Active Tectonics Along the Nebrodi-Peloritani Boundary (Ne Sicily): a New Potential Seismogenic Source S

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GNGTS 2011 SESSIONE 1.1 loghi CPTI08/CFTI4 e di rivedere la valutazione delle intensità assegnate secondo la scala EMS98. Quest’ultima operazione è stata svolta con una sistematica e minuziosa rilettura delle descrizioni di danni, organizzata attraverso l’omogeneizzazione dei termini usati per descriverli e integrata con gli elementi necessari per ricostruire l’assetto urbanistico degli insediamenti colpiti in base alla loro consistenza numerica, tipologia edilizia, vulnerabilità. La disamina delle fonti porta ad assegnare una Imax 9 EMS alla sola località di Castelnuovo, mentre per altre sette località risulta un’intensi- tà ben al di sopra della soglia del danno. La magnitudo macrosismica calcolata con il codice Boxer 4.0 (Gasperini et al., 2010) è pari a Mw 5.5. Al termine dello studio il terremoto del 6 ottobre 1762 risulta considerevolmente arricchito per quanto riguarda il quadro complessivo delle conoscenze e lievemente ridimensionato sia nella magnitudo sia nella intensità massima raggiunta; la distribuzione delle intensità è probabilmente “viziata” da effetti locali, a conferma di quanto già emerso in precedenti studi (Lanzo et al., 2011). L’ipotesi che l’evento del 1762 possa essere stato un predecessore (Tertulliani et al., 2009) di quel- lo del 2009 sembra da riconsiderare, risultando più credibile che si sia trattato di un evento collo- cato su un segmento di faglia più meridionale. Più in generale, l’esperienza fatta con questo studio ci conferma che anche in zone ad alta sismicità dove i terremoti sono noti e ben documentati, dopo decenni di studi di sismologia storica, il margine di miglioramento delle conoscenze può essere, in certi casi, ancora ampio. Bibliografia Gasperini P., Vannucci G., Tripone D., and Boschi E.; 2010: The location and sizing of historical earthquakes using the attenuation of macroseismic intensity with distance, Bull. Seismol. Soc. Am., 100, 2035–2066, doi: 10.1785/0120090330. Gruppo di lavoro CPTI; 2009: Catalogo Parametrico dei Terremoti Italiani, versione parziale “CPTI08aq”, a cura di A. Rovida, http://emidius.mi.ingv.it/CPTI08. Guidoboni E., Ferrari G., Mariotti D., Comastri A., Tarabusi G. and Valensise G.; 2007: CFTI4Med, Catalogue of strong earthquakes in Italy (461 B.C.-1997) and Mediterranean area (760B.C.-1500), http://storing.ingv.it/cfti4med Tertulliani A., Rossi A., Cucci L., Vecchi M.;2009: L’Aquila (Central Italy) earthquakes: the predecessors of the April 6, 2009 event, Seism. Res. Lett. 80, 6, 1008-1013, doi: 10.1785/gssrl.80.6.1008. Lanzo G., Silvestri F., Costanzo A., d’Onofrio A., Martelli L., Pagliaroli A., Sica S., Simonelli A.; 2011: Site response studies and seismic microzoning in the Middle Aterno valley (L’Aquila, Central Italy), Bull. Earthquake Eng., 9, 1417–1442, DOI 10.1007/s10518-011-9278-y.

ACTIVE TECTONICS ALONG THE NEBRODI-PELORITANI BOUNDARY (NE SICILY): A NEW POTENTIAL SEISMOGENIC SOURCE S. Catalano, F. Pavano, G. Romagnoli, G. Tortorici Dipartimento di Scienze Geologiche, Università di Catania Introduction. In NE Sicily, the geodetic data (Hollenstein et al., 2002) indicate the occurrence of a crustal sector that diverges from the rest of the Sicily. Several sites located in the Nebrodi- Peloritani Mountains and the Eolian Islands, in fact, show N-ward directed GPS vectors that, if combined with those of the rest of the Sicily, pointing towards the NW, would constrain a NNE- SSW oriented active extension, at rate of about 4.5 mm/a (inset in Fig. 1). This evidence conflicts with the available geological data, reporting for the Nebrodi-Peloritani Boundary diffuse Plio-Pleis- tocene NW-SE oriented dextral faults, associated with NE-SW oriented thrust and folds and E-W oriented oblique thrust (Ghisetti & Vezzani, 1982; Lentini et al., 2000). A morphostructural study, based on analyses of 1:10,000 and 1:33,000 scale aerial photographs and satellite images and field analyses of the terraced deposits, has been carried out along the Tyrrhenian coast of NE Sicily in order to recognize the location and the geometry of the tectonic margin that separates the diverging crustal domains. The study has been integrated with mesoscale kinematic analyses on fault planes. The aim of the study is to define the displacement-rate and the related seismogenic potential that can be released along this active extensional boundary. Marine terraces and tectonic uplift in NE Sicily. The Tyrrhenian coast of the NE Sicily is bordered by a flight of Late Quaternary marine terraces, which is composed by several polycyclic

GNGTS 2011 SESSIONE 1.1 surfaces that ranges from the OIS 15 (570 ka) to the OIS 3 (60 ka) (Catalano & Di Stefano, 1997) (Tab. 1). The tectonic uplifting of the region post-dates the deposition of the Early-Middle Pleistocene (900-600 ka) marine sequence that now crops out at an elevation of about 580 m a.s.l., in the area of the Naso village, near Capo d’Orlando (Fig. 1). The present elevation of these deposits, corrected for their depth of deposition, constrain a total vertical displacement of about 680 m, in the last 600 ka. This implies an averaged uplift-rate of about 1.1 mm/a. This estimation is consistent with the vertical displacement of the strandlines of the marine terraces that indicates an almost uniform uplift-rate (1.1 ± 0.05 mm/a), constant for the entire 80 km-long segment of the coast, from Messina to Capo d’Orlando (Tab. 1).

Tab. i - Marine terraces.

A sharp decrease in the elevation of the strandlines of the marine terraces has been recognised in the coast between Capo d’Orlando and S. Marco d’Alunzio (Fig. 1). In this segment of the coast, a single marine platform represents each of the OISs from 13 (520 ka) to 7 (240 ka). A polycyclic abrasion surface has been recognised only for the OIS 5 (125-80 ka), while the marine terrace of the OIS 3 (60 ka) has been cancelled by the retreat of the Holocene sea-cliff (tab 1). The elevation of the strandlines in the S. Marco d’Alunzio section indicate an averaged uplift-rate of about 0.7 mm/a, almost constant in the last 520 ka. This implies that a relative vertical motion of about 0.4 ± 0.05 mm/a could be related to the occurrence of active tectonics in the region between Capo d’Or- lando and S.Marco d’Alunzio Quaternary tectonics in the Capo d’Orlando-San Marco d’Alunzio region. In the region between S. Marco d’Alunzio and Capo d’Orlando (Fig. 1) Early-Middle Pleistocene deposits, composed of basal calcarenites evolving to batial marly clays (Catalano & Di Stefano, 1997), culminate in the hill of Naso. Along the southwestern slope of this culmination, discrete segments of a NW- SE oriented dextral shear zone have been reactivated by extensional motions, generating the Naso Fault Zone (NFZ in Fig. 1). The normal faulting has involved the deposits of the Pleistocene marine succession, whose basal levels are downthrown towards the southwest, for about 40 m. The vertical motion along the fault zone has been associated with the deposition of syn-tectonic slope deposits that consist of a prevalent sandy matrix, including metric blocks of the Pleistocene calcarenites. These deposits, which drape ancient fault scarps, being involved by the later normal fault motions (Fig. 2), are terraced along an outer-edge that, resting at an elevation of about 130 m a.s.l., can be correlated, towards the coast, with that of the Tyrrhenian (125-80 ka) polycyclic abrasion surface. The normal fault segments located to the south of Naso are concealed by a thick Recent

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Fig. 1 - Active tectonics and Pleistocene deposits at the Nebrodi-Peloritani Boundary. The focal mechanism of the 06.23.2011 event is from ISIDe Working Group (INGV, 2011).

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Fig. 2 - Panoramic view of the free face along the SFZ (a); Detail of the free face, showing a basal light-coloured strips, along the SFZ (b); Late Pleistocene slope deposits displaced along the NFZ (c).

(Holocene) talus that draps graded slope profile. More to the south, in the area running from S. Mar- co d’Alunzio to Galati Mamertino, a further NW-SE oriented, 8.5 km-long, fault zone (San Marco d’Alunzio Fault Zone; SFZ in Fig. 1) is composed of several discrete NW-oriented fault segments, showing well defined rejuvenated scarps as an evidence of their Holocene activity. The remobilised fault segments range in length from 1 to 4 km. The longest segment, located between the villages of Frazzanò and S.Marco d’Alunzio (Fig. 1), is characterised by a 3-5 m high, SW facing fresh bedrock scarp, marked at the base by a light coloured strip (Fig. 2). Kinematic indicators observed on the fault plane constrain a normal sense of motion (pitch= 80°). This normal fault is associated with a minor 2 km-long antithetic segment, marked by a 3 m-high rejuvenated scarp. This antithetic fault is characterised by striations indicating a normal (pitch= 80°) sense of motion, superimposed on ancient indicators of dextral movement (pitch= 160°). More to the south, further four NW-SE oriented segments, showing 2 m-high fresh bedrock scarps have been recognised. Between the villages of Galati Mamertino and Tortorici (Fig. 1), two SW dipping steep scarps extends for about 1500 m and 800 m, respectively. Between Longi and the Catafulco Gorge, two antithetic, NE facing fresh scarps have been recognised for length of 500 m and 1500 m, respectively. Finally, to the southwest of Tortorici, two measurement sites have been located along a N10 oriented, E-dipping, 1600 m long fresh bedrock scarp. On these sites striations and accretion and frac- ture steps indicate both dextral (pitch= 30°) and more recent normal (70-80°) motions. Discussion and conclusion. In NE Sicily, an active NW-SE oriented extensional fault zone has been recognised to the south of Capo d’Orlando, at the Nebrodi-Peloritani Bouandary. It is composed of main SW-dipping fault segments that are distributed in a 10 km-wide belt, with associated minor NE-facing antithetic planes. The active faults have originated from the re-activation of part of the pre-existing Plio-Pleistocene dextral shear zones. The active normal fault belt separates two distinct sectors of NE Sicily, which are characterised by different rate of tectonic uplifting. The maximum vertical displacement has been recognised along the NFZ, at the northeastern border of the faulted zone, where the deposition of Late Pleistocene (125-80 ka) syn-tectonic clastic deposits

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Fig. 3 - Geometry at depth of the NFZ and conjugate faults and their relation with the swarm seismicity of June- August 2011 (data from ISIDe Working Group - INGV, 2011) has been associated to a cumulative vertical offset of 40 m. It is to remark that the faults of the NFZ are associated with graded slopes, draped by Holocene deposits. This implies that the fault activity along the NFZ was confined to the Late Pleistocene, with a vertical displacement-rate estimated from 0.6 to 0.4 mm/a. Evidence of diffuse Holocene remobilisation can be recognised within the faulted belt, where several aligned fault segments show a distinctive sharp free face, at places marked at the base by a light coloured strip. These morphostructural features are usually referred as the result of the vertical displacements that cumulated in the last climatic period favourable for the slope stabilisation. According to several Authors, the present-day climatic stability began from 15 ka (Bar-Matthews et al. 1997) to 10 ka (Gvirtzman & Wieder 2001). As a consequence the 5 m-high fresh bedrock scarps, developed along the 8.5 km long SFZ, would indicate a maximum Holocene vertical displacement-rate ranging from 0.3 to 0.5 mm/a. It is to note, that both the long-term and the short-term estimations of the displacement-rate along the extensional belt coincide with the dif- ference in the uplift-rate measured across the faulted zone. The active extensional fault belt, actual- ly, accommodates the entire vertical relative motions between the two adjacent crustal blocks of NE Sicily. The seismogenic potential of this regional tectonic lineament can be evaluated considering the maximum length (8.5 km) of the Holocene fault scarps, which would constrain a maximum expected M≈ 6.1 (Wells & Coppersmith, 1994). The recurrence of events could be estimated at about 0.270 ka, by the comparison between the maximum expected co-seismic displacement (160 mm) with the maximum estimated Holocene displacement-rate (0.6 mm/a), assuming a dip of the fault of 60°. The displacements along the high-angle faults accommodate only a small portion of the total extension measured by geodetic data. This apparent incongruence could be explained, assuming a listric geometry for the NFZ (Fig. 3). This master fault would accommodate at depth the total amount of extension that, towards the surface, is partly transferred along discrete high-angle fault zones (e.g. SFZ) and partly dispersed along minor shear planes within the hangingwall of the NFZ. An example of the widespread dispersion of the extensional shearing is suggested by the swarm of the low-magnitude (≤ 4.1) seismic events of the June-August 2011 (INGV, 2011) that evidenced a diffuse fracturing confined in the rider bounded by the NFZ and the SFZ. It is noteworthy that the main events of this swarm, showing focal mechanism consistent with the kinematics of the active faults, are distributed along NE-dipping antithetic planes. This suggests the occurrence of shallow fracturing along minor structures, accommodating discrete motion along the low-angle sole-fault.

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References Bar-Matthews M., Ayalon A.,, Kaufman A; 1997: Late Quaternary Paleoclimate in the eastern Mediterranean Region from Stable Isotope Analysis of Speleothems at Soreq Cave, Israel. Quat. Res., 47, 155–168. Catalano S., and Di Stefano A.; 1997: Sollevamento e tettogenesi Pleistocenica lungo il margine tirrenico dei Monti Peloritani: integrazione dei dati geomorfologici, strutturali e biostratigrafici. Il Quaternario, 10, 337-342 Ghisetti F., Vezzani L.; 1982: Different styles of deformation in the calabrian arc (southern italy): implications for a seismotectonic zoning. Tectnonophysics, 85, 149-165 Gvirtzman G., Wieder M.; 2001: Climate of the last 53,000 years in the eastern Mediterranean, based on soil-sequence stratigraphy in the coastal plain of Israel. Quat. Sci. Rev., 20, 1827–1849. Hollestein Ch., Kahle H.-G., Geiger A., Jenny S., Geos S., Giardini D.; 2003: New GPS constraints on the Africa-Europe plate boundary zone in southern Italy. Geophysical Research Letters, 30, NO.18, 1935. ISIDe Working Group (INGV, 2011): Italian Seismological Instrumental and parametric database: http://iside.rm.ingv.it Lentini, F., Catalano, S., Carbone, S.,; 2000: Carta geologica della Provincia di Messina. Ed. S.El.Ca, Firenze. Wells D.L., Coppersmith, J.K.; 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bull. seism. Soc. Am., 84, 974–1002.

NUOVO CENTRO ACQUISIZIONE DATI DELLA RAN G. Costa 1, L. Filippi 2, E. Zambonelli 2, A. Ammirati 2, M. Nicoletti 2, M. Dolce 2 1 Seismological Researcher Group (SeisRam) - Dipartimento di Geoscienze, Università degli Studi di Trieste 2 Dipartimento della Protezione Civile, Roma Introduzione. La rete accelerometrica italiana (RAN) del Dipartimento della Protezione Civile (DPC) (Gorini et al., 2010, Zambonelli et al., 2010) è costituita da 463 stazioni. L’attuale distribuzione spaziale della rete consente il monitoraggio sismico delle aree urbane ad elevata pericolosità sismica del territorio italiano. La geometria della rete ben si integra con stazioni accelerometriche di reti locali presenti sul territorio nazionale gestite da altre amministrazioni pubbliche, università ed istituti di ricerca In particolare la RAN ben si integra in Friuli Venezia Giulia con la rete accelerometrica (RAF) della regione gestita dal dipartimento di Geoscienze dell’Università di Trieste (UTS-DIG) (Costa et. al.,.2009) e in Campania la rete ISNET dell’AMRA (Weber et al., 2007). La RAN include 192 stazioni della storica rete italiana, installate all’interno delle cabine di tra- sformazione ENEL, che hanno registrato i terremoti più forti che si sono verificati in Italia in epo- ca strumentale (Friuli 1976, Irpinia 1980, Umbria-Marche 1997-1998, Molise 2001). I vecchi strumenti analogici sono stati sostituiti e ad oggi tutte le stazioni della RAN sono provviste di strumenti digitali, sono sincronizzate al GPS e sono connesse in tempo quasi-reale alla sede del DPC a Roma attraverso le tecnologie di trasmissione dati GSM/GPRS. Presso la sede del DPC è installa- to un sistema hardware e software che utilizza l’applicativo Antelope per la ricezione, l’archiviazio- ne e l’analisi del dato in tempo reale ed in modo automatico. Grazie ad accordi interistituzionali ed apposite convenzioni il sistema del DPC riceve, attraverso Antelope, anche i dati accelerometrici registrati dalle reti RAF e ISNET (Fig. 1). Personalizzazioni del software Antelope per il suo utilizzo a fini di protezione civile. Le caratteristiche tecniche degli strumenti digitali installati sulla RAN e sulle reti integrate alla rete nazionale e l’acquisizione ed elaborazione dei dati centralizzata, hanno consentito ai funzionari del- l’Ufficio Rischio Sismico e Vulcanico (SIV) del DPC e ai ricercatori del Seismological Research and Monitoring group (SeisRaM) dell’UTS-DIG di definire ed implementare procedure automati- che, nell’ambiente software Antelope. Tali procedure permetteranno al DPC di estrarre informazioni utili da dati strumentali, il più pos- sibile attendibili, subito dopo un terremoto di magnitudo locale (Ml) maggiore o uguale a 4 al fine di aumentare l’efficacia della risposta del DPC in caso di emergenza sismica. Il software Antelope riceve i dati in tempo reale o quasi reale, li elabora e li archivia insieme ai risultati dell’elaborazione, in un database relazionale. Lo stesso software provvede automaticamente alla localizzazione dell’evento ed alla sua associazione con le localizzazioni provenienti da altre agenzie. Per le registrazioni associate ad eventi sismici, il gruppo SeisRaM ha elaborato una procedura