First study of fault trench stratigraphy at Mt. Etna volcano, Southern Italy: understanding Holocene...

First study of fault trench stratigraphy at Mt. Etnavolcano, Southern Italy: understanding Holocene surface

faulting along the Moscarello fault

Ra�aele Azzaroa,*, Domenico Bellab, Luca Ferreli c, Alessandro MariaMichetti c, Federica Santagatid, Leonello Servae, Eutizio Vittori e

aCNR-GNDT, Gruppo Nazionale per la Difesa dai Terremoti (National Group for Defense from Earthquakes), c/oIstituto Internazionale di Vulcanologia, Piazza Roma 2 Ð 95123, Catania, Italy

bCentro Scienti®co per l'Ambiente, via Nino Martoglio 13 Ð 95024, (CT), Acireale, ItalycCNR-GNDT, Gruppo Nazionale per la Difesa dai Terremoti (National Group for Defense from Earthquakes), c/o

ANPA, via Vitolieno Brancati 48-00144 Roma, ItalydScuola di Specializzazione Archeologia Classica, UniversitaÁ di Lecce, Lecce, Italy

eANPA Agenzia Nazionale per la Protezione dell'Ambiente (Italian Environmental Protection Agency), via VitalianoBrancati 48 Ð 00144 Roma, Italy

Abstract

Paleoseismology, the study of past earthquakes based on their geological record in the stratigraphyand landscape, is a successful newly developing ®eld of research. The application of fault trench studiesin volcanic environments is one of the youngest branches of paleoseismology. In this paper, we presentthe results of the ®rst exploratory trenches excavated at Mt. Etna in Sicily, the largest Europeanvolcano. Modern surface faulting at Etna is a very well known feature, which poses signi®cant hazard tothe local community, both in terms of ground displacement of essential lifelines and ground shackingfrom frequent damaging earthquakes. However, while the geomorphology and the seismicity of theactive fault in the Etna region consistently show very high rates of tectonic activity, the Holocenecumulative throw and slip-rates, along with the nature (coseismic vs. creeping fault slip), dimension andtiming of the displacement events, are still poorly constrained. For this purpose, we selected as a samplearea the Moscarello fault, one of the most outstanding segment of the Timpe system of active normalfaults in the volcano's lower eastern ¯ank. Displaced landforms and volcanic units at the FondoMacchia basin, in the central sector of this fault, indicate some hundreds of meters of vertical o�set inthe last ca. 80 kyr, with a long-term slip-rate substantially higher than 1.5±2.0 mm/yr. According to the

Journal of Geodynamics 29 (2000) 187±210

0264-3707/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.PII: S0264-3707(99)00055-1

* Corresponding author. Tel: +39-95-448084; fax: +39-95-435801.E-mail address: ra�[email protected] (R. Azzaro).

historical sources and instrumental observations, the Moscarello fault ruptured four times in the last 150years during shallow (H< 5 km) and moderate magnitude (M < 4.8) earthquakes. These events wereassociated with severe damage in a narrow epicentral area (macroseismic intensities up to the IX±Xgrade of the MSK scale) and extensive surface faulting (end-to-end rupture length up to 6 km, verticalo�sets up to 90 cm). This clearly indicates very high modern rates of deformation along this fault. Weconducted trench investigations at the Fondo Macchia site, in a point where eyewitnesses observed ca.20 cm of coseismic vertical displacement after the April 21, 1971, Ms � 3:7, earthquake. The excavatedsections provided direct stratigraphic evidence for a vertical slip-rate of 1.4±2.7 mm/yr in the last ca. 6kyr. This should be regarded as a minimum slip-rate for the central section of the fault. We explored asingle scarp at a single site, while we know from recent historical observations that several parallelscarps may rupture coseismically at Fondo Macchia. Thus, the relevant deformation rate documentedfor the modern period might be likely extended back in the past to a time-span of some thousands ofyears at least. As expected, for such a volcanic environment, the activity rates of the Moscarello faultare also signi®cantly higher than for the Apennines normal faults, typically showing slip-rates lowerthan 1 mm/yr. The agriculturally reworked trench hangingwall stratigraphy did not allow to recognizeindividual displacement events. Nevertheless, the sedimentary structures observed in the trench footwallstrongly suggest that, as for the last 150±200 years of detailed historical record, fault behavior at FondoMacchia is governed by coseismic surface displacement rather than fault creep. This research con®rmsthat paleoseismology techniques can be e�ectively applied also in active volcanic environments, typicallycharacterized by rheology and, consequently, seismicity and fault dynamics very di�erent from those ofother tectonic environments in which paleoseismology has been ®rstly developed and is today extensivelyapplied. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

Contemporary surface faulting phenomena tied to numerous and fast slipping active faultsare extremely well documented at Mount Etna volcano in Eastern Sicily, the largest volcano inEurope (Figs. 1 and 2). This poses signi®cant hazard to the local community, both in terms ofground displacement of essential lifelines and ground shacking from frequent damagingearthquakes (Lo Giudice, 1988; Azzaro, 1999; Azzaro et al., 1998). In the last decade,paleoseismological studies have been developed along several active normal faults in theApennines, however, fault exploratory trenching in the Etna area was totally missing. Wedescribe the results of the ®rst study of an Etnean fault conducted through the analysis ofspeci®cally excavated stratigraphic sections. In this paper, we use the term active fault toindicate capable faults, faults that show the potential of producing displacement at or near theground surface (IAEA, 1991; Azzaro et al., 1998). Surface faulting in the Apennines isinvariably related with earthquakes of Mw > 5:5±6:0 and focal depth H < 10±15 km, asobserved also in other regions a�ected by active normal faulting (e.g. Wells and Coppersmith,1994; Yeats et al., 1997). For instance, the September 26, 1997, Col®orito earthquakes of Mw

5.7 and 6 were accompanied by discontinuous surface ruptures for a total length of ca. 12 kmand vertical displacement up to 8 cm (Cello et al., 1998); while the November 23, 1980, Irpiniaearthquake of Mw 6.9 was accompanied by well developed fault scarps for a total length of ca.40 km and vertical displacement up to 50 cm (Pantosti and Valensise, 1990). Slip-rates ofnormal faults in the Apennines range from 0.1 to 1.0 mm/yr and return periods vary from a

R. Azzaro et al. / Journal of Geodynamics 29 (2000) 187±210188

Fig. 1. Regional geological setting of eastern Sicily and high degree isoseismals for some major crustal earthquakes

of the region.

R. Azzaro et al. / Journal of Geodynamics 29 (2000) 187±210 189

Fig. 2. Tectonic sketch map of Mt. Etna's eastern ¯ank and distribution of shallow, M > 3:0 earthquakes (fromCamassi and Stucchi, 1997).


Fig. 3. Simpli®ed geological map of the Timpe fault zone (modi®ed from Monaco et al., 1997) and historicalearthquake surface ruptures along the Moscarello fault.


few hundreds to few thousands of years. No fault creeping has been observed in the Apenninesto present-day knowledge.Conversely, in a volcanic environment, such as that of Mt. Etna, surface faulting shows very

di�erent features and is associated to a variety of processes (e.g. Smith et al., 1996). Theoccurrence at Etna of both coseismic and aseismic surface rupture events is well known, alsoalong the same fault (RasaÁ et al., 1996; Gresta et al., 1997; Azzaro, 1999). Fault creepinga�ects extensively most of the faults mapped in Fig. 2, with modern (last 200 yr) creep ratesranging from 0.5 to 2.3 cm/yr (RasaÁ et al., 1996). At Mt. Etna, fault displacement of theground may result from volcano-related movements due to dyke-induced rifting and ¯ankinstability sliding (Borgia et al., 1992; Lo Giudice and RasaÁ , 1992; Montalto et al., 1996;McGuire et al., 1997). Likewise, purely tectonic faulting certainly plays a signi®cant role(Gresta et al., 1990; Lanzafame et al., 1996). Mt. Etna volcano is located along thenorthernmost extension of the Malta Escarpment (Fig. 1), the regional master fault separatingthe continental crust of the Hyblean Plateau from the oceanic one of the Ionian Basin(Scandone et al., 1981; Lo Giudice et al., 1982; Monaco et al., 1995). According to severalauthors, the Malta Escarpment should be regarded as the causative fault of the February 4,1169 and January 11, 1693 earthquakes of intensity XI grade MSK, arguably the largestseismic event in the Italian history (Barbano, 1985; Lombardo, 1985; Boschi et al., 1995).Macroseismic data clearly show that both the 1169 and 1693 are crustal earthquakes �H <15±20 km) having epicentral areas localized in the coastal sector of southeastern Sicily (Fig. 1).We have no historical information for earthquakes of similar dimensions in the Etna area.

Fig. 4. Plots of magnitude versus length of surface rupture for Etna volcano (from Azzaro, 1998) in comparison

with the worldwide database compiled by Wells and Coppersmith (1994; we did not plot earthquakes lackingsurface rupture). * Worldwide database (from Wells and Coppersmith, 1994), r Etnean database (from Azzaro,1999).


However, we do know that the epicenter of the 1818 crustal earthquake of intensity IX gradeMSK is along the eastern ¯ank of Etna volcano (Imposa and Lombardo, 1985; Figure 1), andthis event produced also ground ruptures in the epicentral area (Azzaro, 1999).Much more commonly, coseismic surface faulting is here associated with very shallow �H <

5 km), moderate �2:9RMR4:8� earthquakes, which occur with short recurrence timing (tens ofyears). Such earthquakes produce macroseismic intensities reaching even the IX±X grade MSKalong elongated, narrow zones (Fig. 3) (Azzaro et al., 1989; Lo Giudice and RasaÁ , 1992)including signi®cant surface faulting, with end-to-end rupture length up to 5±6 km and verticalo�sets up to 50±90 cm (Azzaro, 1999). Fig. 4 shows the striking di�erence in the relationsbetween magnitude and surface rupture length for these earthquakes compared to worldwidecrustal surface faulting earthquakes. This point was already made by Riuscetti and Distefano(1971), who studied in detail the surface ruptures produced by the April 21, 1971, Ms � 3:7,Fondo Macchia earthquake.The interaction between regional tectonic processes and local volcano-tectonic dynamics

must be carefully evaluated in the study of surface faulting at Etna volcano. Nevertheless, oncethese complications have been properly taken into account, paleoseismological faultinvestigations potentially provide (i) much more detailed information on the Holocene sliprates of the Etnean faults than the traditional geological approaches can produce and (ii)important constraints on fault behaviour, segmentation, seismic potential and ground rupturehazard. In fact, exploratory trenching of faulted stratigraphy allows the analysis of asubstantially larger time-span with respect to the two centuries of available detailed historicalseismicity data.The present study takes advantage for the ®rst time from this fossil database. We show the

results of the ®rst two trenches recently carried out in the volcano's lower eastern ¯ank alongthe Moscarello normal fault, which is one of the most outstanding tectonic feature of theTimpe system (Timpe is the local name for cli�s, steep slopes). We selected this fault as theinaugural sample area of our paleoseismological research program at Mt. Etna. The reason isthe excellent local record of historical surface faulting events, including the well studied April21, 1971, earthquake, coupled with a very good geological and geomorphological evidence ofrecent activity. Thus, the aim of this note is to compare short-term fault behaviour knownfrom the last 200 yr of good quality historical information with long-term history of surfacefaulting as reconstructed from faulted Holocene stratigraphy along the Moscarello fault.

2. Geological, morphological and seismotectonic setting of the Moscarello fault

The Timpe fault system consists of a 20 km long and 8±10 km wide belt of mainly NNW±SSE trending extensional structures (Fig. 2). From the morphological point of view, this systemis segmented into individual, steep, fault escarpments up to 5±8 km long and up to 200 m high,o�setting late Pleistocene to Holocene volcanics and alluvial deposits (Romano, 1979). Nearthe town of Acireale, the Timpe system controls the shoreline pattern, while, moving to thenorth, gently displaces the slope of the Etna volcano toward the sea by east-facing, step-faultswith associated antithetic structures, that locally de®ne graben-like features (Lo Giudice et al.,1982; Monaco et al., 1997). Typical tectonic features indicating very recent activity, such as


linear scarps, fresh fault planes exposed at the base of oversteepened range fronts, andtrapezoidal facets, a�ect late Pleistocene to Holocene volcanic units, including historical lava¯ows (Monaco et al., 1997). The drainage pattern in the area between the villages of Giarreand Linera displays impressive active tectonic landforms such as de¯ected streams, linear andperched valleys, and river piracy (Adorni and Carveni, 1993; Stewart et al., 1993; Carveni andBella, 1994). Geological and seismological evidence indicates the Moscarello fault as one of themost active faults of the Timpe system. Such a structure develops between the villages ofGuardia and Sant'Al®o for an overall length of 10 km (Fig. 3). It is a N208W trending, eastdipping normal fault showing no relevant strike-slip components of displacement, asdemonstrated for instance by the lack of signi®cant lateral o�set in the faulted Holocene lava¯ows near Dagala. The fault o�sets the Pleistocene volcanics of the `Ancient Alkaline Centers'(`AAC' in the following) and `Trifoglietto' units (Cantarella and Cristofolini, 1981), the alluvialfan deposits locally named `Chiancone' (Calvari and Groppelli, 1996), the late Pleistocene to

Fig. 5. Simpli®ed geologic cross-sections (topographic pro®les drawn from the 1:10.000 scale map of the RegioneSicilia) oriented perpendicular to the Moscarello fault scarp, showing vertical surface o�sets of the `Chiancone'deposits (5a; dating from Calvari and Groppelli, 1996) and `Trifoglietto' volcanics (5b; dating from Gillot et al.,

1994). The depth of the `Trifoglietto unit' top surface in the hangingwall, even if not well constrained, is certainlysigni®cant. The 125 m of morphological o�set at 5b is thus a minimum value for the true vertical displacement atFondo Macchia. Pro®le locations in Fig. 3.


Fig. 6. Geologic and topographic setting of the central sector of the Moscarello fault. Pattern of coseismic surface

ruptures along this part of the fault since 1855 and locations of trenches A and B are also shown.


Holocene products of the `Ellittico' eruptive centers and the middle Holocene to historical lava¯ows of the `Recent Mongibello' (Romano, 1979; Cristofolini et al., 1991).Faulted late Quaternary landforms along strike clearly show the recent displacement history

of the Moscarello fault. At the southern termination near the village of Guardia (Fig. 3), theMoscarello fault ruptured to the ground surface during the 1865 and 1911 earthquakes.However, in this sector the fault has virtually no geomorphic expression, except at fewlocalities where subdued scarps a�ects latest Pleistocene to Holocene volcanics. The scarcity ofmorphological evidence demonstrates that cumulative o�set along this section of the fault islow, and clearly indicates the prolongation of the fault trace into a segment boundary. Movingnorthward, near the village of Dagala the fault o�sets the Holocene paleosurface of the`Chiancone' alluvial fan with a distinct, ca. 12 m high scarp (Fig. 5a). Radiocarbon dating ofthe `Chiancone' deposits (Calvari and Groppelli, 1996) indicates an age of ca. 7 kyr for thedisplaced paleosurface, which yield a vertical slip rate for this sector of the Moscarello fault of1.7±1.8 mm/yr. Vertical displacement and slip-rate reach maxima at the center of the fault(Fig. 6), characterized by a prominent, ca. 125 m high footwall range-front (Timpa diMoscarello) and a well-developed hangingwall basin (Fondo Macchia), bounded to the east byantithetic faults (Fig. 3). Geological and morphological data indicate for this area a verticalo�set of several hundreds of meters. Here the fault juxtaposes the footwall Pleistocene volcanicsequence (that culminates with the `Trifoglietto' unit, dated ca. 80 kyr to 60 kyr by Gillot etal., 1994; Fig. 5b) with hangingwall recent alluvial and colluvial deposits, whose thickness isunknown but is likely signi®cant. The Timpa di Moscarello is a 30±408 steep fault escarpment.The local drainage network, pattern of sediment deposition and path of lava ¯ows is clearlycontrolled by the progressive uplift of the footwall range-front and related hangingwall basinsubsidence (Adorni and Carveni, 1993; Stewart et al., 1993). For instance, Fig. 6 shows thepath of two historical lava ¯ows (1284±1285 and 1689 A.D.) that followed deeply incised streamchannels perpendicular to the fault escarpment and eventually expanded in the Fondo Macchiabasin. It is reasonable to infer that several prior lava ¯ows did the same during the latestPleistocene to Holocene and are now buried below the recent alluvial ®lling of the basin.Moreover, Figs. 3 and 6 clearly show that the northern part of the `Chiancone' alluvial fan hasalso fed the hangingwall basin of Fondo Macchia. The `Chiancone' deposits have a latestPleistocene to Holocene age and a thickness in the order of hundreds of meters (Calvari andGroppelli, 1996). In addition, the age of the Pleistocene volcanics top surface in the footwall isnot well constrained. Since, it is not clear to what extent it is erosional or depositional innature, this surface might result quite younger than the age of the exposed `Trifoglietto' rocks.Therefore, the long-term slip-rate of ca. 1.5±2.0 mm/yr obtained using an age of 60±80 kyr

for the footwall top surface and a vertical throw equal to the morphological o�set of 125 m(Fig. 5b; see also Monaco et al., 1997), in fact give us a minimum value that signi®cantlyunderestimate the real slip-rate in this central sector of the fault.Moving north from Fondo Macchia, the northernmost sector of the fault (Fig. 3) shows

progressively decreasing amount of displacement in Pleistocene to historical volcanic units. Asfor the opposite termination of the fault near Guardia, cumulative vertical throw here is in theorder of few meters. The geomorphic evidence of the fault eventually vanishes just near thevillage of S. Al®o. Despite stable-sliding movements have been frequently observed along thenearby structures (RasaÁ et al., 1996; Gresta et al., 1997), the lack of modern evidence for creep


along the Moscarello fault suggests that this fault is presently active only by coseismic, stick-slip movements. The seismicity associated with the Moscarello fault is due to shallowearthquakes mostly occurring in the Fondo Macchia sector, where some of the mostdestructive events of the Etna area were located (19.07.1865 and 15.10.1911, Fig. 2). In the lasttwo centuries this structure indeed has generated, with an average recurrence timing of ca. 50yr, four earthquakes with epicentral intensities ranging VII to X grade MSK, all producingsurface faulting (Table 1). Detailed historical reports have allowed us, except for the oldestevent, to map the zones of surface rupture (Figs. 3 and 6) and to estimate maximumdisplacements. In particular, the strongest events (1865 and 1911, M � 4:5� produced an end-to-end rupture length of ca. 6 km, extending also to the southernmost section of the fault,which, as already pointed out, has virtually no morphological expression. The maximumvertical fault displacement Ð reaching 90 cm through two parallel scarplets (individual o�sets70 and 20 cm) developing in a strip less than 100 m wide Ð occurred in the middle-northernsector of the structure (RiccoÁ , 1912). In the Fondo Macchia area, ground ruptures with verticalo�set not exceeding 30 cm were also observed after the 1855 and 1971 earthquakes. Thesurface rupture zone, typically some tens of meter wide, a�ects the foot of the scarp and showsalso relevant extensional components revealed by open ®ssures, hollows and graben-likestructures, the latter developing up to some meters wide and 60 m long. Horizontal, strike-slipcomponent of motions, suggested by en echelon cracks and minor splays from the main ®ssure,has been reported only for the 1865 and 1971 earthquakes (Silvestri, 1865; Riuscetti andDistefano, 1971). However, as already pointed out, from aerial photograph interpretation and®eld geomorphology there is no evidence for laterally o�set landforms that can indicaterelevant long-term strike-slip displacement.The maximum vertical o�sets reported in Table 1 refer to single site measurements. Except

for the 1971 event, however, we do not have enough historical information to exactly describethe coseismic slip distribution along strike. Therefore, we cannot use the values in Table 1 tocalculate a representative, average slip-rate of the fault in the last 150±200 yr. Nevertheless, itis important to remark that the Fondo Macchia sector of the fault was ruptured during all the

Table 1

Historical surface ruptures

Date Imaxa

(MSK)

Mmb Ms

c Mld Length

(km)

Extensional component

(cm)

Vertical displacement

(cm)

References

1855/01/26 VII 3.9 1.0 ± ± Grassi, 1865

1865/07/19 IX 4.5 6.0 60±100 30 Grassi, 1865; Silvestri, 18651911/10/15 X 4.3 4.5 6.0 10±30 90 RiccoÁ , 19121971/04/21 VII 4.1 3.7 4.0 3.0 ± 25 Riuscetti and Distefano, 1971

a Imax is maximum intensity in the MSK (Medvedev±Sponheur±Karnik) macroseismic scale, according to Camassiand Stucchi (1997).b Mm is macroseismic magnitude, calculated using the relationship by Azzaro and Barbano (1997).c Ms is surface-wave magnitude.d Ml is local magnitude.


four strong earthquakes reported in Table 1. This indicates for this central, most activesegment of the fault a very high modern slip-rate, in the order of several mm/yr.Thus, while the geomorphology and the seismicity of the Moscarello fault consistently show

very high rates of tectonic activity, the Holocene cumulative throw and slip-rates, along withthe nature, dimension and timing of the displacement events, are still poorly constrained.

3. Trenches

3.1. Trench site selection

The selection of the excavation sites was carried out following geomorphological mapping by®eld inspection and aerial photograph interpretation. Two excavations were conducted at theFondo Macchia site (Fig. 6), along the middle-northern segment of the Moscarello fault, whichpresented the most promising condition for trenching. In fact, this area displays (i) a basinwhich has allowed a continuous and thick sedimentation and (ii) a prominent footwall range-front probably depending on the higher slip rate and faulting events related to this faultsegment. The ultimate selection of excavation sites was driven by the indications of eye-witnesses of the last rupturing episode (1971) as well as by logistics, since the whole area hostsintensive citrus plantations.

3.2. Trench A

The ®rst trench was excavated across the 0.4±2.7 m high, N trending scarplet, whichdiscontinuously oversteepens the base of the Timpa di Moscarello fault escarpment (Figs. 6and 7). At the selected site eyewitnesses observed a vertical displacement of ca. 20 cm (westside up) following the April 1971 earthquake. The trench, perpendicular to the scarp, exposeda narrow rupture zone composed by a main fault plane Ð striking N3508±3558 and dipping758±908E, without kinematic indicators Ð and minor fault strands (Fig. 8). The main faultseparates two di�erent stratigraphic sequences. The footwall stratigraphy consists of alluvial

Fig. 7. Topographic pro®le based on theodolite and electronic distance meter (EDM) survey. Trench A has been

excavated at the foot of the fault scarp, where a ca. 20 cm coseismic o�set was observed during the 1971earthquake.


Fig. 8. Logs of the north and south walls (mirror image of the original log) in trench A (see location in Fig. 6).Reference grid in meters. Circled numbers (1a±6) refer to stratigraphic horizons described in the text (Fig. 10 alsouses the same numbers). Sample dating details in Table 2.


fan deposits, mostly unconsolidated sandy conglomerate (including few rounded boulders ofnearly metric size) with sandy and silty-clayey levels (Unit A). On the basis of the sedimentaryfacies, such a deposit may be associated with the top of the late Pleistocene to Holocene`Chiancone' formation as described by Calvari and Groppelli (1996). Along the two trenchwalls the rupture zone shows a di�erent geometry of the fault planes (Fig. 8). Near the rupturezone in the southern wall, Unit A clearly shows erosional surfaces at several stratigraphichorizons, which are seldom preserved in the northern wall. Since there is no evidence of othergeomorphic processes that can have produced similar features, these surfaces must be relatedto episodes of scarp retreat triggered by the formation of a fault scarp free-face. Within therupture zone in the northern wall, the horizon 3 is clearly backtilted to the west, a feature thatis not preserved in the southern wall. Also, in the fault zone most of the clast long axes arelined parallel to fault dip, and no evidence of in®ltrating water cementing the material andcrack ®ll is present.The hangingwall stratigraphy displays a ca. 4 m thick sequence of massive, ®ne-grained

colluvial deposits (Unit C), reworked by agricultural activities. Such a sequence containspottery sherds, which have been analysed in order to obtain archaeological dating (seeAppendix A). Based on their typological and workmanship features, the fragments comingfrom pots made on the wheel and tiles, may be dated between 1,100 B.C. (FMR 19), at thelower colluvial level (Unit C), and the modern period (FMR 6±7±8), at the higher levels. This

Table 2Radiocarbon samples

Trench Sampleb d13C Radiocarbon agec Calibrated aged Description

(Years B.P.) (Calendar) (Years B.P.)

A FMR11a ÿ27.6 3150250 1520±1300 B.C 3470±3250 Colluvial unit C, charcoal

FMR10a ÿ25.0 1540240 430±620 A.D. 1520±1350 Colluvial unit C, charcoalFMR16 ÿ26.2 49102180 4300±3100 B.C. 6250±5050 Alluvial unit A, TOC from organic soilFMR17 ÿ26.6 48102200 4000±2900 B.C. 5950±4850 Alluvial unit A, TOC from organic soil

FMR18 ÿ25.9 36202260 2900±1300 B.C. 4850±3250 Alluvial unit A, TOC from organic soilFMR25 ÿ25.5 2280240 400±340 B.C 2350±2290 Colluvial unit C, charcoal

320±200 B.C. 2270±2150

FMR14 ÿ26.0 23902110 800±200 B.C. 2750±2150 Colluvial unit C, TOC from organic soilFMR20 ÿ23.9 4020240 2620±2450 B.C. 4570±4400 Colluvial unit C, charcoalFMR15 ÿ25.9 3310290 1780±1400 B.C. 3730±3350 Colluvial unit C, TOC from organic soil

B FM 5a ÿ25.8 23902120 800±200 B.C. 2750±2150 Alluival unit A, TOC from organic soil

FM 9 ÿ24.7 31352105 1700±1050 B.C. 3650±3000 Alluvial unit A, TOC from organic soilFM 14 ÿ26.3 2850280 1260±820 B.C. 3210±2770 Alluvial unit A, TOC from organic soilFM 10 ÿ24.6 2245290 520±40 B.C. 2470±1990 Alluvial unit A, TOC from organic soil

FM 6 ÿ25.4 1920280 100 B.C.±260 A.D. 2050±1690 Alluvial unit A, TOC from organic soilFM 12 ÿ25.7 2200295 410 B.C.±20 A.D. 2360±1930 Colluvial unit C, TOC from organic soilFM 7 ÿ25.1 17302115 50±600 A.D. 1900±1350 Colluvial unit C, TOC from organic soil

a AMS dating.b Sample locations are shown in Figs. 8 and 9.c Analyses made by Geochron Lab. Cambridge, Massachusetts, USA.d Calculated using Stuiver and Reimer (1993) at 2s con®dence level.


hangingwall sequence is lacking any sedimentary structures related to single events of scarpformation and erosion. However, we know that at least for the 1971 event a ca. 20 cm highcoseismic fault scarp was observed at this site, and probably similar scarps formed duringmany other previous earthquakes. Therefore, the lack of scarp-related colluvial wedges is mostlikely due to agricultural activities that in the last millennia continuously disturbed the originalstratigraphic fabric. A 1.5 m deep, hand-made drilling carried out in the hangingwall of thetrench has crossed the Unit C colluvium up to 5.6 m of depth without reaching the Unit Adeposits outcropping in the footwall.A Standard Penetration Test (SPT) drilling conducted in the fault hangingwall at a distance

of 7 m south of trench A allowed to recognize at 4 m of depth a sudden change in thelithological and geotechnical properties of the drilled deposits, which can be correlated with thefacies change from Unit A to Unit C (Ferreli et al., 1998). Projecting the SPT drilling dataalong the hangingwall section of trench A, the top of Unit A should be located just below thebottom of the hand-dug borehole, that is about 5.6 m below the ground surface. Taking intoaccount a possible northward dip of the top of Unit A horizon, we can estimate that thishorizon could have a maximum depth of 7 m below the ground surface in the hangingwallsection of trench A.

Fig. 9. Log of the south wall in trench B (see location in Fig. 6). Reference grid in meters. Sample dating details in

Table 2.


4. Discussion

4.1. Slip-rate at the trench site

Taking into consideration the existing uncertainties about the age and throw of the faulteddeposits, the data shown in Fig. 8 allow to reliably constrain the vertical component of theMoscarello fault slip rate at the studied site. For this purpose, we now discuss the existingconstraints for the age and vertical displacement of horizon 2b, that we use as a referencehorizon for assessing the slip rate in trench A.The sedimentation of horizons 5 and 4 and the base of horizon 3 from Unit A in trench A

occurred between 6,250 and 3,250 years ago, based on the dating of samples FMR 16, 17 and18 in Fig. 8. Radiocarbon ages regularly decrease from the bottom (FMR 16) to the top (FMR18) samples. Thus, the age of horizon 2b, the highest horizon of Unit A exposed in trench A,must be younger than 4850±3250 yr B.P., the dating of FMR 18. Fig. 10 shows ourreconstruction of the stratigraphic events observed in trench A.This includes several events of deformation, erosion and deposition (`c', `d' and `e' in Fig.

10) occurred after the deposition of sample FMR 18 and before the deposition of horizon 2b.It is, therefore, possible that the age of horizon 2b is some hundred years younger than 4850±3250 yr B.P. The minimum age of horizon 2b is constrained by the dating of the base of UnitC. The stratigraphy of trench B (Fig. 9) clearly indicates a regular passage from Unit A toUnit C. Radiocarbon and archaeological dating of Unit C in trench A displays someinconsistency (see Fig. 8, Table 2 and Appendix A). For instance, the two charcoal fragmentsFMR 11 and FMR 25, sampled from the same stratigraphic level, yield ages with a di�erenceof about one millennium. This is probably related with the detrital nature of the charcoalsamples, and with the agricultural reworking of the soil. However, the general sequence of theC14 and archaeological dating indicate a regular trend from ages ranging between 4,570 to3,350 yr B.P. at the base of horizon 1a (FMR 20 and FMR 15), to ages of 2,750±2,150 B.P. atthe top of horizon 1a, up to the medieval to modern ages of the samples from horizon 1b.Therefore, the youngest dating from Unit A (FMR 18, 4850±3250 yr B.P.) clearly overlapswith the oldest dating from Unit C (FMR 20, 4570±4400 yr B.P.). Thus, in trench A the`missing' stratigraphy is not very thick. This corroborates the idea, coming from the previouslydescribed hand-dug borehole and STP drilling, that the top of horizon 2b is at a shallow depthbelow the bottom of the trench hangingwall.In conclusion, we think that a reasonable approximation for the dating of horizon 2b is an

age ranging between 4,000 and 3,000 yr B.P., and will use these values in the following.As shown in Fig. 8, the minimum vertical displacement of horizon 2b is 5.6 m. The

maximum vertical displacement is constrained by both the result of the STP drilling and thestratigraphic evidence collected in the two trenches. As already pointed out, the STP drillingshows that in the hangingwall section of trench A the top of Unit A is located at a maximumdepth of ca. 7 m below the ground surface (Ferreli et al., 1998). It is important to note,however, that the top of horizon 2b in the footwall section is an erosional surface. Thus, thetop of horizon 2b does not coincide with the top of Unit A. On the other hand, it isreasonable to suppose that in the hangingwall section the continuous stratigraphic passagefrom Unit A to Unit C could be preserved. For instance, this passage is well exposed along the


walls of trench B (Fig. 9). This allows to infer that the hangingwall section of trench A mayinclude some horizon from Unit A that was deposited after the horizon 2b. As mentionedbefore, the overlapping ages obtained from radiocarbon dating of samples in the top footwalland bottom hangingwall clearly indicate that the thickness of possible post-horizon 2b Unit Adeposits in the hangingwall should be less than 1 m. In conclusion, our best estimate of themaximum vertical displacement for horizon 2b is ca. 8 m.Therefore, using the extreme values for horizon 2b age and throw as discussed above, the

vertical component of the Moscarello fault slip rate at this site ranges between 1.4 (5.6 m in 4kyr) and 2.7 (8 m in 3 kyr) mm/yr.

4.2. Late Holocene displacement history at the trench site

The data collected in trench A provide suitable information to reconstruct the nature andevolution of the displacement at this site. Fig. 10 shows our model of the fault scarp growth inthe last ca. 6 kyr. The lack of sedimentary structures in the hangingwall section prevents fromevaluating in detail the number and size of the single displacement events. However, theerosional surfaces preserved in the footwall sequence, and the relations among displaceddeposits and fault planes in the rupture zone clearly demonstrate the occurrence of repeatedscarp formation èvents' related to instantaneous surface faulting during the deposition of UnitA. Based on the stratigraphic and erosional structures observed in the trench footwall, theseèvents' are reconstructed in Fig. 10a±f. As previously noted, our data do not allow todiscriminate if one èvent' represents one or more episodes of surface faulting. However, it isde®nitely possible to rule out that fault creep phenomena played a major role in the scarpgrowth during the deposition of Unit A. Surface faulting events and scarp growth in thisperiod essentially took place through recurrent high-intensity (MSK IX±X or greater)earthquakes. Fig. 10g shows a dislocation of 3.6 m that should be obviously seen as the resultof several displacement events. Once again, there is no stratigraphic evidence suitable foridentifying the di�erent stages of the fault growth during the deposition of Unit C. Therefore,we can hypothesize that in the last ca. 3 kyr creep movements dominated the fault slip alongthe scarp at this site. However, this is in complete disagreement with the fault behaviourtypically associated with signi®cant releases of seismic energy, as indicated by (i) the historicalinformation in the last 150±200 yr and (ii) the previously illustrated footwall stratigraphicfeatures in the ca. 6 to 3 kyr B.P. time interval of Unit A deposition. For this reason, thealternative hypothesis that coseismic fault slip characterized the scarp growth also during UnitC deposition appears to be more reasonable. The lack of scarp-related sediments can beexplained because the stratigraphy of the trench hangingwall section is completely disturbed byagricultural reworking. In particular, this is also supported by the observation that there is noevidence in the trench for the surface faulting e�ects described at this site after the 1971earthquake. At this end, it should be noted that the coarse grained gravels representing most ofthe deposits exposed in the trench footwall are almost totally lacking in the hangingwallsection. Such feature can be partially due to the soil selection likely produced by the long-dated, intense agricultural activities. However, this can also be an indication that the coseismicdisplacement during Unit C deposition never produced scarps higher than ca. 40±50 cm, that isabout the present day thickness of the organic soil along the trench. Such a value corresponds


to the typical vertical o�set associated with a single coseismic fault strand as observed after the1971 and, partially, the 1911 earthquakes. In this way, perhaps the top of Unit A has beennever signi®cantly exposed along the fault scarp earthquake free-faces in the last ca. 3 kyr,which prevented substantial erosion and redeposition of the footwall gravel deposits in thefault hangingwall.

5. Conclusions

Paleoseismological investigations at the Fondo Macchia site have provided, the ®rst time foran active fault at Etna volcano, direct stratigraphic evidence for constraining the latestHolocene vertical slip-rate of the Moscarello normal fault. The observed value of 1.4±2.7 mm/yr is a minimum slip-rate for this central sector of the fault. We explored a single scarp at asingle site, while we learned from observations of modern historical earthquakes that severalparallel scarps may rupture coseismically in the Fondo Macchia area. Also, this value of 1.4±2.7 mm/yr is almost the same as the 1.5±2.0 mm/yr of minimum vertical long term slip-rateobtained using the age and throw relations shown in Fig. 5a. Therefore, both the trenchstratigraphy for the last ca. 6 kyr and the geomorphic evidence for the last ca. 80 kyr clearlyshow that the vertical slip-rate at Fondo Macchia is substantially greater than 1.4±2.7 mm/yr.It is important to remark that vertical slip-rates from exploratory trenching in the Apenninesare typically lower than 1 mm/yr (Giraudi and Frezzotti, 1995; Pantosti et al., 1993, 1996;Michetti et al., 1995, 1996, 1997; Galadini et al., 1997). As expected for a volcanic environmentsuch as at Mt. Etna, the activity rates of the Moscarello fault are therefore signi®cantly higherthan for the Apennines normal faults.In conclusion, the relevant deformation rate documented for the modern period might be

extended back in the past to a time-span of some thousands of years at least. Also, thesedimentary structures observed in the trench footwall strongly suggest that, as for the last150±200 yr of detailed historical record, fault behavior at Fondo Macchia is governed bycoseismic surface displacement rather than fault creep. Stratigraphic trench evidence possiblyindicates that the coseismic vertical displacements occurred in the last millennia are not highbut may be compared with those of earthquakes similar to the 1971 one. Finally, if thepaleoseismic slip rate is compared with the variation in vertical separation along strike, it ispossible to recognize di�erent grades of tectonic activity in a few kilometers (from FondoMacchia as far as Guardia), thus implying likely fault segmentation. Further investigations,including also seismic re¯ection and electrical prospections useful for characterizing thesubsurface geology of the Fondo Macchia basin, will be performed in order to better de®netotal vertical displacement, slip rates and slip per event on the Moscarello fault.Nevertheless, the present study con®rms that paleoseismology techniques can be applied with

success also in active volcanic environments (e.g. Smith et al., 1996), typically characterized byrheology and, consequently, seismicity and fault dynamics very di�erent from those of othertectonic environments in which paleoseismology has been ®rstly developed and is todayextensively applied.


Acknowledgements

The authors are indebted to Prof. L. Villari for the continuous support to this research. Wethank P. Del Carlo and M. Coltelli who assisted us in the stratigraphic interpretation ofvolcanoclastic deposits; thanks are also due to M.S. Barbano, M. Cosentino, B. Antichi and R.Rigano for help and discussion in the ®eld. B. Puglisi assisted in the micro topographicpro®ling operations. Ing. L. Archetti technical and administrative service was of invaluablehelp for a successful trenching. D. Wells kindly provided data plotted in Fig. 4. We areespecially grateful to Avv. A. Ruggiero and F. Proietto Batturi for kind permission to entertheir properties. This study was funded by ANPA, GNDT and GNV.

Appendix A

A preliminary analysis based on typological and technical-stylistical identi®cation hasindicated that the ceramics found in the Fondo Macchia trenches do not display, from thearchaeological point of view, any meaningful characteristics. The pottery in question is formedby very small hand-made sherds (mainly body sherds of vessels) having no decorations,symbols or seals useful for dating the fragments. The only valuable elements are ultimately thetype of the pottery and their workmanship.

A1. Trench A, south wall

Sample FMR 19: Fragment (4 � 3 cm) of a pot made of bright plain pottery. It is madewithout a wheel, full of mica, and with just one trace of almost invisible red paint; thethickness of the sherd is medium/thin, and it has on one side a small semicircular recess (about1 cm in diameter). The pot seems to be an artifact of an indigenous production since thevessel's shape does not bear any similarity to the usual Greek shapes; the kind of workmanshipand the paste of the pottery only allow us to date the sherd between 1100 B.C. and VI centuryB.C..

A2. Trench A, north wall

Sample FMR 6: Two sherds (ca. 11 � 7 cm) of pot with a medium thickness, made on thewheel, with a bright clay in the inner side and a light slip on the surface; it presents aworkmanship that could probably be modern.Sample FMR 7: Two fragments (ca. 12 � 7 cm) of tiles made of bright paste; it presents a

workmanship that could probably be modern.Sample FMR 8a: One fragment (ca. 6 � 4 cm) of tile made of bright paste; it presents a

workmanship that could probably be modern.Sample FMR 8b: one fragment (ca. 11 � 9 cm) of tile made of bright paste; it presents a

workmanship that could probably be modern.


A3. Trench B, south wall

Sample FM 8: Three body sherds (ca. 4 � 4 cm) of a vessel of medium thickness and madeon the wheel; it presents a workmanship that could probably be modern.Sample FM 13: One fragment (ca. 5 � 5 cm) of tile of medium thickness; it presents a

workmanship that could probably be modern.

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