent., records it on February 8) and likely in other places of the Po Plain. However, due to the scarcity of news, it is impossible to provide certain parameters or to associate it to a seismogenic structure. It could be really occurred everywhere around Ferrara and, for instance, also in the eastern part of the region hit in 2012. DBMI11 reports also two minor earthquakes (Io 6 MCS) within the 2012 mesoseismic area. The first one occurred on December 6, 1986 (Mw 4.35), with effects of 6 MCS in Bondeno, Finale Emilia, Gavello and Scor- tichino; the second one on May 8, 1987 (Mw 4.56), with similar effects in Camposanto, Finale Emilia, Massa Fi- nalese and San Felice sul Panaro. Taking into account their epicentral location (see Fig. 4), both could be relat- ed to the same seismogenetic structure of the 2012 events. It is worth noting that, due to the 1987 event, several fractures opened in the field of Rivara (San Fe- lice sul Panaro), forming en-echelon linear settlements, 10-20 cm wide and 1-2 m deep which were not attribut- ed to any liquefaction phenomena, but to mechanisms associated to sinkhole formation (Bianchi et al., 2008). Finally, on October 15, 1996, at the western boundary of the investigated area, an earthquake with Mw 5.41 (CPTI11. Io 7 MCS: Massucci et al., 1996; De Canini et al., 1997) affected mainly Bagnolo in Piano and Correggio where severe damages were observed to few ancient buildings (i.e., the Torrazzo in Bagnolo and San Francesco church in Correggio), light cracks in many brick-masonry buildings, and also in two rein- forced-concrete (r.c.) frame structure (i.e., cracks in the brick-infill of the pilotis), beside the fall of chimneys and tiles. At that time, the earthquake was associated to the rupture of the left-lateral ramp of the Cavone anticline (De Canini et al., 1997), i.e., the western prolongation of the Mirandola structure. Further north, in 1806 a similar earthquake caused damage in Correggio, and also in Brescello, Campagnolo Emilia, Novellara and Viadana (effects estimated as 7 MCS in ISMES, 1985), and also in other localities hit by the 2012 sequence, as Reggiolo and Carpi (6-7 MCS). From all the above, we can conclude that at least in the past ~600 years (see the discussion of catalogue completeness in Stucchi and Albini, 2000), the epicen- tral area of the 2012 events: 1) has likely never generated earthquakes with energy similar to that associated to the 2012 sequence (i.e., Mw≥ 5.8); 2) has likely never been affected by damage caused by external earthquakes, with the only exception of those related to the 1570 Ferrara event in the east- ernmost portion, and to the 1806 and 1996 events in the westernmost portion; 3) has been sometime affected by local, Mw≤4.6 events, probably generated by the same 2012 seismogenetic structures. Therefore, it is likely that the 2012 mainshocks is the real first, and somewhere the last, anti-seismic test- ing for all the historical buildings of the region. 3. THE MACROSEISMIC SURVEY As aforementioned, the attribution of the site in- tensity (Is) has been achieved by applying the MCS macroseismic scale, that is the intensity scale of the whole Italian macroseismic database (DBMI11; ~86000 datapoints), and not the more recent European macro- seismic scale (EMS; Grünthal, 1998). Indeed, as the MCS scale does not fully account for the vulnerability of each single building, it allows a more expeditious appli- cation during the survey, providing information directly correlated to the damage level, i.e. that of immediate interest to the civil protection purposes, both in terms of rescue planning and emergency management (Molin, 2009; Galli et al., 2009). In particular, we adopted the methodology proposed by Molin (2003; 2009), who dif- ferentiated five damage levels (1-5: light, moderate, se- vere, destruction, collapse), providing the percentages of damage level (5%, 25%, 50%, 75%, 100%) that are representative for each MCS degree (Fig. 5), as implicit- ly contained in the original scale (Sieberg, 1930). Tab. 2 - From left to right, damage progression (levels 1-5) to be adopted for an homogenous application of the MCS scale for Is ≥ 5 (from Molin, 2009). For instance, the MCS 6 degree has to be as- signed to those localities in which the ~50% of buildings presents level 1 damage, ~25% level 2, and where the 5% has bee affected by severe damage (level 3). Anal- Galli P. et al. 110 ogously, the 8 MCS degree is reached when a quarter of the buildings has been destroyed (level 4), including a ~5% of total collapse (level 5), whereas half of the vil- lage presents severe damage (level 3). This means that in a locality with 1000 building, 500 should be severely damaged, 250 of which destroyed and 50 collapsed. 3.1. May 20 earthquake survey The macroseismic survey started in the morning of May 20 from the epicentral area, reaching in the days after localities farther and farther in the provinces of Mo- dena, Ferrara and Mantua, for a total of 52 visited locali- ties belonging to 30 municipalities. Even if necessary incomplete, the results of this first survey have allowed to define the area of the most severe effects (Is≥ 6 MCS), which is WNW-ESE elongated west to the in- strumental epicentre, in full agreement with the focal mechanism of the mainshock (Fig. 5). In detail, the area was extended less than 25 km in this direction, matching entirely the hangingwall of the Fig. 5 - Intensity datapoint distribution of the May 20 mainshock (white circle, proportional to MCS degree). The background image roughly indicates the areal shaking in MCS terms. Dashed line is the interpolated 6 MCS isoseismal; dashed blue line is the buried front of Ferrara thrusts (see Fig. 1) to which the focal mechanisms are referred (TDMT, 2012 and QRCMT, 2012)°. Tab. 3 - Instrumental (left), and macroseismic (right) parameters of the two mainshocks of the Emilia 2012 sequence (INGV source, and this paper). May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 111 outer Ferrara thrusts front (FTT in Figs. 1, 5, and asso- ciated focal mechanisms), i.e. the seismogenetic struc- ture responsible for this part of the sequence. Outside this area, both to the west and to the east, we observed a raising of the intensities, respec- tively in the Moglia area and Sant’Agostino, San Carlo and Mirabello. The maximum intensity (Imax) estimated for this event is 7 MCS, assigned to Mirandola and San Felice sul Panaro (Fig. 5), with Is 6-7 observed at Finale Emilia, Canaletto, Mortizzuolo and San Carlo, and Is≤6 MCS assigned to all the other localities. The epicentral intensi- ty evaluated also through the Boxer4 algorithm (Gaspe- rini et al., 1999) is Io 7 MCS, while the equivalent mo- ment magnitude resulted much more low (Mw 5.1) than the instrumental value (Mw 6.11). The epicentral coordi- nate derived by Boxer4 identify a point located ~5 km south of the instrumental epicentre (Figs. 4, 5). Earth- quake parameters have been summarized in Tab. 3. Most of the heavy damages, including total and partial collapses, affected tall, historical buildings, as churches (Fig. 6), bell-towers, towers, castles (Figs. 7-8), palaces, ancient farmhouses, beside many industrial warehouses in the outskirts of Mirandola and Sant’Ago- stino (Fig. 9). The residential housing estates, both in brick- masonry (reinforced and not) and in r.c., generally suf- fered low grade damage (levels 1-2), with sparse severe damages (level 3) and very rare partial collapses (usual- ly of the roof-ledges, roofs and loading dock of crum- bling houses) and destructions (level 4), beside diffuse fall of chimneys, tiles and plaster, almost everywhere focused within the old downtown. This fact explains why this earthquake, although it has happened in the night, has caused a limited number of victims (9), almost all night-shift workers in the industrial warehouses. Severe damages in r.c. buildings have been observed in one case inside Mirandola and in other three in its northern outskirts (light crushing of pillars and cracks on the brick-curtain walls), as in some apartment houses in Cavezzo (Fig. 10). In some cases, more than the seismic waves, damages have been induced by the huge liquefaction phenomena affecting the deposits below the buildings, with consequent loss of the bearing capacity, and differ- ential settlement and/or tilting of the foundations. Lique- faction occurred extensively in the villages located over the paleo-beds of the main rivers, as in Sant’Agostino, San Carlo and Mirabello, all founded along the aban- doned Reno fluvial-ridge. Here, besides the damages to buildings, the surficial breaks related to the liquefaction settlement processes affected also roads and pipelines. By looking at Figure 5, it emerges that the intensity bulge at the eastern side of the mesoseismic area is merely due to the effects of liquefaction on. In the far, main cities of the Plain, we did not ob- serve severe damage, not even affecting isolated build- ings. In Mantua and Modena everybody has awakened getting into a panic, and rushing out in the streets; how- ever, damages were limited to levels 1-2 to very few houses. Ferrara, in turn, experienced levels 1-2 to sev- eral old houses downtown, mostly related to pre-existing cracks. We have recorded the fall or rotation of a dozen Fig. 6 - View of the Mirandola Cathedral (15th century) in the morning of May 20. At that time, only few architectural elements at the top of the façade were fallen down, beside the incipient façade detachment. Inside, not visible, also part of the roof has fallen down. The church was damaged again, more seriously, by the May 29 mainshock. Fig. 7 - Partial collapse of the Estense castle (15th century) in Finale Emilia (Is 6-7 MCS) in the morning of May 20. Galli P. et al. 112 of chimneys, tiles, and sparse architectural elements hanging on the churches façades, beside the partial collapse of a small tower over the Es- tense Castle. Actually, we had trouble in evaluating the site intensity in many localities, because of the extreme difference of damage level existing be- tween recent residential housing (usually 1-2 storeys, reinforced brick-masonry villas) and the historical-monumental buildings. As mentioned before, while on one hand many churches, cas- tles, bell-towers, towers and ancient palaces experienced severe damage (levels 3-4) or col- lapse (levels 4-5), on the other hand, private houses generally were not damaged at all, es- pecially all those outside the small old town- centre. As in the 2009, Mw 6.3 L’Aquila earth- quake (Galli et al., 2009), we have choose to take more into account the effects recorded by the buildings inside the villages old-centre (al- most all in brick unreinforced masonry), and to exclude entirely damages to industrial ware- houses. In doing this, we think that the assigned intensities can be roughly comparable to those of the historical earthquakes contained in the Italian seismic database (DBMI11). 3.2. May 29 earthquake survey Soon after the second mainshock (Mw 5.96), we re-started the macroseismic survey, visiting again the villages hit by the May 20 event, and extending it during the days after up to 190 localities belonging to 87 municipalities. In the whole, our observations cover the area of six Provinces of Emilia Romagna, Lombardy and Veneto Regions, i.e. 52 of Modena, 32 of Ferrara, 22 of Bologna, 14 of Reggio Emilia, 53 of Mantua and 11 of Rovigo (Fig. 11). Generally speaking, we have observed a growth of the effects in the western part of the area, with an increase of 1-2 MCS degrees in some villages; severe damage in Reggiolo (Is 6- 7 MCS), Novi di Modena (Is 7 MCS), Concordia sulla Secchia (Is 7 MCS; with some partial col- lapse and severe damage to the porch- buildings; Fig. 12), Moglia (Is 7 MCS), and Rov- ereto (Is=Imax 7-8 MCS; maximum intensity assigned in this earthquake) where several par- tial collapses and heavy damages occurred both in the brick-masonry houses of the old-centre and in some recent reinforced-masonry and r.c. buildings (Figs. 13-15). In the other localities, generally west of Mirandola, the intensity in- crease has been < 1 MCS degree. In Mirandola, the mainshock of May 29 and the successive strong aftershock of the same morning (Mw 5.3) caused the collapse of the 14th century Saint Francis church, which was only slightly damaged by the May 20 shock. Moreover, it caused further collapses of the Ca- thedral and several heavy damages and partial collapses to the ancient houses downtown, be- sides to some r.c. buildings, the same already struck on May 20. In Cavezzo also, the increas- ing damage level has yield an higher intensity (at least Is 7 MCS), mainly justified by the col- Fig. 8 - Finale Emilia (Is 6-7 MCS): collapse of the Medieval clock-tower, and of the Cathedral tympanum in the May 20 morning. The tower collapsed defi- nitely during the seismic sequence. Fig. 9 - Collapse of the Sant’Agostino’s ceramics warehouse. Fig. 10 - Left, Cavezzo (Is 6 MCS), May 22: level 3-4 damage along the joint of a r.c. building inside the old-center; this building collapsed entirely because of the mainshock of May 29. Right, Mirandola (Is 7 MCS), May 21: crushing of a pillar of a r.c. building in the old-centre. May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 113 lapse of the pilotis of three r.c. buildings and to the diffu- sion of level 3 damage to several brick-masonry houses in the old-centre. Southward, outside the 6 MCS isoseismal (Fig.11), both Crevalcore and Cento suffered more effects within the old-centre which presents now diffuse level 2 dama- ge at about a quarter of the buildings, with some heavy damage (level 3) and sparse partial collapse of under- roof walls. In turn, the damage area substantially did not extended northward. Anyway, we have assigned Is 5-6 MCS to several localities near the Po River and, excep- tionally, also north of it (Castelmassa). Heavy damage to isolated monumental structures (usually the churches or the bell-towers) have been observed in many villages to which we necessarily assigned Is 5 MCS. Indeed, these were the only buildings damaged by the seismic events that, instead, did not affect the remaining hous- es. However, the intensity datapoints listed in Table 4 keeps the memory of these effects in the column DJ, where labels A-B-C indicate heavy damage and/or col- lapse to buildings, towers/bell-towers and churches. In the whole, the 6 MCS area extended westward, reaching a length of 35 km, i.e. 10 km more then the May 20 event. Also, two extreme intensity bulges exist to west and to east. The first one is now focused on Reggiolo (it was on Moglia, before), whereas the second is always that occupied by San Carlo and Mirabello. Considering also these areas, the 6 MCS is elongated WNW-ESE of ~55 km, with a N-S width of 15-20 km. By observing Fig. 11, and assuming a rough similar vulner- ability in all the surveyed localities, it is probable that the mentioned intensity bulges represents as many cases of geological amplification. In one of these, we have rec- ognized the strong contribution of the liquefaction phe- nomena occurred extensively all along the paleo- channel of the Reno River, while in the others cases, only future geological analyses will hopefully reveal the causes of the seismic shaking increase. Also for this earthquake, our intensity estimate refer mainly to the old-centre of the surveyed localities, as almost every- where the May 29 mainshock struck more these than the modern outskirts (generally affected by sparse 1-2 level damage), apart some impressive exceptions as the one of Rovereto (collapse of the pilotis of a new rein- Fig. 11 - Intensity datapoints distribution of the whole 2012 sequence (June 15 updating; white circles, proportional to intensity). Also in this figure, the colour image in background suggests qualitatively the areal shaking in terms of MCS intensity. Black dashed line is the 6 MCS degree isoseismal as interpolated from the intensity data. Dashed and dotted blue lines are the Ferrara and Cavone-Mirandola thrust fronts, respectively, i.e. the structures responsible for the whole sequence. Black stars are the main events, with associated focal mechanisms (TDMT, 2012 and QRCMT, 2012). The two empty stars are the macroseismic epicentres of the May 20 (east) and of the cumulated May 20-29 mainshocks (west). Note the four intensity bulges located outside the mesoseismic area (Reggiolo, Crevalcore, Cento and San Carlo) which indicate as many possible local amplification areas. Galli P. et al. 114 forced brick-masonry 3-storey villas; Fig. 15) or Fossoli (level 3 damage to the first-second storey of some r.c. buildings). In some villages (e.g., Cento, Concordia, Crevalcore, Moglia, Reggiolo, San Giacomo delle Segnate) damage is diffused in the porch-houses, i.e the typi- cal building style of almost all the main streets of the Po Plain settlements. Here we often observed damage level 2 and 3 affecting the columns and/or the pillars of the arches, with consequent damage of the overlaying wall of the first-second sto- rey. We have tried to “close” the intensity points distribution both southward (Mode- na and Bologna Provinces) and northward (Ferrara, Rovigo and Mantua), identifying several localities affected by light damag- es in the Mantua Province, south to the Po River, with the exception of Castel- massa (left bank of Po River, Rovigo Province), where we have observed sev- eral level 2 cracks to the porch-houses of the old-centre and to the churches and the theatre. On the other hand, we did not find significant damage increase east of San Felice sul Panaro, apart from the two quoted cases of Crevalcore and Cento, at southeast. Fig. 13 - Left, Rovereto (Is=Imax 7-8 MCS): level 3 damage affecting a r.c. building. Right, Mirandola (Is 7 MCS): partial collapse of the under-roof walls of an unreinforced brick-masonry building of the old-centre (photos of May 29). Fig. 12 - Concorda sulla Secchia (Is 7 MCS). Collapse of inhabited, old unreinforced brick-masonry buildings inside the old-centre (photo of June 6). May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 115 Fig. 14 - Cavezzo (Is 7 MCS). Total collapse of the pilotis of a r.c. building in the old-centre (photo of May 29). This building experienced level 2 damage because the May 20 mainshock, and was not inhabited when it crushed on May 29. . Fig. 15 - The impressive collapse of the brick-masonry pilotis of a new building in the outskirts of Rovereto (Is=Imax 7-8 MCS). Galli P. et al. 116 May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 117 Table 4 - MCS intensities (Is) evaluated for the May 20 and for the May 29 (cumulated) Emilia earthquakes. DJ, severe damage to iso- lated buildings (Is<6 MCS). Pop and blds, number of inhabitants and buildings, respectively. Through Table 3, the latter column may suggest the quantity and level of damaged buildings for each locality. Galli P. et al. 118 Finally, it is worth noting that also during the May 29 seismic shaking, several liquefaction phenomena oc- curred, some of them in the same place as May 20, oth- ers in localities that were not previously affected. How- ever, the strength of the phenomenon was certainly lower than the first case, and none further damage has been reported. We have found unreliable estimating the macro- seismic parameters of the only May 29 shock, because of the cumulated effects with the preceding events. Therefore, the epicentral intensity of the cumulated se- quence (mainly May 20 and 29), calculated also through Boxer4 algorithm, is Io 7 MCS, with an equivalent mag- nitude always lower (Mw 5.23) with respect to the in- strumental one (Mw 6.11 + Mw 5.96). The final epicen- tral coordinates (Tab. 3) identify a point located more than 15 km west of both the instrumental and macro- seismic epicentre of May 20, and 5 km west of the in- strumental one of May 29 (Fig. 11). This westward drift of the barycentre of the macroseismic effects with re- spect to the instrumental ones may roughly indicate the rupture direction along the structures responsible at depth for the sequence, i.e. from east to west. 3.3. Liquefaction As aforementioned, both mainshocks - and mainly the first one – have induced hundred liquefaction phe- nomena all over the mesoseismic area. Liquefaction is the transformation of a granular deposit from a solid state into a liquefied state as a consequence of the increased pore-water pressure determined by cyclic shaking (Youd, 1977). The associated features may vary from place to place in geometry, type, and dimension, due to the dif- ferent propagation and amplification of the seismic waves at the surface and to the differing site conditions (grain size and density of deposits, position of the ground-water level). In the eastern Po Plain region, lique- faction processes have already happened, and in par- ticular during the quoted 1570 event, when in Ferrara and in some neighbouring localities (San Paolo bridge, San Pietro church area, Giara del Po, La Punta, Polesino di San Giovanni Battista, Polesino di San Giorgio, Torre della Fossa), up to Ficarolo, on the left bank of Po River, several sand boils, volcanoes and ground settlements occurred (see in Galli, 2000). Indeed, on the basis of its geological properties and seismicity level, this sector of the Po Plain was long since classified as “liquefaction Fig. 16 - Distribution of the liquefactions generated by the 2012 earthquakes vs MCS intensity (empty and black stars, macroseismic and instrumental epicentres, respectively). Almost all the liquefaction cases fall within the 6 MCS area, with the exception of the May 29 clus- ter in Quistello. Note that liquefactions affect systematically the sandy deposits related to the abandoned paleo-bed of the main rivers (gray lines, from Castaldini e Raimondi, 1985), as the case of the paleo-Reno, toward Mirabello, of the paleo-Secchia, toward Cavezzo- San Felice sul Panaro and san Possidonio, or as the case of the abandoned paleo-beds of Po River, further north. May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 119 susceptible zone” (Galli, 1991; Galli and Meloni, 1993; Galli and Ferreli, 1995). Nevertheless, recently, it has been surprisingly defined as scarcely or not prone to liq- uefaction by Fioravante (2008). In May 2012 earthquakes, the most diffuse fea- tures typology (e.g. in Galli, 2000) that we have ob- served can be related both to ground fissuring (“dry” ground fissures; with water emission; water and sand ejection: i.e. sand boils, sand volcanos. Figs. 17-18) and to related ground deformation (linear settlement and lateral spread). All of these occurred in the agricul- tural fields and below buildings, roads and pipeline causing, in the latter cases, from light to severe dam- ages (i.e., differential settlement; fig. 17). The most im- pressive phenomena are surely those happened in the easternmost side of the area, between Sant’Agostino, San Carlo and Mirabello (Fig. 16), all villages built up over the paleo-channel of the Reno River, a fluvial ridge emerging ~5 m above the plain. Here, the liquefaction processes caused huge lateral spread mechanisms all along the fluvial ridge, with hundred meters long fractures arranged in complex en-echelon geometry, with synthetic and antithetic systems (e.g., graben-like features) and offsets reaching some decimetres (Fig. 19). Cracks affected also buildings, walls, pipelines and roads, causing severe damage and throwing over 1-m-thick lay- er of greyish, fine sand (Fig. 20). Water and sand have been ejected from ground fissures, but also from any preferential path met during the upward rising, as wells, shafts, piping systems, manhole, underground structures (car-box, cellars), and even phone booth and toilet bowl. By comparing the areal distribution of the surveyed liquefaction cases (e.g. in Gruppo di Lavoro Liquefazione, 2012) with the MCS in- tensity assigned in this paper (Fig. 16), it emerges that all the cases fall within the 6 MCS degree area, with the only exception of those happened in Quistello (Is 5-6 MCS), near the Po River, whereas all happened less than 20 km from the two epicentres. This is in full agreement with the empirical relationships between Io/Is/Mw and distance for liquefaction (Galli, 2000) that foresee a boundary limit for Mw 6 earthquake of ~40 km (N.B., this limit is respected also assuming the value of the equivalent magnitude calculated through Boxer4 – Mw 5.23 – which yields a distance of 17 km). However, the most striking fact emerging from Figure 16 is that all the liquefaction cases do not match with the maximum intensities; on the other hand, they Fig. 17 - San Carlo: left, differential settlement of a mixed r.c. and masonry building, due to huge liquefaction phenomena below part of the foundation. Right, sand ejected from a water-manhole (photo of May, 20). Fig. 18 - Road San Carlo-Sant’Agostino: sand volcanoes aligned on a ground fissure (photo of May 21). Galli P. et al. 120 Fig. 19 - San Carlo. Coseismic surficial breaks associated to liquefaction (linear ejection of sand and water). Note the echelon geometry of fractures that mimic a graben-like feature (i.e., lateral spreading of the paleo-Reno River ridge). Buildings, roads, walls and pipelenes located above this fractures have been seriously damaged the night of May 20. Fig. 20 - San Carlo. Sand ejection inside a car-box. Remarkably, that the car has been by raised by ~80 cm up to the ceiling of the box due to the powerful push of the liquefied sand below during the night of May 20. This side of the house (see Fig. 17, left) has been se- verely damaged by differential settlement. May 2012 Emilia earthquakes (Mw 6, northern Italy): macroseismic effects … 121 systematically follow the paths of the paleo-river beds (Castaldini and Raimondi, 1985), as the case of the paleo-Reno, toward Mirabello, of the paleo-Secchia, to- ward Cavezzo-San Felice sul Panaro and san Possi- donio, or as the case of the abandoned paleo-beds of Po River, further north (San Martino Spino). Likely, in our opinion, the sandy deposits associated to the depo- sitional facies of river-channel s.s. are dramatically the most liquefaction susceptible among all the other alluvial sediments of the plain. Therefore, as many settlements have been founded along the abandoned fluvial ridges that emerge few meters above the level of the whole Po Plain, this fact might represent a general risk than need to be mitigated by means of appropriate geotechnical tools, both in the 2012 mesoseismic area and elsewhere in the Plain. 4. DISCUSSION AND CONCLUSIONS We applied the MCS scale (Sieberg, 1930) during the survey of the effects induced by the 2012 Emilia seismic sequence to 190 localities around the epicen- tres, in the Provinces of Mantua, Rovigo, Ferrara, Mo- dena and Reggio Emilia. The intensities were estimated by using the methodology proposed by Molin (2009), which roughly quantifies the percentage of buildings af- fected by each of the five level of damage. In this way, as in the 2009 L’Aquila earthquake (see in Galli et al., 2009), we believe to have released a comprehensive intensity datapoints distribution (IDD) of the earthquake which, on one hand, meets the primary purposes of the civil protection intervention (rescue and emergency plan- ning based on real damages, living vulnerability out of consideration), and on the other can be promptly com- pared to the historical earthquakes, i.e. to the IDD con- tained in the Italian seismic database (e.g., DBMI11), all expressed in terms of MCS intensities. In doing this, owing both to the early onset of the MCS survey on May 20 morning, and to its intrinsic ex- peditiousness, we succeed in compiling the IDD also for the first mainshock, which has resulted composed by 52 datapoints (Tab. 4). The highest IDD (HIDD) for both mainshocks fits with the hangingwall of the two seismo- genetic structures that, in turn, have been individuated by matching instrumental (hypocentres, interferograms, focal mechanisms) and geological (maps of the buried structures, seismic reflection profiles) data. As a matter of facts, May 20 HIDD lays entirely south of the outer Ferrara thrusts front, while May 29 HIDD lays south of the Mirandola-Cavone thrust (Figs. 5, 11). Both mainshocks look associated to the frontal thrust rupture of each of the two mentioned structures (Figs. 2-3), even though we can not exclude the involvement of the sole thrust, as suggested also by the fault dip provided by the focal mechanisms (Fig. 1). On the other hand, the macro- seismic epicentre of the cumulated sequence falls west to the instrumental ones, suggesting a rough rupture directivity from east toward west, at least for the May 29 shocks (Fig. 11). Therefore, considering 1) the entire extension of the arched, outer Ferrara thrusts front and of the inter- mediate Cavone-Mirandola buried folds and thrusts (Fig. 11), and 2) the historical seismicity of the same region (Fig. 4), this sequence has partially filled the seismic gap existing between the earthquakes in the Ferrara area to the east (i.e., 1570), and those around Correg- gio to the west (1806, 1996), confirming the supposed residual activity of these buried structures (Galli, 2005). Indeed, this activity has been recently quantified by Scrocca et al. (2007) at 0.15 mm/yr for the Mirandola structure (Late Pleistocene), and this value is consistent with the amount of surface coseismic vertical uplift (11-15 cm) measured by the differential interferograms during the current earthquakes (TRE, 2012). Taking into account the lack of seismic events comparable to the 2012 sequence in the last ~1 kyr, or even hypothesizing that the quoted 1346 earthquake might represent a pos- sible 2012 predecessor (i.e., the literary sources record its effects only for Ferrara, and not for smaller villages), that slip rate could really represent an average value for these structures (11-15 cm every 700-1000 yr yield ~ 0.15 mm/yr), and, in particular, relatable to Mw ≤ 6 earthquakes. Anyway, it is worth noting that this tectonic slip rate, and its related surficial deformation, are from 10 to 50 times lower that the rough sedimentation rate in this region of the Po Plain. By considering the mentioned thickness of the post LGM and post Late Roman depos- its, the sedimentary rates has grown from 1.3 to 6.7 mm/yr in the past thousands years. Thus, at least in the eastern part of the Plain, the supposed diversion of the rivers around the growing anticlines - which some schol- ars claim to be an evidence of active tectonics (Burrato et al., 2003) - seems unreliable. Conversely, it is proba- ble that fluvial paths are here influenced by other geolog- ical/hydrogeological causes that have much higher rates. For instance, the huge general subsidence affecting wide areas of the Po Plain (1-5 mm/yr, with local peaks of 3-4 cm/yr; Bitelli et al., 2010; Carminati et al., 2010), and/or – within this subsidence - the differential rates affecting the areas subtending deep synclinal (e.g., 8000 m of Plio- Quaternary deposits NE of Modena) vs shallow, sub- outcropping anticlines (few hundred meters below Novi di Modena, Concordia, Mirandola, San Felice sul Pa- naro), that have been quantified at 1 mm/yr by Arca and Beretta (1985) for the period 1897-1957 (i.e., -4 mm/yr vs -3 mm/yr). Indeed, from this point of view, the dominant geomorphic processes are not the moderate, 1-kyr spaced earthquakes, but the hundred, often catastrophic alluvial floods occurred continuatively in the past centu- ries in the Po Plain (see in CNR-SICIS, 2012). Living aside this specific issue, we want to evi- dence another aspect linked to the 2012 event. The equivalent magnitude estimated through Boxer4 algo- rithm (i.e., the one on which the whole CPTI11 is based) is, in facts, much lower than the instrumental values. On the basis of our HIDD, we calculated Mw 5.23 vs an in- strumental Mw 6.1 (or Ml 5.8). Probably, if we really had considered only the damage affecting the ancient hous- es in each villages, excluding at all not only the recent outskirts, but also the new buildings inside the old- centres, our intensities would have been likely higher by 0.5-1 degree (N.B., not everywhere! For instance in Cavezzo it would have been lower). Anyway, this in- crease is not yet enough to rise the equivalent magni- tude to the instrumental one. This “anomaly” is evi- denced also by the measured acceleration peaks (e.g., 0.29 g, PGA in Mirandola; 0.23 g, PGA in San Felice sul Panaro and Cento; 0.25 g, PGA in Moglia; acceleromet- Galli P. et al. 122 ric data of DPC-RAN), that would theoretically account for much more damage with respect to those really oc- curred (i.e., according to the existing regression laws, the peak acceleration in Mirandola account for 9 MCS; Faccioli and Cauzzi, 2006). Moreover, until 2004 the area affected by the current seismic sequence was not classified as seismic, and therefore was not subject to compliance with specific technical standards for con- struction. However, as aforementioned, most of the settle- ments have small old-centres with brick-masonry build- ings (where damage mostly focused on high and wide structures), and vast residential outskirts, consisting of low and regular buildings (2-3 storeys), realized with reinforced composite masonry (manufactured bricks) or, rarely, in reinforced concrete. Moreover, even in down- towns, the quality of ancient buildings is higher than that observed in many other areas of Italian territory, such as in Abruzzo (hit by the 2009 earthquake), where it is common to observe houses made with rubble stones or field stones with poor mortar (class of vulnerability A, according to EMS scale). Therefore, the quality of con- structions and, likely, the role played by the deep alluvial deposits on filtering the earthquake frequency contents, might have concurred in mitigating the shaking effects on buildings, despite both the relative high magnitude values and the acceleration peaks. 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