Chapter 5 Geology of the Urban Area of Catania (Eastern Sicily)

Home , Aetna (city), Castello Ursino, Gela

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Chapter 5 Geology of the urban area of Catania (Eastern Sicily)

C. Monaco, S. Catalano, G. De Guidi & L. Tortorici Dipartimento di Scienze della Terra, Università di Catania, Italy

Abstract

The geology of the Catania urban area (Eastern Sicily) is the result of three combined processes related to the Late Quaternary sea-level changes, the volcanic and tectonic processes and finally to human activity. The backbone of the urban area is represented by a flight of marine terraces carved on a sedimentary substratum. This is made up of a Lower-Middle Pleistocene succession of marly clays, up to 600 m thick, upwardly evolving to some tens of metres of coastal sands and fluvial-deltaic conglomerates, referred to as the Middle Pleistocene. The unconformably overlying terraced deposits, of both coastal alluvial and marine origin, are characterized by several metres of thick sands, conglomerates and silty clays. They rest on abrasion platforms distributed at different elevations dating to the sea level high-stands between 200 and 40 kyr. These indicate a strong uplift of the area which can be explained as the effect of the deformation at the footwall of the NNW-SSE trending active normal fault system located at the south-eastern lower slope of Mt. Etna that extends offshore into the Ionian sea. The sedimentary substratum is dissected by deeply entrenched valleys filled with thick lava flows, which represent most of the rocks cropping out in the city. These consist of basaltic lavas which, flowing from NW to SE, invaded the urban area in pre-historical and historical times (e.g. A.D. 252, A.D. 1381, A.D. 1669). In the ancient part of the city the uppermost stratigraphic horizons consist of several metres thick levels of ruins derived by the destruction due to the occurrence of the 1693 earthquake. 1 Introduction

Catania is one of the cities with major seismic hazards in Italy, being located along the Ionian coast of Sicily (Fig. 1), an area prone to high rates of uplift and

84 SEISMIC PREVENTION OF DAMAGE characterized by a high level of crustal seismicity [1, 2]. The kinematics of the area have been attributed to the NNW-SSE trending active normal fault system that, related to the regional WNW-ESE extensional tectonic regime, is located at the south-eastern lower slope of Mt. Etna volcano, extending to the south into the Ionian offshore [3, 4, 5]. Moreover, Catania is located on the lower southern slope of the Mt. Etna volcanic edifice, at the border of the Simeto river plain, and was exposed in pre-historical and historical times to repeated lava flow invasions and major flooding events. The integration of the field mappings with about 500 bore-holes, allowed us to reconstruct the stratigraphy and the detailed geometry of the geological bodies in the subsurface of the urban area of Catania. The field survey was supported by the detailed analysis of 1:10,000 scale aerial photographs, and also based on historical documents such as prints, maps, photographs, etc. The result was the Geological map of the urban area of Catania (1:10,000 scale) [6]. The collected data also allowed us to evaluate the morphotectonic evolution of the area during the Quaternary. This area shows, in fact, a complex interaction between tectonic uplift and Middle Pleistocene-Holocene eustatic sea level changes, resulting in several orders of marine terraces which represent useful markers to evaluate the uplifting in tectonically active regions. 2 Stratigraphy The geology of the urban area of Catania (Fig. 2) is the result of three main interacting processes: the volcanic and tectonic activity affecting since the Late Quaternary a portion of the Plio-Pleistocene Gela-Catania foredeep basin [7], the sea-level changes and, finally, human activity. The stratigraphy of the substratum of the urban area of Catania, reconstructed by the interpretation of the bore-holes combined with the field survey, is mostly made up of a Lower-Middle Pleistocene foredeep clayey and sandy succession, extensively outcropping in the south-western boundary of the city. This is carved by a flight of marine terraces and dissected by deeply entrenched valleys, filled with thick lava flows, which developed as consequence of the Late Quaternary dynamic processes. A detailed description of the stratigraphic succession, from the bottom to the top, follows. 2.1 Marine deposits of the Lower-Middle Pleistocene cycle In the area to the south of Mt. Etna, the Quaternary sedimentary substratum of the volcano extensively outcrops (Fig. 1). It is made up of a Lower-Middle Pleistocene foredeep succession [8] of marly clays (AM), up to 600 m thick, upwardly evolving to some tens of metres of sands and conglomerates (SC). They characterize the newly urbanized hilly area of the south-western boundary of the city but small outcrops on ridges (“dagale”) preserved by the lava flow invasions are also diffused in the centre and to the north of the city (Lago di Nicito, Cibali, S. Sofia-Città Universitaria, Leucatia and Nuovalucello). The AM are bluish silty-marly clays with rare intercalations of yellowish fine-grained sands, containing macrofaunas with Arctica islandica and Clamys septemradiata and microphaunas with Hyalinea balthica, Bulimina etnea and Globorotalia

SEISMIC PREVENTION OF DAMAGE 85 truncatulinoides, indicating a Lower-Middle Pleistocene age [8]. In the area of Mt. S. Sofia, several bore-holes revealed the presence, at the top of the formation, of thin levels of fine-grained reworked volcanoclastic. The sand intercalations thicken upwards to the passage with the SC. These are cross-bedded quartzose coastal sands containing rare silt levels and frequent lenses of poligenic conglomerates interpreted as channel fillings in a fluvial-deltaic environment. Only in the Zia Lisa area does the contact with the underlying marly clays seem to be unconformable. At San Giorgio, a small level of thin-layered whitish pumice of trachyandesitic composition, interbedded with brown sands, has been found [9]. They contain poor macrofaunas with Ostrea edulis and microfaunas with Elphidium spp. and Ammonia spp. [8]. In spite of the insignificant fossiliferous content, this formation has been referred to as the Middle Pleistocene [10] or to the “Mindel-Riss interglacial stage” [11]. However, taking into account the tholeiitic composition of the volcanic clasts contained in the conglomeratic levels [12], the age of SC can be precisely defined by their attribution to the early stage of the Etnean volcanic activity [13], dated 365-235 kyr BP in the area to the west of Catania by Gillot et al. [14]. As SC is, in turn, covered by tholeiitic lava flows west of Catania, the age of the formation is constrained between 365 and 235 kyr ago. 2.2 Middle Pleistocene terraced deposits (T1 –T2) Terraced deposits were observed in the urban area at different elevations (Table 1), unconformably lying on the sediments of the Lower-Middle Pleistocene cycle. They form landward-pinching coastal sedimentary wedges [15], with thin alluvial and/or colluvial caps and basal contacts usually marked by erosional surfaces and levels of paleosoils. These deposits have been roughly divided into two groups, including the terraces older and younger than the reference-horizon represented by the products of the Ancient Alkalic Centres (see below). The first group is composed of the highest terraces located on the hills around the city: the Monte Po and the Barriera terraces. The “Monte Po terrace” (T1) is characterized by coastal brown poligenic conglomerates with sandy matrix, 20-30 m thick, outcropping in the south- western boundary of the city, on the top of Mt. Po and San Giorgio hills, at altitudes ranging between 200 and 150 m. Together with the clays and sands of the Lower-Middle Pleistocene cycle, this forms the southern limb of a large E-W trending anticline (the Terreforti anticline; Monaco [12], Labaume et al. [16]). The T1 conglomerates contain prevailing sedimentary and metamorphic clasts deriving from the erosion of the Maghrebian Chain units, located in central and northern Sicily (Fig. 1), and less than 5% volcanic clasts of Etnean origin. The deposits of the “Barriera terrace” (T2) outcrops in the northern boundary of the city, in the localities Santa Sofia, Barriera and Leucatia, at altitudes of 165- 190 m. They are made up of yellow-brownish sandy clays and silty sands, containing thin levels of volcanic ashes, pebbles (sandstones and basalts) and fragments of bivalve shells. North of Catania, the T2 inner edge is covered by Etnean lava flows, has been observed near Acitrezza (Fig. 1), at altitudes of 265 m a.s.l. [12].

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The ages of the terraces T1 and T2 must be contained between 365 and 180 kyr, as they embody volcanic clasts showing only tholeiitic composition (365-235 kyr; Gillot et al. [14]) and are covered by alkaline volcanic products 180 kyr old (Gillot et al. [14], see below).

Figure 1: Tectonic sketch map of Mt. Etna. Dashed area represents outcropping of volcanic products. Extension direction from Monaco et al. [3].

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Figure 2: Schematic geological map of the urban area of the city of Catania (location in Fig. 1).

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2.3 Volcanic products of the Ancient Alkalic Centres

The present Etnean volcanic edifice, formed since about 180 kyr [14], is constituted by different products of the alkaline suite grouped in the Ancient Alkalic Centres Unit (180-100 kyr), the Trifoglietto Unit (80-60 kyr) and the Mongibello Unit (< 40 kyr) [10, 13, 14, 17, 18]. In the northern boundary of the city, Mt. S. Sofia, Leucatia, Mt. Paolillo, products of the Ancient Alkalic Centres, overlying the T2 terrace deposits, outcrop in small ridges (“dagale”) between the more recent lava flows. They are formed by 132-138 kyr old (sample n. 9 of Gillot et al. [14]) lavas and volcanoclastics whose composition ranges between hawaiitic basalts and basic mugearites [10, 13, 17, 18]. Volcanoclastics (EA) are made up of banks and/or strata more or less cemented, 10 cm to 2 m thick, of arenitic-ruditic epiclastics, showing at places cross- bedding and traces of vegetal fossils. As they contain strata of chaotic alluvial material, they have been interpreted as having been deposited in lagoon or lacustrine environments very close to the sea, subject to violent episodes of alluvial floods [13, 19]. They pass upwards to a 1-5 m thick level of basaltic pebbles in a silty-sandy matrix, covered by greyish and compact lavas (LA), 4-5 m thick, often fractured and highly degraded, characterized by pyroxene and olivine megacrysts. They show a tabular morphology bounded by non-linear scarps which probably represent marine paleocliffs of Tyrrhenian age (see below). The LA lavas are overlaid by a very thin alluvial terraced deposit (AT), composed of brownish sandy-silts and LA pebbles.

2.4 Late Pleistocene terraced deposits (T3 – T7)

The second group of terraces (Table 1), younger than the products of the Ancient Alkalic Centres, are represented by abrasion surfaces and overlying coastal deposits forming relicts preserved on the main divides of the lower hills around the city and downtown (Fig. 2). Here, they are almost completely covered by recent lava flows, and have been detected by the interpretation of several bore- holes and by topographic analysis. The San Giorgio quarter, in the south-west boundary of the city, is mostly built on terraced deposits (the “San Giorgio terrace”, T3), made up of coastal poligenic conglomerates in a brownish silty-sandy matrix, whose inner edge is located at 150 m. To the north, the inner edge of the terrace T3 has been recognized all along the bottom of the Mt. S. Sofia, Leucatia and Mt. Paolillo scarps (Fig. 2), at an elevation of about 165 m. South-west of Mt. S. Sofia (Trappeto), the terrace T3 is characterized by the occurrence of prevalent marine silty-sands with pebble and arenitic-ruditic epiclastic levels, whereas in the area between Mt. S. Sofia and Leucatia it is represented by a large, deeply dissected wave-cut platform located at about 150 m a.s.l., mostly covered by recent lava flows. The “Cibali terrace” deposits (T4) mainly outcrop in the south-western boundary of the city (Fig. 2), south of San Giorgio, whereas a small remnant is located in the Cibali quarter. It is composed of coastal silty sands and poligenic

SEISMIC PREVENTION OF DAMAGE 89 conglomerates. The inner edge, observed south of San Giorgio, is located at 120 m a.s.l.

Table 1: Relationship between coastal-alluvial and marine terraces of Catania (see location in Fig. 2) and the isotopic stages [53], obtained from observations on Etnean volcanic clasts in the terraced deposits.

Terrace Order of Altitude Isotopic Stratigraphic unit of the of the stage volcanic clasts terrace inner edge and time and age in Kyr (m) (Kyr) (Gillot et al. [14]) Mt. Po T1 7.5 (240) Clasts of pre-Etnean tholeiites (365-235) Barriera T2 265 7.1 (200) Clasts of pre-Etnean tholeiites (365-235) San Giorgio T3 165 5.5 (125) Clasts of Ancient Alkalic Centres (180-100) Cibali T4 120 5.3 (100) Clasts of Ancient Alkalic Centres (180-100) Corso T5 85 5.1 (80) Clasts of Ancient Alkalic Indipendenza Centres (180-100) Acquicella T6 45 3.3 (60) Clast of Trifoglietto Unit (80-60) Castello T7 14 3.1 (40) Ursino

The “Corso Indipendenza terrace” (T5) is characterized by up to 30 m thick littoral deposits made up of yellowish cross-bedded sands with fossils of bivalves and levels of poligenic conglomerates. They mainly outcrop in the area of Corso Indipendenza, whereas small remnants, surrounded by recent lava flows, are preserved at Susanna and Cibali. In the south-western boundary of the city (Librino), it is composed of coastal sands and poligenic conglomerates. At places (Corso Indipendenza, S. Maria di Novaluce), bore-hole analysis revealed the presence of dark arenitic epiclastic levels. The inner edge, well exposed at Cibali and Susanna, is located at altitudes of 85 m a.s.l. Since the terraces T3, T4 and T5 are characterized by volcanic clasts belonging to the Ancient Alkalic Centres Unit (180-100 kyr; Gillot et al. [14]), they must be younger than 180 kyr. The “Acquicella terrace” deposits (T6) extensively outcrop in the city centre (Villa Bellini, Monte Vergine, Monastery of San Nicolò L’Arena) and in the southern boundary (Acquicella, cemetery, Villaggio S. Agata), reaching a thickness of 25-30 m. Moreover, they have been recognized in several bore- holes, buried below the recent lava flows. The deposits of the terrace T6 have been widely described by Sciuto Patti [20] who interpreted the coastal-alluvial deposits outcropping at Acquicella and in the cemetery area as volcanic tuffs (“Tufi di Acquicella”). Actually, the T6 deposits are composed of cross-bedded

90 SEISMIC PREVENTION OF DAMAGE sands and silts, containing pebble levels and, at places, as between Via Giusti and Via Acquicella, dark decimetric epiclastic levels showing ruditic and arenitic granulometry. In the Villaggio S. Agata area, they pass upwards to alluvial conglomerates and sands. According to Di Paola Bertucci [21] the origin of the name of the Monastery of San Nicolò L’Arena (“arena” is a synonym of sabbia = sand) is to ascribe it to the earlier presence in the area of large outcrops of dark sands, referable to the epiclastic levels still outcropping in the garden of the Monastery [20], now occupied by the Vittorio Emanuele Hospital. Pebbles are mostly quartzarenitic, whereas few volcanic clasts probably belonging to the Trifoglietto Unit (80-60 kyr; Gillot et al. [14]) have been found in the epiclastic levels (Table 1), suggesting an age younger than 80 kyr for the terrace T6. The inner edge is exposed south of Corso Indipendenza at 45 m a.s.l. Between Monte Vergine and the Monastery of San Nicolò L’Arena (about 40 m a.s.l.), the T6 has been preserved on the top of a hill now surrounded by the 1669 lava flow. In this site, in 729 B.C. Greek colonizers founded the town of Katane [22], the early nucleus of the present city. Actually, in the view of Catania drawn by Tiburzio Spannocchi in 1578 [23, 24], this sector of the medieval town seems to be built on a flat terraced surface bordered by fortifications (Fig. 3). Even the layout of these fortifications, built during the 16th century (before the 1669 eruption) and now mostly removed (see Fig. 2), seems to have been influenced by the shape of the terrace T6, as they partially follow its outer edge [25].

Figure 3: The southern sector of the old city of Catania from the panoramic view to the north drawn by Tiburzio Spannocchi in 1578 [23, 24]. On the left, the medieval town (Monte Vergine) seems to be built on a flat terraced surface (T6). On the right, the Castello Ursino is located on a lower terrace (T7), facing the Ionian sea. Both terraces are bordered by fortifications. The 1669 lava flow had not yet covered the T6 and T7 outer edges, easily visible under the town walls and the Roman Circus (in the foreground). Though the large 13th century Swabian castle of Catania is surrounded by the 1669 lava flow (Fig. 4), several bore-holes along the perimeter and in the

SEISMIC PREVENTION OF DAMAGE 91 courtyard revealed the presence of coastal sediments at altitudes of 10-15 m a.s.l. These deposits, attributed to the “Castello Ursino terrace” (T7), consist of up to 7 m thick silty sands upwardly evolving to sands and poligenic conglomerates. They have also been drilled in the area between Piazza Carlo Alberto and Largo Paisiello, where the inner edge is located at about 15 m a.s.l., and in the quarter of Ognina (Viale Ulisse). In the Castello Ursino area, the terrace T7 is easily recognizable in several drawings reproducing the town of Catania before the 1669 eruption (see Fig. 3 for example). In this area, in fact, the 16th century fortifications around the Castello Ursino probably followed the outer edge of the terrace T7, being the castle built on a flat elevation formerly facing the Ionian sea.

Figure 4: View from the south of Swabian Castle (Castello Ursino). The 1669 lava pressing against the wall is visible.

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2.5 Holocene deposits

In the eastern sector of the city, in the triangle between the shoreline, Via Etnea and Corso Italia, several bore-holes revealed the presence of a large thin- depositional marine platform at –5 m with respect to the present sea level, below the Larmisi lava flow. It is characterized by littoral sands and gravels (SG), up to 5 m thick, and must be formed during an Holocene relative sea level stand (see below). The Holocene is also represented by alluvial and beach deposits mostly outcropping south of the city (Plaia, San Giuseppe La Rena, Airport), where the coastal-alluvial plain of the Simeto river extends. Alluvial deposits (AL), composed of a few metres of silty sands with clayey and gravel levels, have also been drilled in correlation with paleo-valleys invaded by lava flows (e.g. the Acquicella River) and in two flat areas in the city (Via Lago di Nicito and Via Palermo). Here, AL deposits of lacustrine origin formed in response to the valley barrage by lava flows of distinct age (see below). South of the Catania port area, the lava cliffs are substituted by a 20 km long beach (the Plaia), developed during the Holocene in conjunction with the Simeto river alluvial plain. The beach deposits (SP), constituted by yellow quartz sands and fragments of bivalve shells, extends onshore for about 1 km forming an area covered by littoral dunes artificially levelled about 40 years ago. Beaches deposits also outcrop in the small bays of Ognina and Guardia, developed between distinct lava flows which flowed into the sea. In the Ognina harbour they are mostly yellow quartz sands, whereas in the Guardia bay they are represented by basaltic cobbles and black sands.

2.6 Basaltic lava flows of the Recent Mongibello

The heterogeneous Pleistocene substratum of Catania is dissected by deeply entrenched valleys filled with thick lava flows, up to 70 m thick (see the map of lava isopachs), which represent most of the rocks cropping out in the city. These consist of basaltic lavas which, flowing from NW to SE, invaded the urban area in pre-historical and historical times (Recent Mongibello activity, <14 kyr; Kieffer & Tanguy [26]). They were made by massive lava flows irregularly interlayered with clinker and scoria levels, characterized by compositions ranging from basic mugearites to hawaiites-tephrites [10, 12, 17, 18]. At the bottom, the contact with palaeo-soils is characterized by oxidized reddish scoria levels. Seven lava flows have been distinguished on the basis of morphology, macroscopic characters, historical documents [20, 22, 27, 28, 29, 30, 31, 32, 33, 34], and comparison with recent literature [10, 17, 19, 35, 36, 37, 38, 39, 40]. The “Larmisi lava flow” [20] (L1) is the oldest and the largest in the urban area of Catania and extends from Barriera to the sea, where it forms a 2 km long and 10-15 m high cliff (the Larmisi cliff). The age of this lava, above which most of the modern city has been built, is debated: Sciuto Patti [20] subdivided it into two different flows, a southern one (the Larmisi lava) of prehistoric period, and a northern one (the Carvana lava) referred to as the 122 B.C. eruption. This last is attributed to the A.D. 252 eruption by Tanguy [35], on the basis of

SEISMIC PREVENTION OF DAMAGE 93 archeomagnetic analysis. Moreover, following Gemmellaro [31], we think that any lava flow reached Catania during the 122 B.C. event, whose historic descriptions seem to allude to a summit explosion or a destructive earthquake (see also Mercalli [34]). Other authors [10, 37, 38, 40] referred the whole lava flow to the A.D. 252 eruption. In our opinion the Carvana and Larmisi lavas are the products of a single prehistoric eruptive event that gave rise to the lava flow here named “Larmisi”. We estimate an age of 4-5000 yr B.C. for the Larmisi lava flow for two reasons: first of all, the northern portion of this lava flow is characterized by several lava tunnels (Grotta Petralia; Grotta di Novalucello I, Grotta Caflisch, Grotta del Tondo Gioeni, Grotta Angelo Musco, etc.; Centro Speleologico Etneo [42) in which archaeological remnants of the old Bronze age (2000-1400 B.C.) have been found [43, 44, 45, 46, 47], constraining the Larmisi lava age to a period older than 2000-1400 B.C.; moreover, the part facing the sea, between Via Etnea, Corso Italia and the coastline, lies over a 2 km large thin-depositional marine platform (SG) located at –5 m below the sea level. This suggests that the Larmisi lava flow must have flowed over the marine platform formed during the relative sea-level stand which occurred between 8500 and 6500 yr ago (see below), when the sea was stationed a few metres below the present level [48, 49]. The “Fratelli Pii lava flow” (L2), reported in the legend of two heroic brothers who rescued their parents [19], is visible only in small outcrops in the western side of the city, being mostly covered by the 1669 lava flow. It has been generally referred to the 693 B.C. eruption [10, 17, 37, 38, 50], but Tanguy [35], on the basis of archeomagnetic analysis, considers it to be 1000 years older. Nevertheless, historical and morphological considerations suggest an age older than the 693 B.C. for the Fratelli Pii lava flow. From an historical point of view, it is noted that no old manuscript clearly reports lava destruction of the town after the Greeks settling down in the Monte Vergine hill (729 B.C.), where several fragments of Rodian, Ionic and Corinthian pottery have been found ([22; see also Sciuto Patti [20]). From a morphological point of view, along the Acquicella river valley (in front of the cemetery), the bottom of this lava flow is 10 m above the present stream partly filled by the 1669 lava flow. This suggest a very long period of erosion of the sedimentary substratum between the two eruptive events. The “Ognina lava flow” [20] (L3) is very well exposed in the 1.5 km long and 5-10 m high cliff between the Ognina and Guardia bays. Coming down from the Canalicchio area, it is mostly covered by the 1381 lava in the Picanello and Rotolo quarters. The Ognina lava flow has been referred to the 425 B.C. eruption by several authors [10, 17, 33, 37, 38]. North-east of the city (Cezza, Carruba and Cannizzaro areas), the “Cannizzaro lava flow” (L4) outcrops, faces the sea in Ognina bay. The age of this lava flow is uncertain and its stratigraphical relation with the Ognina lava flow is not clear, even if their morphological features are very similar. Nevertheless, these eruptions could have occurred in the same period, as historical documents report two lava flows reaching the sea in the area north of Catania during the 5th century B.C. (see Pindar, 1st Pithic Ode; Thucydide, book

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III, 116, and relative discussion in Tanguy [35, 36]; Tanguy & Kieffer, [39]; Tanguy & Patane’ [40]). Most of the western portion of Catania is built on the “Cifali lava flow” [20] (L5), classically referred to as the A.D. 252 eruption [10, 20, 31, 35, 37, 38]. However, a quite different distribution of this lava flow has been recognized during the field mapping (Fig. 2). This lava flow occurred one year after the St. Agata martyrdom by Romans and threatened Catania, but after 9 days it stopped when the veil of the Saint was carried to the flow front (Act. Sanct. Bollandum, quoted by Recupero [27]; Ferrara [28]; Alessi [30]). The Cifali lava flow is clearly recognizable in several outcrops, being characterized by translucent plagioclase phenocrysts. According to Sciuto Patti [20] and Holm [22], the south-easternmost front of the A.D. 252 lava flow reached the Roman Amphitheatre (the present Piazza Stesicoro). The “Rotolo lava flow” [20] (L6) has been generally attributed to the 1381 eruption [10, 17, 20, 27, 31, 33, 37, 38] on the basis of a manuscript of Simone da Lentini registered in the archives of Catania Cathedral. On the other hand, according to Tanguy [35, 36], it should be moved to the 12th century on the basis of archeomagnetic analysis. The Rotolo lava flow is at present almost completely covered by modern buildings in the northern quarters of Catania (Barriera, Canalicchio, Picanello) but its precise boundaries have been drawn from the Sartorius Von Waltershausen [32] map and from the 1924 Istituto Geografico Militare 1:25,000 scale topographic map. According to Recupero [27], the 1381 eruption started on 6 August from a NNW-SSE trending and 3 km long eruptive fissure (the Cavòli fissure) located NNW of Catania between 475 and 350 m a.s.l. (Fig. 1), along which it formed the Mt. Arsi di St. Maria spatter cones. The lava flow destroyed olive groves in the northern boundary of the city, reaching the coast where two distinct fronts partially filled the small bays of Ognina and Guardia (Fig. 2).

Figure 5: Methodology used to determine the age of marine terraces T2 and T3 in the urban area of Catania, by relationships with dated (Gillot et al. [14]) Etnean lavas and volcanic clasts.

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The 1669 lava flow (L7) was one of the most dramatic and famous eruptions on Etna as it destroyed several villages and reached Catania before flowing into the sea (Fig. 2). At present it is almost completely hidden by the south-western part of the old city and by the modern buildings of the western boundary, where it mostly covers the Fratelli Pii lava flow. For this reason, the lava fronts, showing a well preserved surface morphology uptown (Nesima, Susanna) and outcropping along abandoned quarry fronts downtown (Via Petriera, Curìa, San Cristoforo), have been mostly drawn from the Sartorius Von Waltershausen [32] and Sciuto Patti [20] maps, from the 1924 Istituto Geografico Militare 1:25,000 scale topographic map and, finally, from the numerous old maps and documents reported in Pagnano [25]. However, several bore-holes and lava outcrops, characterized by large pyroxene phenocrysts, are distributed all over the south-western part of the town, allowing us to verify its precise extension. According to historic chronicles, the eruption started on 11 March by the opening of a N-S trending and 12 km long fissure, extending from the Mt. Etna summit to an altitude of 900 m a.s.l., followed by the opening, at lower altitude, of the main vent of Mts. Rossi (Fig. 1). This last gave rise to three main copiously fed lava flows, one of which reached the western boundary of Catania (the Gurna di Nicito pond) on 25 March having travelled some 12 km. After having filled the small depression in six hours, on 12 April it arrived at the walls of Catania which diverted the lava flow to the south where, surrounding the Ursino Castle on 14 April (Figs. 4 and 5), it reached the beach on 23 April, flowing into the sea for 700 m with a 1.5 km large front. The western fortifications resisted until 30 April, when lava rose to the top knocking down a 400 m stretch, and invaded the garden of San Nicolo L’Arena, surrounding the monastery (that was not destroyed) and advancing for about 200 m (Fig. 2). Contrary to the common conviction, the town was not destroyed, only three hundred houses having been overwhelmed, thanks to the authorities constructing a barrage. Lava stopped erupting on 11 July, leaving a completely transformed landscape and economy.

3 Geomorphology

3.1 Morphologic features and urbanization

From a geomorphologic point of view, three distinct sectors constitute the urban area of Catania: the flat southern area, a portion of the Holocene coastal-alluvial plain of the Simeto river facing the Ionian Sea, where the manufacturing district, the airport and bathing establishments are located; the hilly south-western area, characterized by the marine deposits of the Lower-Middle Pleistocene cycle and the overlying flight of coastal-alluvial terraces, where workers’ houses have been built in the last thirty years; the central area, where the morphology incized in the sedimentary substratum, characterized by deeply entrenched palaeo-valleys dissecting the terraced slope, has been flattened by the Holocene lava flows originated from vents located on the southern slope of Mt. Etna. In the last area, in spite of the recurrence of several earthquakes and eruptions, the city and the

96 SEISMIC PREVENTION OF DAMAGE residential quarters have grown since the Greek colonists founded the town of Katane in the 8th century B.C. on the top of the Monte Vergine hill (the present Piazza Dante area; Holm [22]). However, until the 17th century the old city mostly expanded on sedimentary terrain as shown by the 16th century town walls (see Fig. 2), whose southern layout was influenced by the outer edges of terraces T6 and T7 (see also Pagnano [25]). Only after the last eruption (A.D. 1669) and the last strong earthquake (1693), was the city developed above the pre-historical and historical lava flows. It’s worth noting that in the sector within the 16th century walls, the uppermost stratigraphic level consists of several metres thick ruins derived by the destruction due to the occurrence of the 1693 earthquake and by the following levelling of the main streets.

3.2 Coastal morphology

The coastal morphology reflects the complex geological evolution as is characterized by irregular basaltic cliffs in the northern sector, where distinct lavas flowed into the sea, and a linear sandy beach to the south, where the Catania alluvial plain extends. The northern sector is characterized by two basaltic promontories, corresponding to the Larmisi and Ognina lava flows, showing high sea cliffs deriving from the processes of retreat of the coastal profiles due to the continuous sea erosion. The different heights of the two promontories, 10-15 m and 5-10 m respectively, confirms the very different age of the two lava flows (see before). To the south, a low promontory is defined by the 1669 lava flow, at present completely covered by the docks of the Catania port. Bore-hole data and historical documents suggest a seaward shifting of the coastline, up to 2 km, due to the flowing into the sea of the Larmisi, Ognina and 1669 lavas. The 20 km long, N-S trending sandy beach in the southern sector (la Plaia) is the result of the coastal-alluvial deposition of sediments transported by the Simeto river to the Ionian Sea. Before the lava invasions, it probably extended to the north in the same direction, as suggested by bore-hole data and testified by the small sandy beach in Ognina bay.

3.3 Hydrography

Several changes in the hydrography of the Catania urban area have occurred in the last few centuries as the historical chronicles report on an ancient hydrographic network crossing the city. An important freshwater spring is at present located south of Piazza Duomo (the Amenano fountain). It represents the emergence of an old river originally flowing from the north to the south in the site where Via Etnea extends (Holm [22]) and which was then canalized by the Romans then completely covered after the 1693 earthquake (Ferrara [29]). Its hydrogeological basin is probably located to the north of the city where thick lava sequences overlay the impermeable clayey substratum. Moreover, due to the lava flow invasions of the ancient drainage system, a few pond and lacustrine environments developed and disappeared in the pre-historical and historical times, some of which are evinced by the present toponymy (e.g. Via Lago di

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Nicito and Via Botte dell’Acqua). On the basis of historical documents and bore- hole indications, we believe that some ponds formed because of the barrage of water springing up in the Piazza St. Maria di Gesù area [20] by the Cifali lava flows. The remnants of this lacustrine area (the Gurna di Nicito) were definitely covered by the 1669 lava flow. Another lacustrine environment probably developed along the Acquicella river, north of the cemetery. In this area (the eastern termination of Via Palermo) several bore-holes revealed the presence of about 5 m thick alluvial deposits above the Fratelli Pii lava, whereas in Piazza Palestro these deposits are interlayered between the Fratelli Pii and the 1669 lavas. The invasion of the Acquicella river valley by the Fratelli Pii lava flow in pre-historic times (probably before the 693 B.C.) could explain the formation of these deposits that have been successively terraced by the same river. Moreover, because of the deep re-incision of the valley after the Fratelli Pii lava invasion, at present the bottom of this lava is 10 m above the present stream, which in turn has been invaded by a small branch of the 1669 lava flow.

3.4 Age of terraces and uplift rates

The tectonic and morphologic evolution of the area has been coeval to the different stages of the pre-Etnean and Etnean volcanism. So, the terraced alluvial and marine conglomerates contain volcanic clasts of Etnean origin as well as sedimentary clasts deriving from the erosion of the Maghrebian Chain [11, 12 51]. In particular, the different generations of terraces are characterized by associations of volcanic clasts whose petrological features depend on the volcanic stage during which they were involved in the sedimentary processes (Fig. 5). On the basis of petrographic features, integrated with data from literature [10, 12, 13, 17, 18], and consequent attributions of the volcanic clasts to the different Etnean volcano-stratigraphic units, whose age has been determined by Gillot et al. [14], we related (Table 1) both the coastal alluvial terraces and the palaeo-shorelines to the different isotopic stages corresponding to sea-level high-stands of the eustatic curve [52, 53]. Raised marine terraces, in fact, are the result of interaction between tectonic uplift and eustatic sea level changes [5, 15, 54, 55]. The Etnean volcanic clasts of the T2 conglomerates were proved to belong exclusively to the pre-Etnean tholeiites (365-235 kyr; Gillot et al. [14]). Moreover, the terrace T2 is overlaid by the volcanic products of the Ancient Alkali Centres Unit (180-100 kyr; Gillot et al. [14]), whereas the terrace T3 is characterized by volcanic clasts also belonging to the Ancient Alkali Centres (Fig. 5). This suggests a relationship between the terraces T2 and T3 and the high-stands of the eustatic curve which occurred 200 and 125 kyr ago, relative to the isotopic stages 7.1 and 5.5 respectively (Table. 1; Bassinot et al. [53]). Taking into account the petrographic features of the volcanic clasts contained in the other terraces (Table. 1) and considering the flight of terraces as a continuous set of morphological elements, the surfaces T1, T4, T5, T6 and T7 have been ascribed to the other five main high-stands of the eustatic curve which occurred from 240 to 40 kyr (stages 7.5, 5.3, 5.1, 3.3 and 3.1; Bassinot et al [53]).

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The terraces represent useful markers to evaluate the uplift movements occurring in tectonically active regions. Unlike the terrace T1, terraces T2-T7 show very clear inner-edges that, observed at different elevations (Table. 1) and mapped with an uncertainty in the elevation of ±2.5 m, suggest a strong uplift of the area. However, this uncertainty, which basically depends on erosional and depositional processes following the emergence of the terrace, is negligible when estimating the long-term Quaternary uplift rates involving time spans of tens to hundreds of thousands of years. This implies that the elevations of the inner edges reported in this paper are to be considered as mean values useful for estimating the regional uplift of the entire area during the Late Quaternary. So, the distribution of T2-T7 inner edges at elevations of 265, 165 m, 120 m, 85 m, 45 m, 14 m, respectively, suggests a constant uplift rate of the area of about 1.2- 1.3 mm/yr in the last 200 kyr (Fig. 6). Taking into account this value of uplift rate, the thin-depositional platform (SG) located at –5 m below the sea level between Corso Italia, Via Etnea and the shoreline, should have formed during the relative sea-level stand which occurred betwen 8500 and 6500 yr ago (Fig. 6), when the sea stationed ~10 m below the present level [48, 49], and was probably fossilized by the Larmisi lava flow (see before). The high value of uplift-rate, comparable to that reported by Gillot & Kieffer [56] for the same area (1.1-1.6 mm/yr) and by Monaco [12] for the coast to the north-east of Catania (1.4 mm/yr), has to be attributed to the active normal fault system (Fig. 1) located along the Ionian off-shore [5]. However, the normal fault deformation and the strong tectonic uplift of the area since ~200 kyr ago seems to be subsequent to the compressive tectonic regime responsible for the deformation of the terrace T1 which, dipping at 20° to the SSE, represents the southern limb of the Terreforti anticline (see before). This has been interpreted as a blind-thrust propagation fold formed between 240 and 200 kyr at the front of the Maghrebian thrust system [12, 16].

Figure 6: Topographic profile (see location in Fig. 2) of the urban area of Catania (a) and correlation between Late Quaternary coastlines and the peaks of the eustatic curve with relative isotopic stages (from [48, 49, 52, 53]), as modified for an uplift-rate of 1.25 mm/yr (b).

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4 Conclusions

The urban area of Catania rests on a region characterized by intense active dynamics, as seen by the recent geological and morphological evolution, the Etnean volcanic activity and the recurrence of destructive earthquakes. The main long-term evidence of the Late Quaternary dynamics is represented by the strong uplift, estimated at about 1.3 mm/yr, affecting the area. This process caused the terracing of the Middle-Late Pleistocene marine deposits, which rest on the Early-Middle Pleistocene succession. The resulting staircase-shaped slope, housing the town, has been affected, during the last period of the uplifting, by deep incisions of fluvial streams which canalized pre- historical and historical lava flows originating from eruptive fissures located on the southern flank of Mt. Etna. The geological framework is thus characterized by both vertical and lateral strong heterogeneity. The Late Quaternary dynamics and the high level seismicity can be explained as the effects of the NNW-SSE trending active normal fault system located at the Ionian offshore and related to the regional WNW-ESE extensional active regime. So, due to its location, the urban area of Catania is prone to major geological risks deriving from high level seismic, volcanic and hydrogeologic hazards.

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