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SGI Bollettino 20/06-133
Boll.Soc.Geol.It. (Ital.J.Geosci.), Vol. 126, No. 2 (2007), pp. 209-222, 5 figs., 3 tabs.
Queste bozze, corrette e accompagnate dall’allegato preventivo firmato e dal buono d’ordine, debbono essere restituite immediatamente alla Segreteria della Società Geologica Italiana c/o Dipartimento di Scienze della Terra Piazzale Aldo Moro, 5 – 00185 ROMA
Seismogenic evidence of ongoing compression in eastern-central Italy and mainland Sicily: a comparison GIUSY LAVECCHIA (*), RITA DE NARDIS (***), FRANCESCO VISINI (*), FEDERICA FERRARINI (***) & MARIA SERAFINA BARBANO (****)
ABSTRACT We compare the seismogenic role played by two crustal-scale reverse-type shear zones located at the outer border of the ApennineMaghrebian fold-and-thrust belt system: the westward-dipping Adriatic Basal Thrust (ABT) in eastern-central Italy and the northward-dipping Sicilian Basal Thrust (SBT) in southern Sicily. The epicentral and hypocentral distributions and, whenever possible, the kinematics of a merged dataset of pre-instrumental (1600 to 1900), early instrumental (1901 to 1980) and instrumental (1981 to 2005) earthquakes with moment magnitude ≥4.5 sited within the surface areas above the ABT and the SBT depth-contour lines (0-30 km) are analysed. They are both first-order crustal-scale seismogenic structures which strongly control reverse, reverse-oblique and strike-slip seismic activity. Relatively deep (10-to-30 km) seismicity occurring in the foothills region and in mainland Sicily may be associated with brittle shearing of the middle and lower crust ABT and SBT thrusts segments, whereas the shallow seismicity (5.0 in the Marche-Coastal Adriatic region, among which are the 1741 Fabrianese (Maw 6.08) and 1799 Camerino (Maw 5.93) events, and of 10 events with magnitude >5.0 in southern and mainland Sicily, including the 1818 Catanese (Maw 6.00) and 1968 Valle del Belice (Maw 6.12) events. Based on the integration of tectonic and seismological constraints, the boundaries of two seismogenic compressional provinces located above the ABT and the SBT are defined and the associated values of yearly energy release per unit area evaluated. Similar values are obtained for the ABT (~8E+14 erg/y/km2) and for the SBT (~4E+14 erg/y/km2). The recognition of close geometric, kinematic and seismotectonic similarities between the two compressional provinces, further supported by a comparable level of long-term seismic activity, has evident implications both for regional tectonic reconstructions and for seismic hazard assessment purposes.
KEY WORDS: Central Italy, Adriatic Sea, Sicily, seismicity, compression, seimogenesis.
RIASSUNTO Evidenze sismogenetiche di attività compressiva in Italia centro-orientale ed in Sicilia centro-meridionale: un confronto. Il fronte esterno del sistema a pieghe e sovrascorrimenti appenninico-magrebide si estende senza interruzione di continuità
(*) Laboratorio di Geodinamica e Sismogenesi, Dipartimento di Scienze della Terra, Università «G. d’Annunzio» - Campus Universitario - 66013 Chieti Scalo, Italia
[email protected] (**) Dipartimento della Protezione Civile, Servizio Sismico Nazionale, DPC-SSN, Roma. (***) Dipartimento S.T.A.T., Università degli Studi del Molise Contrada Fonte Lappone - 86090 Pesche (Isernia). (****) Dipartimento di Scienze Geologiche, Università di Catania - Corso Italia, 55 - 95129 Catania.
dall’area padana alla Sicilia. Numerosi autori ritengono che l’attività del fronte sia terminata nel Pleistocene inferiore (PATACCA et alii, 1990; BUTLER et alii, 1992; LICKORISH et alii, 1999; DI BUCCI & MAZZOLI, 2002; PATACCA & SCANDONE, 2004), ma altri riportano, almeno per l’Italia centrale e la Sicilia, evidenze di deformazione compressiva tardo pleistocenica-olocenica (BOUSQUET & LANZAFAME, 1986; MONACO et alii, 1996; FREPOLI & AMATO, 1997; LAVECCHIA et alii, 2003; VANNOLI et alii, 2004; MONTONE et alii, 2004; NERI et alii, 2005; CATALANO et alii, 2004 and 2006). La nostra opinione è che l’analisi approfondita dell’assetto sismotettonico dell’area padano-adriatica e di quella siciliana, ed in particolare il confronto tra le due, possa portare un sostanziale contributo alla discussione in atto. In questo lavoro abbiamo, pertanto, analizzato e paragonato le caratteristiche geometriche e sismogenetiche, a scala crostale, del sovrascorrimento basale del sistema appenninico-magrebide nel segmento adriatico, ovest-immergente, da Pescara a Pesaro, ed in quello siciliano, nord-immergente, da Sciacca a Catania. La geometria tridimensionale dei due sovrascorrimenti è stata schematicamente ricostruita attraverso l’utilizzo e l’analisi critica di innumerevoli sezioni geologiche e geofisiche regionali disponibili in letteratura. L’analisi sismologica è stata effettuata tenendo conto delle informazioni fornite dai cataloghi regionali e locali, nonché da pubblicazioni specifiche (BARATTA, 1897; MONACHESI et alii, 1985; POSTPISCHL, 1985; MONACHESI et alii, 1991; MONACHESI & STUCCHI, 1997; RIGANO et alii, 1999; BOSCHI et alii, 2000; AZZARO & BARBANO, 2000; AZZARO et alii, 2000a; AZZARO et alii, 2000b; WORKING GROUP CPTI, 2004; CASTELLO et alii, 2005; BOLLETTINO SISMICO ITALIANO, 2003-2005). Un dataset integrato degli eventi maggiori (magnitudo momento ≥4.5), storici e strumentali, avvenuti nell’intervallo temporale 217 a.C.-2005 d.C., è stato compilato ed analizzato dal punto di vista della completezza. L’analisi critica della forma e dell’estensione del campo macrosismico dei terremoti storici ha permesso di effettuare una stima qualitativa della profondità delle sorgenti sismiche e di attribuire gli eventi nel dataset a sorgenti relativamente superficiali (4) ha evidenziato una cinematica da inversa e trascorrente, con assi di massima contrazione sub-orizzontali ed orientati da NE-SO ad E-O nell’area marchigiana-adriatica e da NNE-SSO a NNO-SSE in Sicilia. L’integrazione delle informazioni geologiche e sismologiche ci ha permesso di definire i confini e la geometria, superficiale e profonda, di due omologhe province sismogenetiche compressive, localizzate al tetto dei sovrascorrimenti basali adriatico e siciliano. Ognuna delle province è stata suddivisa in due sotto-province: una caratterizzata da attività sismica associata a deformazioni di taglio lungo la porzione più superficiale (< circa 10 km) del sovrascorrimento basale e lungo gli splays frontali e laterali, l’altra caratterizzata da attività sismica più profonda (10-25 km), localizzata lungo la porzione medio crostale e nella parte alta della crosta inferiore del sovrascorrimento basale. Le due province sono state quantitativamente comparate dal punto di vista dell’attività sismica osservata durante l’intervallo di completezza dei dati analizzati ed espressa in termini di energia sismica per anno su unità di area. I valori ottenuti, pari a circa 8E+14 erg/y/km2 per l’area marchigiano-adriatica ed a circa 4E+14 erg/y/km2 per l’area siciliana, hanno messo in luce una sostanziale, e finora non nota, equivalenza nel potenziale sismogenetico a lungo termine rilasciato dalle due sotto-province profonde, con evidenti implicazioni in termini di pericolosità sismica.
TERMINI CHIAVE: Italia centrale, Adriatico, Sicilia, sismicità, compressione, sismogenesi.
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1. INTRODUCTION
The Apennine-Maghrebian fold-and-thrust belt system extends with continuity from the Po Plain-Adriatic region to southern Sicily (fig. 1a). It developed since late Oligocene times through the progressive nucleation of inward-dipping crustal slices thrust over the AdriaticPelagian foreland (LAVECCHIA, 1988). According to several authors, the entire belt ended its activity in early Pleistocene times (PATACCA et alii, 1990; BUTLER et alii, 1992; LICKORISH et alii, 1999; DI BUCCI & MAZZOLI, 2002; PATACCA & SCANDONE, 2004), but according to some others it is still undergoing compression, at least in easterncentral Italy and in Sicily. In particular, on the basis of the combined analysis of historical, seismological and geomorphologic data, VANNOLI et alii (2004) outlined a rather regularly segmented active fault system running parallel to the shoreline from Ancona to Rimini (NW-SE), made up of blind thrusts accompanied by anticlinal growth. MONTONE et alii (2004), through the analysis of borehole breakouts, showed that the maximum horizontal stress is generally perpendicular to the strike of the shallow (0-6 km depth) middle Pliocene to Quaternary thrust structures of the Po Plain and Coastal-Adriatic domain. On the basis of the analysis of instrumental seismological data, several authors considered that this domain is undergoing active compression (FREPOLI & AMATO, 1997 among many others). In fact, earthquake focal mechanisms show a predominance of thrust and strike-slip solutions with P-axes generally perpendicular to the trend of the outer thrust system (PONDRELLI et alii, 2004). LAVECCHIA et alii (2003) identified an individual, regional scale structure, responsible for such seismicity in the outermost westward-dipping thrust plane of the Apennine fold-and-thrust belt, which emerges along the Po Plain-Adriatic front. In Sicily, BOUSQUET & LANZAFAME (1986) were the first to document Quaternary E-W contractional deformation, at the external limit of the thrust belt in eastern Sicily. MONACO et alii (1996), based on seismological, structural and morphotectonic observations, suggested Late Quaternary activity of the Sciacca thrust, an inner splay of the outer basal thrust. LANZAFAME et alii (1997) documented field evidence of Late Pleistocene-Holocene N-S compression in the Etna area, with a shortening of a few kilometres. TORELLI et alii (1998) recognised, in a seismic line across the Catania foredeep, a clear incipient northward-dipping reverse shearing, which cuts obliquely through the Pleistocene sediments. CARBONE et alii (2000) pointed out geomorphologic evidence of reactivation of out of sequence thrusts beneath Etna and in the Nebrodi area. Furthermore, CATALANO et alii (2004 and 2006) showed evidence of Late Quaternary tectonic inversion of the northern border faults of the Scordia-Lentini Pleistocene graben, at the front of the Gela-Catania thrust plane, with formation of post 40 ka E-W trending folds. COCINA et alii (1997) and NERI et alii (2005) documented N-S compressional seismogenic activity at depths between 10 and 30 km in eastern Sicily, beneath the Etna volcanic area, whereas CACCAMO et alii (1997) and JENNY et alii (2006) recognise N-S seismogenic compression in western and central Sicily. LAVECCHIA et alii (2006) identified an individual, regional scale structure, possibly responsible for such a compressional seismicity, in a PlioQuaternary north-dipping thrust zone cutting the entire
Sicilian crust and emerging at the outer front of the Sicilian thrust system, along the Sciacca-Catania belt. In this paper, we present a contribution to this discussion focusing attention on the kinematics and the distribution of the historical and instrumental seismicity located in the areas to the rear of the Pescara-Pesaro segment of the Po Plain-Adriatic front and of the Sciacca-Catania segment of the Sicilian front. In particular, we will investigate if the major earthquakes may be linked to reverse shearing along two crust-scale basal thrusts, here respectively called Adriatic Basal Thrust (ABT) and Sicilian Basal Thrust (SBT). With this aim we will reconstruct the ABT and SBT threedimensional shape; we will compile an updated and integrated dataset of all the major events that occurred within the two study areas during the last two thousands years and, through the analysis of the associated macroseismic fields, we will define likely depth ranges of the seismogenic sources. By comparing the 3-D shape of the basal thrusts with the kinematics and with the surface and depth distribution of the overlying earthquakes, we will define the boundaries of homogeneous seismogenic provinces. After verifying the completeness of the seismological dataset, we will also try to compute and quantitatively compare the energy release in the two study areas. 2. THE ABT AND SBT BASAL THRUSTS
The ABT overthrusts the Umbria-Marche meso-cenozoic sedimentary cover and the overlying Plio-Pleistocene foredeep terrains above the Adriatic foreland. It emerges with an eastward convex shape along the Pesaro-Adriatic Sea-Pescara front and deepens westward, cutting through the entire crust and reaching its base beneath the Umbria region (BARCHI et alii, 1998; FINETTI et alii, 2001; LAVECCHIA et alii, 2003). The SBT overthrusts terrains pertaining to the Sciacca-Saccense tectonic unit and to the GelaCatania early-middle Pleistocene foredeep deposits above the Pelagian-Hyblean foreland (GHISETTI & VEZZANI, 1984; CATALANO et alii, 1989; BUTLER et alii, 1992; TORELLI et alii, 1998). It emerges with a southward convex shape along the Sciacca-Gela-Catania front and deepens northward, not only cutting upper crust Plio-Pleistocene sediments, but also mid-crust and lower crust levels as imaged by recent seismic profiles (FINETTI et alii, 2005b). The ABT and SBT are very similar in size and 3-D shape. This is well highlighted in fig. 2 by the geometry of their depth-contour lines, which we have schematically reconstructed through the use of a large number of regional geological and seismic cross-sections sections available in the literature (fig. 1b-1c and references therein). In central Italy, the isobaths have an arched shape that substantially follows the surface ABT trajectory. They are more closely spaced near the northern and southern ends and, on average, dip about 20° westward reaching a depth of about 30 km beneath the Apennine mountain chain. In Sicily, also, the isobaths have an arched shape that substantially follows the surface SBT trajectory; they are more closely spaced near the western and eastern concavity and more distant in the central sector. This feature reflects the SBT dip-attitude, which is steeper in proximity to the MazaraSciacca and Gela-Catania areas, and flatter in the Agrigento-Canicattì-Licata area. On average, the SBT dips about 25° northward reaching a depth of about 30 km beneath the northern coast of Sicily.
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SEISMOGENIC EVIDENCE OF ONGOING COMPRESSION IN EASTERN-CENTRAL ITALY AND MAINLAND SICILY: A COMPARISON
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F2 hru T B as a l a and c) Fig. 1 - a) Location map of the study areas. b) Traces of regional geological (black lines) and seismic cross-sections (bold grey lines and dashed lines) used RC ME 16 Outer frontthe of thedepth-contour ApennineOuter upper 3 to reconstruct linesfront of of the Adriatic 9 13 Maghrebian thrust belt crust extension TR 6 PA Basal Thrust and of the Sicilian Basal Thrust in fig. 2. TS. C9 P 38°N DI M 4 seismic reflection KEY for b: CROP03 (near vertical EBRO N N Active compressional CH2 km 14 A 0 BARCHI et200 F1 Corleone profile) alii, 1998 and LAVECCHIA et alii, axes 15 L E U Marsala Belice 11 2004; DSS-1 (Deep Seismic Sounding profile PiombinoB A 1b Y Valley 1 E Regalbuto Mazara C Ancona) PONZIANI et alii, 1995; DSS-2 (Deep Seismic H Etna T 10 A Sounding profile Latina-Pescara) in P AROTTO et alii, L Mt. P CL 2003; 1) LAVECCHIA et alii, 1994; 2 and 3) CALAMITA et Sciacca 2 EN CT M2 alii, 2002; 4) CALAMITA et alii, 2004; 5) SCISCIANI et alii, Raddusa 3A2 5 AG 2000; 6 and 7) VEZZANI & GHISETTI, 1998; 8) SALVINI et Scordia 7 Canicattì Mineo alii, 1997; 9) CIPOLLARI et alii, 1999; 10) LAVECCHIA et Lentini C12 alii, 1987; 11 and 20) LAVECCHIA et alii, 2004; 12) CAN 12 1 c A LAMITA et alii, 1999; 13) MENICHETTI, 1991; 14, 15 and 37°N Niscemi 11 E Licata C L 16) COWARD et alii, 1999; 17, 18 and 19) CALAMITA et U Gela B A Y E alii, 1991; M16) FINETTI et alii, 2005a. AN=Ancona, M2 H T M2 3A1 8 A AQ=L’Aquila, AP=Ascoli Piceno, CE=Cesena, FO=Forlì, 1 L P FI=Firenze, MC=Macerata, PE=Pescara, PG=Perugia, PS=Pesaro, RI=Rimini, SI=Siena, TE=Teramo, TR=Terni. 17 0 km 40 KEY for c: 1) BEN AVRAHAM et alii, 1990; 2) CATALANO 14°E et alii, 1996; 3, 6 and 12) NIGRO & RENDA, 2001; 4) CATALANO et alii, 2000; 5, 8, 10 and 15) LICKORISH et alii, 1999; 7) BEN AVRAHAM et alii, 1990; 9) PAROTTO & PRATURLON, 2004; 11, 13 and 14) BELLO et alii, 2000; 16) NIGRO & RENDA, 2001; 17) GUARNIERI et alii, 2002; C9 and C10-CASSINIS, 1983; C11b, C11c and C12) CASSINIS et alii, 2003 and 2005; P-PEPE et alii, 2000; M23A1, M23A2, M28A, M6A, M26, M21, M24, F1 and F2-FINETTI et alii, 2005b; CH1 and CH2-CHIRONI et alii, 2000. AG=Agrigento, CT=Catania, CL=Caltanissetta, EN=Enna, ME=Messina, PA=Palermo, TP=Trapani, RC=Reggio Calabria. – a) Localizzazione delle aree di studio. b e c) Tracce delle sezioni geologiche regionali (linea nera) e dei profili sismici (linea grigia e linea nera tratteggiata) utilizzate per costruire le isobate dei sovrascorrimenti basali adriatico e siciliano riportati in fig. 2. LEGENDA per b: CROP03 (profilo di sismica a riflessione) BARCHI et alii, 1998 e LAVECCHIA et alii, 2004; DSS-1 (profilo sismico a rifrazione Piombino-Ancona) PONZIANI et alii, 1995; DSS-2 (profilo sismico a rifrazione Latina-Pescara) PAROTTO et alii, 2003; 1) LAVECCHIA et alii, 1994; 2 e 3) CALAMITA et alii, 2002; 4) CALAMITA et alii, 2004; 5) SCISCIANI et alii, 2000; 6 e 7) VEZZANI & GHISETTI, 1998; 8) SALVINI et alii, 1997; 9) CIPOLLARI et alii, 1999; 10) LAVECCHIA et alii, 1987; 11) LAVECCHIA et alii, 2004; 12) CALAMITA et alii, 1999; 13) MENICHETTI, 1991; 14, 15 e 16) COWARD et alii, 1999; 1, 18 e 19) CALAMITA et alii, 1991; 20) LAVECCHIA et alii, 2004; M16-FINETTI et alii, 2005a. Sigle: AN=Ancona, AQ=L’Aquila, AP=Ascoli Piceno, CE=Cesena, FO=Forlì, FI=Firenze, MC=Macerata, PE=Pescara, PG=Perugia, PS=Pesaro, RI=Rimini, SI=Siena, TE=Teramo, TR=Terni. LEGENDA per b: 1) BEN AVRAHAM et alii, 1990; 2) CATALANO et alii, 1996; 3, 6 e 16) NIGRO & RENDA, 2001; 4) CATALANO et alii, 2000; 5, 8 e 10) LICKORISH et alii, 1999; 7) BEN AVRAHAM et alii, 1990; 9) PAROTTO & PRATURLON, 2004; 12) NIGRO & RENDA, 2001; 11, 13 e 14) BELLO et alii, 2000; 15) LICKORISH et alii, 1999; 17) GUARNIERI et alii, 2002; C9 e C10-CASSINIS, 1983; C11b, C11c e C12-CASSINIS et alii, 2003 e 2005; P-PEPE et alii, 2000; M23A1, M23A2, M28A, M6A, M26, M21, M24, F1 e F2-FINETTI et alii, 2005b; CH1 e CH2-CHIRONI et alii, 2000. AG=Agrigento, CT=Catania, CL=Caltanissetta, EN=Enna, ME=Messina, PA=Palermo, TP=Trapani, RC=Reggio Calabria.
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TABLE 1 Parametric data of the focal mechanisms drawn in fig. 2 (events with Mw≥4.0 and depth 4.0 e profondità
