Seismogenic Evidence of Ongoing Compression in Eastern-Central Italy and Mainland Sicily: a Comparison

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Queste bozze, corrette e accompagnate dall’al- legato preventivo firmato e dal buono d’ordine, SGI Bollettino 20/06-133 debbono essere restituite immediatamente alla Segreteria della Società Geologica Italiana c/o Dipartimento di Scienze della Terra Piazzale Aldo Moro, 5 – 00185 ROMA Boll.Soc.Geol.It. (Ital.J.Geosci.), Vol. 126, No. 2 (2007), pp. 209-222, 5 figs., 3 tabs.

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 dall’area padana alla Sicilia. Numerosi autori ritengono che l’attività del fronte sia terminata nel Pleistocene inferiore (PATACCA et alii, We compare the seismogenic role played by two crustal-scale 1990; BUTLER et alii, 1992; LICKORISH et alii, 1999; DI BUCCI & MAZ- reverse-type shear zones located at the outer border of the Apennine- ZOLI, 2002; PATACCA & SCANDONE, 2004), ma altri riportano, almeno Maghrebian fold-and-thrust belt system: the westward-dipping Adriatic per l’Italia centrale e la Sicilia, evidenze di deformazione compressi- Basal Thrust (ABT) in eastern-central Italy and the northward-dipping va tardo pleistocenica-olocenica (BOUSQUET & LANZAFAME, 1986; Sicilian Basal Thrust (SBT) in southern Sicily. The epicentral and MONACO et alii, 1996; FREPOLI & AMATO, 1997; LAVECCHIA et alii, hypocentral distributions and, whenever possible, the kinematics of a 2003; VANNOLI et alii, 2004; MONTONE et alii, 2004; NERI et alii, merged dataset of pre-instrumental (1600 to 1900), early instrumental 2005; CATALANO et alii, 2004 and 2006). (1901 to 1980) and instrumental (1981 to 2005) earthquakes with La nostra opinione è che l’analisi approfondita dell’assetto sismo- moment magnitude ≥4.5 sited within the surface areas above the ABT tettonico dell’area padano-adriatica e di quella siciliana, ed in partico- and the SBT depth-contour lines (0-30 km) are analysed. They are both lare il confronto tra le due, possa portare un sostanziale contributo first-order crustal-scale seismogenic structures which strongly control alla discussione in atto. In questo lavoro abbiamo, pertanto, analizza- reverse, reverse-oblique and strike-slip seismic activity. Relatively deep to e paragonato le caratteristiche geometriche e sismogenetiche, a sca- (10-to-30 km) seismicity occurring in the foothills region and in main- la crostale, del sovrascorrimento basale del sistema appenninico-ma- land Sicily may be associated with brittle shearing of the middle and grebide nel segmento adriatico, ovest-immergente, da Pescara a lower crust ABT and SBT thrusts segments, whereas the shallow seis- Pesaro, ed in quello siciliano, nord-immergente, da Sciacca a Catania. micity (<10 km) close to the ABT and SBT surface tip line is associated La geometria tridimensionale dei due sovrascorrimenti è stata with reverse, oblique and strike-slip shearing of the two upper crust schematicamente ricostruita attraverso l’utilizzo e l’analisi critica di thrust segments and their frontal and lateral splays. Despite that the innumerevoli sezioni geologiche e geofisiche regionali disponibili in seismic activity within the two studied areas, and especially in Sicily, is letteratura. L’analisi sismologica è stata effettuata tenendo conto commonly considered minor, several highly damaging earthquakes delle informazioni fornite dai cataloghi regionali e locali, nonché da have struck both areas. In the last four centuries, there is knowledge of pubblicazioni specifiche (BARATTA, 1897; MONACHESI et alii, 1985; 37 events with magnitude >5.0 in the Marche-Coastal Adriatic region, POSTPISCHL, 1985; MONACHESI et alii, 1991; MONACHESI & STUC- among which are the 1741 Fabrianese (Maw 6.08) and 1799 Camerino CHI, 1997; RIGANO et alii, 1999; BOSCHI et alii, 2000; AZZARO & BAR- (Maw 5.93) events, and of 10 events with magnitude >5.0 in southern BANO, 2000; AZZARO et alii, 2000a; AZZARO et alii, 2000b; WORKING and mainland Sicily, including the 1818 Catanese (Maw 6.00) and 1968 GROUP CPTI, 2004; CASTELLO et alii, 2005; BOLLETTINO SISMICO Valle del Belice (Maw 6.12) events. ITALIANO, 2003-2005). Un dataset integrato degli eventi maggiori Based on the integration of tectonic and seismological con- (magnitudo momento ≥4.5), storici e strumentali, avvenuti nell’in- straints, the boundaries of two seismogenic compressional provinces tervallo temporale 217 a.C.-2005 d.C., è stato compilato ed analizza- located above the ABT and the SBT are defined and the associated to dal punto di vista della completezza. L’analisi critica della forma values of yearly energy release per unit area evaluated. Similar val- e dell’estensione del campo macrosismico dei terremoti storici ha ues are obtained for the ABT (~8E+14 erg/y/km2) and for the SBT permesso di effettuare una stima qualitativa della profondità delle (~4E+14 erg/y/km2). The recognition of close geometric, kinematic sorgenti sismiche e di attribuire gli eventi nel dataset a sorgenti re- and seismotectonic similarities between the two compressional lativamente superficiali (<8-10 km) o profonde (10-30 km). L’esame provinces, further supported by a comparable level of long-term seis- dei pochi meccanismi focali disponibili in letteratura relativi ad mic activity, has evident implications both for regional tectonic eventi maggiori (M>4) ha evidenziato una cinematica da inversa e reconstructions and for seismic hazard assessment purposes. trascorrente, con assi di massima contrazione sub-orizzontali ed orientati da NE-SO ad E-O nell’area marchigiana-adriatica e da KEY WORDS: Central Italy, Adriatic Sea, Sicily, seismicity, NNE-SSO a NNO-SSE in Sicilia. compression, seimogenesis. 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. RIASSUNTO Ognuna delle province è stata suddivisa in due sotto-province: una caratterizzata da attività sismica associata a deformazioni di taglio Evidenze sismogenetiche di attività compressiva in Italia lungo la porzione più superficiale (< circa 10 km) del sovrascorri- centro-orientale ed in Sicilia centro-meridionale: un confronto. mento basale e lungo gli splays frontali e laterali, l’altra caratterizza- Il fronte esterno del sistema a pieghe e sovrascorrimenti ap- ta da attività sismica più profonda (10-25 km), localizzata lungo la penninico-magrebide si estende senza interruzione di continuità porzione medio crostale e nella parte alta della crosta inferiore del sovrascorrimento basale. Le due province sono state quantitativa- mente comparate dal punto di vista dell’attività sismica osservata durante l’intervallo di completezza dei dati analizzati ed espressa in (*) Laboratorio di Geodinamica e Sismogenesi, Dipartimen- termini di energia sismica per anno su unità di area. I valori ottenu- 2 to di Scienze della Terra, Università «G. d’Annunzio» - Campus Uni- ti, pari a circa 8E+14 erg/y/km per l’area marchigiano-adriatica ed a 2 versitario - 66013 Chieti Scalo, Italia [email protected] circa 4E+14 erg/y/km per l’area siciliana, hanno messo in luce una (**) Dipartimento della Protezione Civile, Servizio Sismico sostanziale, e finora non nota, equivalenza nel potenziale sismogene- Nazionale, DPC-SSN, Roma. tico a lungo termine rilasciato dalle due sotto-province profonde, (***) Dipartimento S.T.A.T., Università degli Studi del Molise - con evidenti implicazioni in termini di pericolosità sismica. Contrada Fonte Lappone - 86090 Pesche (Isernia). (****) Dipartimento di Scienze Geologiche, Università di Cata- TERMINI CHIAVE: Italia centrale, Adriatico, Sicilia, sismicità, nia - Corso Italia, 55 - 95129 Catania. compressione, sismogenesi. 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 210

210 G. LAVECCHIA ET ALII

1. INTRODUCTION Sicilian crust and emerging at the outer front of the Sicil- ian thrust system, along the Sciacca-Catania belt. The Apennine-Maghrebian fold-and-thrust belt sys- In this paper, we present a contribution to this discus- tem extends with continuity from the Po Plain-Adriatic sion focusing attention on the kinematics and the distribu- region to southern Sicily (fig. 1a). It developed since late tion of the historical and instrumental seismicity located in Oligocene times through the progressive nucleation of the areas to the rear of the Pescara-Pesaro segment of the inward-dipping crustal slices thrust over the Adriatic- Po Plain-Adriatic front and of the Sciacca-Catania segment Pelagian foreland (LAVECCHIA, 1988). According to sev- of the Sicilian front. In particular, we will investigate if the eral authors, the entire belt ended its activity in early major earthquakes may be linked to reverse shearing along Pleistocene times (PATACCA et alii, 1990; BUTLER et alii, two crust-scale basal thrusts, here respectively called Adria- 1992; LICKORISH et alii, 1999; DI BUCCI & MAZZOLI, 2002; tic Basal Thrust (ABT) and Sicilian Basal Thrust (SBT). PATACCA & SCANDONE, 2004), but according to some oth- With this aim we will reconstruct the ABT and SBT three- ers it is still undergoing compression, at least in eastern- dimensional shape; we will compile an updated and inte- central Italy and in Sicily. In particular, on the basis of grated dataset of all the major events that occurred within the combined analysis of historical, seismological and the two study areas during the last two thousands years geomorphologic data, VANNOLI et alii (2004) outlined a and, through the analysis of the associated macroseismic rather regularly segmented active fault system running fields, we will define likely depth ranges of the seismogenic parallel to the shoreline from Ancona to Rimini (NW-SE), sources. By comparing the 3-D shape of the basal thrusts made up of blind thrusts accompanied by anticlinal with the kinematics and with the surface and depth distrib- growth. MONTONE et alii (2004), through the analysis of ution of the overlying earthquakes, we will define the borehole breakouts, showed that the maximum horizon- boundaries of homogeneous seismogenic provinces. After tal stress is generally perpendicular to the strike of the verifying the completeness of the seismological dataset, we shallow (0-6 km depth) middle Pliocene to Quaternary will also try to compute and quantitatively compare the thrust structures of the Po Plain and Coastal-Adriatic energy release in the two study areas. domain. On the basis of the analysis of instrumental seismological data, several authors considered that this domain is undergoing active compression (FREPOLI & 2. THE ABT AND SBT BASAL THRUSTS AMATO, 1997 among many others). In fact, earthquake focal mechanisms show a predominance of thrust and The ABT overthrusts the Umbria-Marche meso-ceno- strike-slip solutions with P-axes generally perpendicular zoic sedimentary cover and the overlying Plio-Pleistocene to the trend of the outer thrust system (PONDRELLI et alii, foredeep terrains above the Adriatic foreland. It emerges 2004). LAVECCHIA et alii (2003) identified an individual, with an eastward convex shape along the Pesaro-Adriatic regional scale structure, responsible for such seismicity in Sea-Pescara front and deepens westward, cutting through the outermost westward-dipping thrust plane of the Apen- the entire crust and reaching its base beneath the Umbria nine fold-and-thrust belt, which emerges along the Po region (BARCHI et alii, 1998; FINETTI et alii, 2001; LAVEC- Plain-Adriatic front. CHIA et alii, 2003). The SBT overthrusts terrains pertaining In Sicily, BOUSQUET & LANZAFAME (1986) were the to the Sciacca-Saccense tectonic unit and to the Gela- first to document Quaternary E-W contractional deforma- Catania early-middle Pleistocene foredeep deposits above tion, at the external limit of the thrust belt in eastern the Pelagian-Hyblean foreland (GHISETTI & VEZZANI, Sicily. MONACO et alii (1996), based on seismological, 1984; CATALANO et alii, 1989; BUTLER et alii, 1992; structural and morphotectonic observations, suggested TORELLI et alii, 1998). It emerges with a southward convex Late Quaternary activity of the Sciacca thrust, an inner shape along the Sciacca-Gela-Catania front and deepens splay of the outer basal thrust. LANZAFAME et alii (1997) northward, not only cutting upper crust Plio-Pleistocene documented field evidence of Late Pleistocene-Holocene sediments, but also mid-crust and lower crust levels as N-S compression in the Etna area, with a shortening of a imaged by recent seismic profiles (FINETTI et alii, 2005b). few kilometres. TORELLI et alii (1998) recognised, in a The ABT and SBT are very similar in size and 3-D shape. seismic line across the Catania foredeep, a clear incipient This is well highlighted in fig. 2 by the geometry of their northward-dipping reverse shearing, which cuts obliquely depth-contour lines, which we have schematically recon- through the Pleistocene sediments. CARBONE et alii (2000) structed through the use of a large number of regional pointed out geomorphologic evidence of reactivation of geological and seismic cross-sections sections available in out of sequence thrusts beneath Etna and in the Nebrodi the literature (fig. 1b-1c and references therein). In central area. Furthermore, CATALANO et alii (2004 and 2006) Italy, the isobaths have an arched shape that substantially showed evidence of Late Quaternary tectonic inversion of follows the surface ABT trajectory. They are more closely the northern border faults of the Scordia-Lentini Plei- spaced near the northern and southern ends and, on aver- stocene graben, at the front of the Gela-Catania thrust age, dip about 20° westward reaching a depth of about plane, with formation of post 40 ka E-W trending folds. 30 km beneath the Apennine mountain chain. In Sicily, COCINA et alii (1997) and NERI et alii (2005) documented also, the isobaths have an arched shape that substantially N-S compressional seismogenic activity at depths follows the surface SBT trajectory; they are more closely between 10 and 30 km in eastern Sicily, beneath the Etna spaced near the western and eastern concavity and more volcanic area, whereas CACCAMO et alii (1997) and JENNY distant in the central sector. This feature reflects the SBT et alii (2006) recognise N-S seismogenic compression in dip-attitude, which is steeper in proximity to the Mazara- western and central Sicily. LAVECCHIA et alii (2006) iden- Sciacca and Gela-Catania areas, and flatter in the Agri- tified an individual, regional scale structure, possibly gento-Canicattì-Licata area. On average, the SBT dips responsible for such a compressional seismicity, in a Plio- about 25° northward reaching a depth of about 30 km Quaternary north-dipping thrust zone cutting the entire beneath the northern coast of Sicily. 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 211

SEISMOGENIC EVIDENCE OF ONGOING COMPRESSION IN EASTERN-CENTRAL ITALY AND MAINLAND SICILY: A COMPARISON 211

Po Plain N b 12°E 14°E W E FO CE 14 15 M16 Adria 44°N ti S 16 5 c RI B a PS 10 PS s Montegrimano a l T FI h Adriatic Sea Senigallia

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t AN PE E Po Plain N Cagli AR N 20 W E T Frontone 13 Rome 03 R MC P. San Adr iat S Fabriano Giorgio ic A 17 B CROP DSS 1 Fermo a PG Camerino PS s a L 11 l 43°N T 18 h Adriatic Sea Sarnano Offida

Tyrrhenian u

s A

t AP Sea PE Acquasanta P Terme

E TE 19 Atri Rome 1 N TR Pietracamela 2 N Farindola PE 12 AQ I 9 CROP 11 Sc CT N TyrrhenianS 2 7 t ic s 4 E 3 ili ru 0 km 40 DSS 8 Seaan Th Basal a 6 S 5

Outer front of the Apennine- Outer front of upper c 13°E 15°E Maghrebian thrust belt crust extension CH1 C10

Active compressional M26 0 km Sc200 CT 5 S axes ic st F2 il ru M6A ian h Basal T

Fig. 1 - a) Location map of the study areas. ba) and c) Traces of regional geological (black lines) and seismic M28A cross-sections (bold grey lines and dashed lines) used 16 ME RC Outer front of the Apennine- Outer front of upper 3 to reconstruct the depth-contour lines of the Adriatic 9 13 Basal ThrustMaghrebian and of thrust the belt Sicilian Basalcrust Thrust extension in fig. 2. TR 6 PA TS. 4 P M C9 38°N KEY for b: CROP03 (near vertical seismic reflection 0 km 200 Active compressional F1 NEBRODI14 CH2 profile) BARCHI et alii, 1998 and LAVECCHIAaxes et alii, Corleone 15 2004; DSS-1 (Deep Seismic Sounding profile Piombino- Marsala Belice 11 Valley Ancona) PONZIANI et alii, 1995; DSS-2 (Deep Seismic Mazara C11b 10 RegalbutoHYBLEANEtna Sounding profile Latina-Pescara) in PAROTTO et alii, Mt. CL PLATEAU 2003; 1) LAVECCHIA et alii, 1994; 2 and 3) CALAMITA et 2 Sciacca EN alii, 2002; 4) CALAMITA et alii, 2004; 5) SCISCIANI et alii, M23A2 5 Raddusa CT 2000; 6 and 7) VEZZANI & GHISETTI, 1998; 8) SALVINI et AG Scordia 7 Canicattì Mineo alii, 1997; 9) CIPOLLARI et alii, 1999; 10) LAVECCHIA et C12 Lentini alii, 1987; 11 and 20) LAVECCHIA et alii, 2004; 12) CA- 1 12 LAMITA et alii, 1999; 13) MENICHETTI, 1991; 14, 15 and 37°N Licata Niscemi 16) COWARD et alii, 1999; 17, 18 and 19) CALAMITA et C11c Gela INETTI alii, 1991; M16) F et alii, 2005a. AN=Ancona, M23A1 HYBLEAN AQ=L’Aquila, AP=Ascoli Piceno, CE=Cesena, FO=Forlì, 8 M21 FI=Firenze, MC=Macerata, PE=Pescara, PG=Perugia, PLATEAU M24 PS=Pesaro, RI=Rimini, SI=Siena, TE=Teramo, TR=Terni. 40 KEY for c: 1) BEN AVRAHAM et alii, 1990; 2) CATALANO 0 km 17 14°E et alii, 1996; 3, 6 and 12) NIGRO & RENDA, 2001; 4) CA- TALANO 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. 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 212

212 G. LAVECCHIA ET ALII

TABLE 1

Parametric data of the focal mechanisms drawn in fig. 2 (events with Mw≥4.0 and depth <40 km occurring since 1968 above the ABT and the SBT). KEY: G85=GASPARINI et alii, 1985; A&J87=ANDERSON & JACKSON, 1987; F&A=FREPOLI & AMATO, 2000; P&AL=PONDRELLI et alii, 2004; F&A97=FREPOLI & AMATO, 1997; R89=RIGUZZI et alii, 1989; P02=PONDRELLI et alii, 2002; S03=SANTINI, 2003; C73=CONSOLE et alii, 1973. – Parametri focali degli eventi con magnitudo momento (Mw) >4.0 e profondità <40 km a partire dal 1968. I meccanismi relativi sono proiettati ivi fig. 2. KEY: G85=GASPARINI et alii, 1985; A&J87=ANDERSON & JACKSON, 1987; F&A=FREPOLI & AMATO, 2000; P&AL=PONDRELLI et alii, 2004; F&A97=FREPOLI & AMATO, 1997; R89=RIGUZZI et alii, 1989; P02=PONDRELLI et alii, 2002; S03=SANTINI, 2003; C73=CONSOLE et alii, 1973.

Strike Dip Rake Strike Dip Rake Name Y M D Lon Lat Depth Mw Ml Ref A A A B B B 1972 02 04 13.440 43.720 216 66 -147 112 60 -27 4.8 4.5 G85 5 C73 1972 02 04 13.370 43.730 319 83 48 128 77 -28 4.6 4.3 G85 5 C73 1972 02 04 13.360 43.720 225 63 -164 222 42 171 4.3 4 G85 Ancona 5 C73 sequence 1972 02 05 13.400 43.720 246 35 -166 145 82 -55 4.6 4.3 G85 10.4 C73 1972 02 06 13.430 43.710 212 80 -143 115 54 -11 4.6 4.3 G85 8.3 C73 1972 06 14 13.470 43.690 3 34 70 -170 301 81 -19 4.8 4.5 G85 1987 07 03 13.843 43.174 5 140 56 80 338 35 104 4.6 4.2 R89 P.S. Giorgio 1987 07 03 13.849 43.201 5 9 55 104 205 36 71 4.1 3.6 R89 sequence 1987 07 03 13.868 43.200 5 185 40 78 18 50 100 4.3 3.9 R89 1987 07 03 13.848 43.2050 5 152 60 56 25 43 134 4.3 3.9 R89 Montefeltro 1987 07 05 12.160 43.760 10 294 36 55 155 61 113 4.2 3.8 G89 Northern 1991 12 15 13.067 43.633 9.3 130 30 60 343 64 107 4.0 3.1 F&A97 Marche Southern 1995 07 23 14.033 42.717 19.2 99 40 60 316 56 113 4.1 3.6 F&A97 Marche Pesaro 2000 05 05 13.192 44.014 5 195 25 30 77 78 112 4.3 4.1 S03 Montegrimano 2000 08 01 12.440 43.890 18 212 28 42 84 72 112 4.3 4.5 P02

1968 01 15 12.983 37.750 13.0 270 50 90 90 40 90 5.5 A&J87 Belice 1968 01 16 12.976 37.857 36.0 250 58 80 88 33 106 5.2 A&J87 sequence 1968 01 25 12.966 37.687 3.0 270 64 85 101 26 100 5.2 A&J87 Mazara del 1981 06 07 12.470 37.670 18.1 48 29 48 274 69 110 4.9 4.4 P&Al Vallo Maletto 1987 08 13 15.060 37.900 35.9 352 42 -10 89 83 -132 4.8 4.3 P&Al Nebrodi 1992 09 27 14.667 37.900 23.3 60 90 40 330 50 180 4.1 4.2 F&A Caltanissetta 1995 04 11 13.983 37.583 8.7 170 80 30 74 60 169 4.2 4.1 F&A Etna 1995 02 10 14.967 37.783 20.5 85 65 10 350 81 155 4.1 3.7 F&A

3. FOCAL MECHANISMS AND INSTRUMENTAL SEISMICITY JACKSON, 1987), in both cases the average P-axis being nearly N-S and sub-horizontal. The pure thrusting solu- A selection of the focal mechanisms available in the tion is better supported by the E-W elongation of the literature for the events with moment magnitude ≥4.0, macroseismic field and by the northward-deepening of which have occurred since 1968 within the ABT and the hypocentral depths, from close to the surface down SBT surface projection areas, between the depth-con- to the base of the crust (MONACO et alii, 1996). An tour lines 0-to-30 km (grey areas in fig. 2), is given in almost pure E-W compressional solution, is also shown fig. 2 and tab. 1. by the 1981 Mazara del Vallo event, sited at a depth of In central Italy, the 1972 Ancona seismic sequence about 18 km, nearly 40 km westward of the Belice main (Mmax=4.8) is characterized by a prevailing strike-slip shock (fig. 2, tab. 1). The few other events located in kinematics (GASPARINI et alii, 1985), whereas the 1987 central and eastern Sicily (1995 Nebrodi 1992, Caltanis- Porto S. Giorgio sequence (Mmax=4.6) shows prevailing setta 1995 and Etna 1995) show strike-slip and reverse- reverse solutions. All available mechanisms are compati- oblique kinematics with an average N-S trend of P-axes. ble with NE-SW to E-W trending sub-horizontal P-axes. Furthermore, the focal mechanism inversion of the mid- In Sicily, the focal solutions of the highly destructive to-lower crust background seismicity (Ml 2.2-3.7) 1968 Belice seismic sequence (Mmax=5.5) were evaluated recorded by local and national networks at Mt. Etna either as right lateral transpression on a NNE-striking indicate nearly N-S regional compression (CASTELLANO plane (MCKENZIE, 1972; GASPARINI et alii, 1985) or as et alii, 1997; COCINA et alii, 1997; NERI et alii, 2005) (see pure thrusting on a north-dipping plane (ANDERSON & inset in fig. 2). 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 213

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1875 1786 Good agreement may be also observed between the 12°E 1672 1916 May 14°E active deformation pattern from focal mechanisms and 44°N 1916 Aug Pesaro, 1972 sequence, Ancona 2000 44°N that deduced from the borehole breakouts sited in the Montegrimano 1924 areas above the shallower portions of the ABT and SBT 2000 PS U Urbino (MONTONE et alii, 2004). U 1930 Montefeltro 1897 AN From two regional seismic catalogues available on 1987 1269 line (Catalogo della Sismicità Italiana 1981-2002 in Northern U 1690 Marche, 1991 U 56 B.C. 1987 sequence, P.S.Giorgio CASTELLO et alii, 2005 and BOLLETTINO SISMICO ITALIA- 0 km 40 U 1741 MC NO, 2003-2005), we extracted the events with Ml≥3.0 Camerino which occurred within the ABT and SBT surface areas Source depth range and seismotectonic assignation 1799 (grey areas in fig. 2), at depths <40 km, in the period of the major earthquakes 1873 43°N

1981-2005. The earthquakes sited beneath the Etna vol- “Shallow” source (< 10 km), 100 B.C. canic area at depth <10 km, were excluded from the Compressional Province 1943 TE dataset, given that they would be most likely linked with “Deep” source (10-30 km), Compressional Province the volcanic dynamics rather than with the regional tec- Atri Southern Shallow source (< 10-15 km), 1950 PE Marche, 1995 tonic process (CASTELLANO et alii, 1997; AZZARO, 1999; Extensional Province 1461 CH ATANE RIVITERA ONACO P & P , 2001; M et alii, 2005) that we Uncertain depth U U U 10 km 5km 1762 15 km are investigating in this paper. The ABT dataset consists and/or seismotectonic 20 a 13°E 30 km 25 km km of about 60 events with 3.0≤Ml≤5.0 within an area of assignation ~14500 km2; the SBT dataset consists of about 130 events Instrumentalseismicity Historicalseismicity with 3.0≤Ml≤4.6, within an area of ~16200 km2. Above the 1981-2005 360B.C.-1980 Ancona 1972 Local Magnitude (M l) Magnitude moment (Maw or Mw) and ABT, 40% of the selected events is sited at depths less Belice 1968 than 10 km and 50% between 10 and 30 km; above the 3.01-3.5 3.51-4.0 4.01-4.49 4.5-4.9 4.5-4.9 5.0-5.4 5.5-5.9 6.0-6.6 SBT, 20% of events occur at depth less than of 10 km and 13°E 14°E 15°E 80% between 10 and 30 km. Belice sequence,1968 In eastern-central Italy and in north-eastern Sicily Caltanissetta, Nebrodi Maletto, Etna,1995 good quality microseimic data (Ml<4.5) registered by 1995 1992 1987 ME local networks are also available (MONACHESI et alii, PA TP 2004; NERI et alii, 2005). The event depth distribution Calatafimi 30 kmkm U shows an inward deepening of the seismogenic source, 25 kmkm down to a depth of about 30 km, which follows well the Marsala Troina 20 kmkm 1968 15 kmkm westward deepening of the ABT and the northward deep- U EN 10 kmkm Raddusa 5km5 km ening of the SBT. This geometry is highlighted in fig. 3, Sciacca 361 CL CT U 1818 where selected hypocenters (see source of data in the Mazara del Vallo, Canicattì figure caption) have been projected along the traces of 1981 Mineo AG Caltagirone 1624 two interpretive crust-scale sections, one drawn along the Mt. Etna detail trace of the DSS profile 1978 (after LAVECCHIA et alii, Gela 37°N 2003) and the other across the Etna volcanic edifice. This latter section has been reconstructed integrating surface data with speculative interpretation of deep crust sections available in the literature such as the land side extension , 0 km 40 b of the CROP M26 seismological profile (FINETTI et alii, 14°E 15°E 2005b), the Castel di Tusa-Caltanissetta-Hyblean plateau 10 km

Fig. 2 - Epicentre distribution of historical and instrumental earthquakes (depth <40 km) within the areas above the 0-to-30 km depth-contour lines (dotted lines) of the Adriatic Basal Thrust (a) and of the Sicilian Basal Thrust (b). The maps are in Italy Lambert Conformal Conic projection. The shape of the depth-contour lines is based on the critical review and re-interpretation of the regional sections whose traces are given in fig. 1b and c. The instrumental data are extracted from the CSI 1981-2002 seismic catalogue in (CASTELLO et alii, 2005) and from the BOLLETTINO SISMICO ITALIANO, 2003-05, assuming Ml>3.0, GAP<200°, ErrH and ErrZ<4 km, RMS<0.8 s, phase readings >8 and depth <40 km. The historical earthquakes are mainly extracted from the CPTI04 parametric catalogue (WORKING GROUP CPTI, 2004) with integration from other catalogues and detailed papers (see tabs. 2a and 2b and reference therein). The different epicentral colours of the major (Maw ≥4.5) earthquakes represent the inferred depth range of the associated seismogenic source and the proposed seismotectonic assignation. Focal mechanisms refer to major (magnitude moment ≥4.0) earthquakes since 1968 (parametric data and references are given in tab. 1). The inset in fig. 2b shows a selection of focal mechanisms computed by NERI et alii (2005) for microseismic events (2.2≤Ml≤3.7) recorded in the period 1988-2001 at depth >10 km beneath the Etna volcano; the arrows represent the average compressional stress axis computed by NERI et alii (2005) from the focal mechanism stress inversion. – Distribuzione epicentrale della sismicità storica e strumentale localizzata a profondità <40 km nell’area compresa tra le isobate 0-30 km (linee tratteggiate) dei sovrascorrimenti basali adriatico (a) e siciliano (b). La proiezione delle mappe è nella rappresentazione conica conforme di Lam- bert. La ricostruzione dell’andamento delle isobate è basata sulla revisione critica delle sezioni regionali le cui tracce sono riportate in fig. 1 b e c. I dati strumentali sono stati estratti dal catalogo sismico CSI (CASTELLO et alii, 2005) per l’intervallo 1981-2002 e dal Bollettino Sismico Italiano per l’intervallo 2003-05, assumendo Ml>3.0, GAP<200°, ErrH e ErrZ<4 km, RMS<0.8 s, numero di fasi >8 e profondità ipocentrale <40 km. I terremoti storici derivano principalmente dal catalogo parametrico CPTI04 (WORKING GROUP CPTI, 2004) con integrazioni (vedi il testo e riferimen- ti bibliografici associati) nelle tabb. 2a e 2b). Gli epicentri dei terremoti maggiori (Maw≥4.5) sono stati colorati diversamente a seconda della ipo- tizzata profondità della sorgente sismogenetica e della proposta provincia sismotettonica di appartenza. I meccanismi focali si riferiscono ai maggiori terremoti (magnitudo momento ≥4.0) a partire dal 1968 (i parametri focali e la bibliografia sono dati in tab. 1). Il riquadro in basso a sinistra mostra una selezione di meccanismi focali elaborati da NERI et alii (2005) relativamente alla microsismicità (Ml tra 2.2 e 3.7) registrata nel periodo 1988-2001 nell’area del Mt. Etna, a profondità >10 km; le frecce rappresentano l’orientazione media dell’asse dello sforzo di massima compressione associato calcolato dagli stessi autori sulla base dell’inversione dei meccanismi focali. 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 214

TABLES 2a and b Merged dataset of historical and instrumental earthquakes with moment magnitude ≥5.0 and depth <40 km sited above the Adriatic Basal Thrust and the Sicilian Basal Thrust (grey areas in fig. 2). The starred references are referred to events reviewed for this paper: ABAR2000 (AZZARO & BARBANO, 2000), ABAR2000a (AZZARO et alii, 2000a), A&J87 (ANDERSON & JACKSON, 1987), BAR1897 (BARATTA, 1897), BMING (BOLLETTINO MACROSISMICO, 1982-1999), CFTI (BOSCHI et alii, 2000), CST03 (WORKING GROUP CSTI, 2003), DOM (MONACHESI & STUCCHI, 1997), G85 (GASPARINI et alii, 1985), C73 (CONSOLE et alii, 1973), P&AL (PONDRELLI et alii, 2004), POS85 (POSTPISCHL, 1985), R89 (RIGUZZI et alii, 1989), R99 (RIGANO et alii, 1999); the remaining ones are extracted from the CPTI04 parametric catalogue (WORKING GROUP CPTI, 2004). The column «Depth range» gives qualitative information about the structural level of the seismogenic source: D=deep (mid-to-lower crust) source, S=shallow (upper crust) source, S- Et=shallow source earthquake associated to Etna volcanic dynamics. The column SP attributes the earthquakes to different Seismogenic Provinces: SC=shallow compressional province, DC=deep compressional province, EV=Etnean volcanic province, T=tensional province. Imx=maximum intensity and Io=epicentral intensity in Mercalli-Cancani Sieberg scale × 10. Mw=instrumental moment magnitude, Maw=moment magnitude mainly derived from the macroseismic field (for further explanation see text and WORKING GROUP MPS, 2004). – Dataset integrato dei maggiori terremoti noti (magnitudo momento ≥5.0), storici e strumentali, localizzati nelle aree sovrastanti le isobate da 0 a 30 km dei sovrascorrimenti basali adriatico e siciliano (aree grigie in fig. 3). Le citazioni con l’asterisco si riferiscono ai lavori di dettaglio e/o a database da noi appositamente consultati: ABAR2000 (AZZARO & BARBANO, 2000), ABAR2000a (AZZARO et alii, 2000a), A&J87 (ANDERSON & JACKSON, 1987), BAR1897 (BARATTA, 1897), BMING (BOLLETTINO MACROSI- SMICO, 1982-1999), CFTI (BOSCHI et alii, 2000), CST03 (WORKING GROUP CSTI, 2003), DOM (MONACHESI & STUCCHI, 1997), G85 (GASPARINI et alii, 1985), C73=CONSOLE et alii, 1973, P&AL (PONDRELLI et alii, 2004), POS85 (POSTPISCHL, 1985), R89 (RIGUZZI et alii, 1989), R99 (RIGANO et alii, 1999); mentre le rimanenti indicano gli eventi estratti direttamente dal CPTI04 (WORKING GROUP CPTI, 2004). La colonna «Depth range» fornisce una indicazione qualitativa sul range di profondità della sorgente sismogenetica; nel caso dei terremoti pre-strumentali questa informazione è stata ricavata dall’analisi della forma e dell’estensione del campo macrosismico, in relazione alla magnitudo e all’anno di occor- renza dell’evento: D (deep source) = sorgente localizzata nella crosta medio-inferiore (circa 10-30 km), S (shallow source) = sorgente localizzata nella crosta superiore (<~10 km), S-Et=sorgente superficiale associata alle dinamiche vulcaniche del Monte Etna; Imx=massima intensità e Io=intensità epicentrale nella scala Mercalli-Cancani Sieberg × 10. Mw=magnitudo momento strumentale; Maw magnitudo momento equivalente (vedi testo e WORKING GROUP MPS, 2004); SP=Provincia Sismogenetica: SC=provincia sismogenetica compressiva superficiale, DC=provincia sismogenetica compressiva profonda, EV=Provincia Vulcanica Etnea, T=provincia distensiva (tensionale). Table 2a Major crustal earthquakes (magnitude moment ≥ 5.0) above the Adriatic Basal Thrust from 217B.C. to 2005 A.D. H Depth n° Y M D H M Lat Lon Locality References Imx Io Maw Mw SP (km) range 1 -100 43.17 13.5 D Picenum CFTI 85 85 5.84 DC 2 -56 4 43.43 13.67 S Potentia CFTI 85 85 5.84 SC 3 1269 9 43.558 13.564 S Ancona DOM 80 80 5.6 SC 4 1308 1 25 16 44.07 12.57 S Rimini CFTI 75 75 5.37 SC 5 1389 4 43.837 13.018 S Fano DOM 70 70 5.17 SC 6 1461 11 26 21 30 42.308 13.543 S Aquilano DOM 100 100 6.46 T 71472 44.059 12.567 S Rimini DOM 70 70 5.17 SC 81474 8 18 43.603 13.507 S Ancona DOM 70 70 5.17 SC 9 1480 42.919 13.835 S Monteprandone DOM 75 75 5.37 SC 10 1502 9 6 43.462 13.087 D Cupramontana DOM 70 70 5.17 DC 11 1540 4 8 43.16 13.716 S Fermo DOM 65 65 5.03 SC 12 1626 5 12 17 45 43.333 13.5 D Macerata est POS85 70 5.17 DC 13 1672 4 14 15 45 43.93 12.58 S Riminese CFTI 80 80 5.6 SC 14 1690 12 23 0 20 43.55 13.6 S Anconetano CFTI 85 85 5.73 SC 15 1712 3 28 43.513 12.734 Frontone DOM 75 75 5.31 U 16 1727 12 14 19 45 43.61 12.818 D S.Lorenzo in campo DOM 70 70 5.18 DC 17 1741 4 24 9 20 43.425 13.004 D Fabrianese DOM 90 90 6.08 DC 18 1762 10 6 12 10 42.3 13.58 S Aquilano CFTI 95 90 5.9 T 19 1786 12 25 1 43.98 12.58 S Riminese CFTI 80 80 5.67 SC 20 1788 4 18 43.837 13.018 S Fano DOM 65 65 5.03 SC 21 1799 7 28 43.147 13.123 D Camerino DOM 95 90 5.93 DC 22 1805 5 9 1 43.451 13.48 S Macerata DOM 65 65 5.03 SC 23 1809 8 25 12 13 43.333 13.5 D Macerata est POS85 70 5.17 DC 24 1838 6 23 43.805 13.007 S Pesaro DOM 65 65 5.03 SC 25 1841 4 14 4 43.791 12.568 D Cagli DOM 65 65 5 DC 26 1870 2 8 43.55 13.471 S Numana DOM 70 70 5.11 SC 27 1873 3 12 20 4 43.08 13.25 D Marche meridionali CFTI 90 80 5.88 DC 28 1875 3 17 23 51 44.07 12.55 S Romagna sud-orient. CFTI 80 80 5.74 SC 29 1888 7 8 16 42.667 13.75 D Bellante POS85 70 5.17 DC 30 1897 9 21 43.706 12.966 D Adriatico centrale DOM 70 70 5.5 DC 31 1908 11 16 16 28 43.155 13.596 D Adriatico centrale DOM 40 5.37 DC 32 1911 3 26 13 51 44.061 12.507 S Rimini DOM 60 60 5.18 SC 33 1916 5 17 12 50 44 12.63 S Alto adriatico CFTI 80 80 5.85 SC 34 1916 8 16 7 6 43.97 12.67 S Alto adriatico CFTI 80 80 5.92 SC 35 1917 11 5 22 47 43.506 13.586 S Numana DOM 65 60 5.36 SC 36 1921 8 28 10 45 43.12 13.253 D Sarnano DOM 70 70 5.06 DC 37 1922 6 8 7 47 43.148 13.286 D Caldarola DOM 65 60 5 DC 38 1924 1 2 8 55 43.736 13.141 S Senigallia DOM 75 75 5.59 SC 39 1928 5 30 20 1 43.706 13.122 S Adriatico centrale DOM 50 55 5.06 SC 40 1930 10 30 7 13 43.659 13.331 S Senigallia DOM 85 90 5.94 SC 41 1943 3 25 15 40 43.074 13.584 D Offida DOM 60 60 5.02 DC 42 1943 10 3 8 28 42.935 13.639 D Offida DOM 90 85 5.81 DC 43 1950 9 5 4 8 42.516 13.657 D Gran Sasso DOM 80 80 5.73 DC 44 1951 8 8 19 56 42.704 13.546 D Monti della Laga DOM 75 70 5.28 DC 45 1951 9 1 43.028 13.287 D Sarnano DOM 70 70 5.31 DC 46 1962 1 23 17 31 43.921 12.806 S Adriatico DOM 60 65 5.03 SC C73 47* 1972 2 4 2 42 43.72 13.44 5 SMedio Adriatico (Ancona) G85 4.8 SC CPTI04 5.18 G85 4.8 48* 1972 6 14 18 55 43.69 13.47 3 SMedio Adriatico (Ancona) SC CPTI04 5.4 49 1972 11 26 16 3 42.966 13.454 D Montefortino DOM 80 75 5.34 DC R89 4.56 50* 1987 7 3 10 22 43.174 13.843 5 SPorto San Giorgio SC CPTI04 5.18 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 215

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Table 2b Major crustal earthquakes (magnitude moment ≥ 5.0) above the Sicilian Basal Thrust from 217B.C. to 2005 A.D. H Depth n° Y M D H M Lat Lon Locality References Imx Io Maw Mw SP (km) range 117 37.8 15.2 D Reggio C.-Sicil. CFTI 85 85 5.14 DC 2 361 37.5 14 D Sicilia CFTI 100 100 6.6 DC 3 1536 3 23 37.502 15.087 S-Et Catania DOM 65 65 5.03 EV 4 1578 37.508 13.083 S Sciacca DOM 70 70 5.17 SC 5 1624 10 3 17 37.27 14.75 S Mineo CFTI 90 80 5.57 SC 6 1643 7 17 37.783 14.599 S Troina DOM 65 65 5.03 T 7 1716 12 1 37.502 15.087 S Catania DOM 70 70 5.17 SC 8 1718 2 20 37.599 14.619 D Sic. Or. (Regalbuto) DOM 65 65 5.4 DC 9 1740 6 13 37.582 12.84 S Sciacca DOM 75 75 5.37 SC 10 1818 2 20 18 15 37.6 15.13 D Catanese CFTI 95 90 6 DC 11 1822 4 6 37.747 14.398 S Nicosia DOM 65 65 5.03 T 12* 1823 3 27 37.931 12.329 D Favignana R99 70 70 5.17 DC 13* 1828 5 18 37.8 12.433 D Marsala R99 70 70 5.17 DC 14* 1865 7 19 1 37.702 15.153 S-Et Area Etnea ABAR2000 90 90 5.03 EV 15* 1878 10 4 0 46 37.266 14.691 S Mineo ABAR2000 65 65 5.03 SC 16 1907 5 8 37.5 15 S-Et Catania sud POS85 70 5.17 EV 17* 1914 5 8 18 1 37.659 15.149 S-Et Area Etnea ABAR2000 95 95 5.3 EV 18 1934 9 11 1 19 37.439 14.58 D Madonie DOM 65 65 5.03 DC 19* 1959 12 23 9 29 37.428 14.89 D Piana di Catania ABAR2000 65 65 5.23 U 20* 1968 1 14 12 28 37.804 13.012 19 D Valle del Belice A&J87 5.2 DC 21* 1968 1 14 13 15 37.676 12.966 1 S Valle del Belice A&J87 5.1 DC 22* 1968 1 15 1 33 37.817 13.006 34 D Valle del Belice A&J87 5.2 DC A&J87 5.5 23* 1968 1 15 2 1 37.75 12.983 13 D Valle del Belice DC CPTI04 6.12 24* 1968 1 16 16 42 37.857 12.976 36 D Valle del Belice A&J87 5.2 DC 25* 1968 1 25 9 56 37.687 12.966 3 S Valle del Belice A&J87 5.2 DC

seismic section (BELLO et alii, 2000) and the WARR sec- example the 1968 Belice and 1972 Ancona seismic tion across eastern Sicily (CHIRONI et alii, 2000) (see sequences, these values are substantially different. This traces of the sections in fig. 1c). may be inferred on one side to the possible low reliability of the earthquake parameters calculated in the early instrumental period and on the other side to the difficulties in 4. MAJOR EARTHQUAKES the evaluation of the intensity of single events belonging to seismic sequences because of cumulative damage effects. In order to have the most possible complete picture For all the selected events, we have attempted to valu- on the major earthquakes that have occurred in historical ate the depth range of the seismogenic source. We fixed and present times within the surface areas above the 0-to- two depth ranges from geological considerations, an upper 30 km ABT and SBT depth contour lines (grey areas in crust range (<~10 km), labelled S (Shallow), and a mid-to- fig. 2), we compiled an integrated dataset of the seismic lower crust one (10-30 km), labelled D (Deep), and we events with magnitude moment ≥4.5 for the time interval chose the appurtenance of each earthquake to one or the 217 B.C.-2005 A.D.. The main source of data was the other of the two ranges through the analysis of the shape CPTI04 parametric catalogue, which contains informa- and extent of the macroseismic field (BARATTA, 1897; tion on the Italian historical earthquakes from 217 B.C. to POSTPISCHL, 1985; MONACHESI & STUCCHI, 1997; BOSCHI 2002 (WORKING GROUP CPTI, 2004); it was integrated et alii, 2000). Simply speaking, we looked at the distance and updated with data from other regional catalogues and difference in intensity between the macroseimic epi- (BARATTA, 1897; POSTPISCHL, 1985; MONACHESI et alii, centre and the other data points¸ also taking into consider- 1985; MONACHESI et alii, 1991; DOM by MONACHESI & ation the year of occurrence and the geographic location, STUCCHI, 1997; CFTI by BOSCHI et alii, 2000), from spe- e.g. the historical context. Some examples of the macro- cific papers on reviewed historical earthquakes (RIGANO seismic fields which we defined as «shallow» and «deep» et alii, 1999; AZZARO & BARBANO, 2000; AZZARO et alii, are shown in fig. 4. Based on indications from the macro- 2000a; AZZARO et alii, 2000b) and from regional instru- seismic catalogue of Mt. Etna (AZZARO et alii, 2000a), the mental catalogues (Catalogo Sismico Italiano 1981-2002 earthquakes related to the volcanic dynamics (swarm by CASTELLO et alii, 2005 and BOLLETTINO SISMICO ITA- sequences in the first 8-10 km of depth) were separately LIANO, 2003-2005). grouped and labelled as Shallow-Etna (S-Et) in tab. 2b. All the events of the dataset are projected onto the epi- Evidently, it was not possible to define the depth range of centre map of fig. 2, whereas a selection of the events with all the events and the uncertain ones were labelled U. magnitude moment ≥5 is given in tabs. 2a and 2b. The Once the events of the merged datasets were projected magnitude moment is labelled Mw when derived from onto the ABT and SBT depth-contour maps (fig. 2), exclud- instrumental data and Maw when derived from the macro- ing the S-Et events, it was evident that earthquakes belong- seismic field, in the case of the historical earthquakes, or ing to the shallow source group prevailed in the area corre- through a weighted average procedure which considers sponding to the surface projection of the upper crust thrust both the macroseismic and the instrumental information, segment (depth-contours 0-to-10 km), whereas the deep in the case of early instrumental and instrumental events source group prevailed above the lower crust segments (WORKING GROUP MPS, 2004). When available, we have (depth-contours 10-to-30 km). Just a few events classified reported both the Mw and Maw values in tabs. 2a and 2b. In as shallow sources were found to be located within the the case of early instrumental earthquakes, such as for areas above the deep crust portions of the ABT and SBT. 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 216

216 G. LAVECCHIA ET ALII

Fig. 3 - Map and section view of the major Plio-Quaternary structural elements (thrusts and normal faults) in eastern central Italy (a) and in north-eastern Sicily (b) compared with the surface and depth distribution of good quality instrumental data recorded from local networks (see references within the figure). Cross sections AA’ is drawn along the trace of the DSS profile 1978 (after LAVECCHIA et alii, 2003). Cross- section B-B’ has been interpreted considering data from the literature (see references in the caption of fig. 1c); the base of the crust and the possible location at depth of the thrusts are mainly deduced from FINETTI (2005) and FINETTI et alii (2005b), also taking into account information from CHIRONI et alii (2000). In both sections an inward deepening of the hypocentres of the events sited close to the outermost basal thrust is evident. – Geometria, in pianta ed in sezione, dei principali elementi strutturali attivi (sovrascorrimenti e faglie normali) in Italia centro-orientale (a) ed in Sicilia nord-orientale (b), confrontati con la distribuzione in pianta ed in profondità di dati strumentali di buona qualità registrati da reti locali (bibliografia in fig.). La sezione AA’ coincide con la traccia del profilo DSS 1978 ed è tratta da LAVECCHIA et alii (2003). La sezione B-B’ è interpretata tenendo conto delle innumerevoli sezioni geologiche e geofisiche attraverso la Sicilia orientale le cui tracce sono riportate in fig. 1c; l’andamento della base crosta è derivato principalmente da FINETTI, 2005; FINETTI et alii (2005b) e CHIRONI et alii (2000).

These events (striped epicentres in fig. 2), although lying and active deformation field, as well as the reconstructed within the areas above the ABT and SBT, are not linked to three-dimensional geometry of the Adriatic and Sicilian the compressional regime investigated here, but rather to basal thrust planes (figs. 2 and 3, tabs. 1, 2a and 2b), a the extensional tectonics which is ongoing at upper crust seismogenic zoning of the two analysed areas can be pro- levels in the areas which overlie the deepest ABT and SBT posed with a same interpretative clue (fig. 5). The consid- segments (GHISETTI & VEZZANI, 1999 and 2002; LAVEC- ered seismicity is associated with the west-dipping ABT CHIA et alii, 2003; BONCIO et alii, 2004; PACE et alii, 2006; and the north-dipping SBT and in both cases it occurs NERI et alii, 2005; JENNY et alii, 2006). close to and along the thrust plane within two distinct seismogenic layers: sited within the upper crust (<10 km) and within the mid-to-lower crust (10-25 km) (LAVECCHIA 5. THE COMPRESSIONAL SEISMOGENIC PROVINCES et alii, 2003; LAVECCHIA et alii, 2006; PACE et alii, 2006). We chose the depth threshold of 10 km on the basis of Taking into account the surface and depth distribu- geological and geophysical investigations which allow the tion of the historical and instrumental seismicity and the transition between the upper crust (cover) and the middle available information on the kinematics of the geological crust (basement) to be located at about this depth 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 217

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Fig. 4 - Macroseismic fields of historical earthquakes and examples of attribution to shallow (H<~10 km) and to deep (H=10-30) seimogenic source. Key: A, B, H=intensity data from DOM (MONACHESI & STUCCHI, 1997); C, D, E, F=intensity data from CFTI (BOSCHI et alii, 2000); G=intensity data from AZZARO & BARBANO (2000). – Esempi di piani quotati per terremoti storici superficiali (H<~10 km) e profondi (H>10) nelle due aree in studio. Sulla colonna di sinistra sono riportati 4 esempi per l’Appennino centrale: A e B=dati di intensità tratti dal DOM (MONACHESI & STUCCHI, 1997); C e D=dati di intensità tratti dal CFTI (BOSCHI et alii, 2000). Sulla colonna di destra sono riportati 4 esempi per la Sicilia orientale ed occidentale: E ed F=dati di intensità tratti dal CFTI; G=dati di intensità tratti da AZZARO & BARBANO (2000); H=dati di intensità tratti dal DOM. 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 218

218 G. LAVECCHIA ET ALII

a Adriatic Intra-Apennine Basal extensional province Compressional province Thrust D S Seismogenic Province (1741, 1799, 1873, (1672, 1690, 1786, 1875, 1897, 1943, 1950) 1916, 1924, 1930) Umbria-Marche coast S SW fold-and-thrust system NE 0 D 10 20

Adriatic basal thrust km 0 km 40 10 km MOHO 25 km 40 0km20

Nebrodi-Peloritani. extensional province Compressional province b D S (361, 1818,1968) (1624) coast Panormide-Imerese 25 NNE N coast thrust system S km0 0 1010 km 10 S D Sicilian S 20 20 AL Basal Sicilian basal thrust

Thrust zone MOHO km Seismogenic km Province 0 km 40 40 40 0km20

(.... 1818,.....) 123 4 56 7 8 9

Fig. 5 - Seismogenic interpretation and comparison between the ABT and SBT compressional seimogenic provinces. The maps on the left side (a and b) delineate the surface boundaries of the provinces and their subdivision into an upper crust (light green) and a mid-lower crust (dark green) sub-province, respectively labelled S (e.g. shallow) and D (deep). The black and white arrows represent the P- and T-axes from the focal mechanisms of fig. 2 and tab. 1. The interpretative crustal profiles on the right side show the section view geometry of the seismogenic provinces. KEY: 1) upper crust compressional seismogenic province; 2) Agrigento-Licata zone; 3) mid-to-lower crust compressional seismogenic province; 4) extensional seismogenic domains; 5) normal and normal-oblique faults; 6) Active normal and normal-oblique faults; 7) thrust faults; 8) active thrust faults; 9) earthquakes with Maw≥5.5 associated with the provinces. – Intrepretazione sismogenetica e confronto, in pianta ed in sezione, tra le province localizzate al tetto dell’ABT e del SBT. Le mappe nei riquadri a sinistra (a e b) schematizzano i limiti delle province e la loro relativa suddivisione nelle sub-province S («shallow» - superficiale, verde chiaro) e D («deep»-profonda, verde scuro), caratterizzate rispettivamente da deformazione sismica nella crosta superiore ed in quella intermedia e profonda. Le frecce nere e bianche rappresentano la proiezione degli assi P e T dei meccanismi focali riportati in fig. 2 e tab. 1. Le sezioni crostali schematizzano l’interpretazione proposta, legata all’attivazione di sorgenti sismogenetiche compressive lungo i sovrascorrimenti basali siciliano ed adriatico nella crosta superiore ed in quella medio-inferiore. LEGENDA: 1) sub-provincia compressiva nella crosta sup.; 2) zona asismica Agrigento-Licata; 3) sub-provincia compressiva nella crosta medio-inferiore; 4) dominio sismogenetico in distensione; 5) faglie normali ed oblique; 6) faglie normali ed oblique attive; 7) sovrascorrimenti; 8) sovrascorrimenti attivi; 9) terremoti storici con Maw≥5.5 attribuiti alle province.

(PAUSELLI & COSTANZO, 2002; CHIRONI et alii, 2000; Within the last four centuries, a significant number of LAVECCHIA et alii, 2003). We propose to subdivide the moderate to high magnitude earthquakes (11 earthquakes area above the ABT into two seismogenic compressional with 5≤Maw<5.5 and 8 earthquakes with Maw≥5.5), among sub-provinces, called «Coastal-Adriatic Province» and which the Senigallia 1930 event (Maw=5.94), characterises «Pede-Apennine Province» which respectively represents the Coastal-Adriatic Province. More damaging historical the surface projection of the ABT upper crust segment earthquakes, among which Fabriano 1741 (Maw=6.08), (depth 0-10 km) and ABT deeper crust segment (depth 10- Camerino 1799 (Maw=5.99) and Southern Marche 1873 25 km). With the same criteria, the area above the SBT (Maw=5.88) are associated to the Pede-Apennine Province may be subdivided into two sub-provinces called «Main- (PACE et alii, 2006). Similarly, moderate earthquakes (3 land Sicily Province» and «Southern Sicily Province». The earthquakes with 5≤Maw<5.5 and one with Maw≥5.5) char- section-view geometry of the sub-provinces is shown in acterise the western sector of Southern Sicily Province the interpretative sections of fig. 5, which highlight well (Sciacca area) and its eastern sector (Niscemi-Caltagirone the structural and seismogenic similarity between the two and Mineo areas), whereas the central Agrigento-Licata investigated areas. sector (dotted area in fig. 5b) is almost completely aseis- 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 219

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TABLE 3 Quantitative comparison of the seismic activity level within the ABT and the SBT seismogenic provinces. The number of earthquakes refers to events sited above the 0-to-25 km ABT and SBT depth contour lines and occurring during the time interval of catalogue completeness for magnitude classes. The areal extents of the Shallow (S) and Deep (S) sub-provinces have been measured in the maps of fig. 2a and b, between the 0-10 and 10-25 depth contour lines, respectively. The energy released (ER) by the earthquakes numbered in this table was calculated using the GUTENBERG & RICHTER (1942, 1956) empirical relationship, that relates the energy to the instrumental magnitude. It was computed for classes of magnitude normalised to the time window of completeness of each class (∆TC) divided by the areal extent (A) of the seismogenic provinces and sub-provinces. – Confronto quantitativo fra il livello di attività sismica associato alle province sismogenetiche localizzate al tetto del sovrascorrimento basale adriatico (ABT) e di quello siciliano (SBT). Il numero di terremoti per classi di magnitudo è derivato dalla lista di tab. 2. L’energia rilasciata (ER) è stata calcolata per classi di magnitudo normalizzate nell’intervallo di completezza di ciascuna classe (∆TC), ed è stata divisa per l’estensione areale (A) della provincia sismogenetica. ER è stata ricavata dall’applicazione dalle relazioni empiriche di GUTENBERG & RICHTER (1942, 1956), tra energia e magnitudo.

Seismogenic Provinces Areal Number of eqs. Number of eqs. Observed Mmax ER / ∆TC ER / ∆TC / A (SP) Extent (km2) 4.5 ≤ M <5 M≥ 5 (Maw ± Daw) erg / y erg / y / km2 Adriatic Basal Thrust SP Sub-province S 6700 km2 15 19 5.94±0.03 5.5E+18 8.2E+14 Sub-province D 7700 km2 24 18 6.08±0.11 6.6E+18 8.5E+14 Entire province 14400 km2 39 37 6.08±0.11 1.2E+19 8.4E+14 Sicilian Basal Thrust SP Sub-province S 6900 km2 11 4 5.57±0.19 3.5E+17 5.1E+13 Sub-province S - AL zone 6900 - 3.100 km2 11 4 5.57±0.19 3.5E+17 9.2E+13 Sub-province D 9300km2 17 6 6.12±0.07 4.7E+18 5.0E+14 Entire province 16200 km2 28 10 6.12±0.07 5.4E+18 3.3E+14 Entire province- AL zone 16200- 3.100 km2 28 10 6.12±0.07 5.4E+18 4.1E+14

mic. Four moderate earthquakes with 5≤Maw<5.5 and two thus selected, we computed the energy (E measured in highly damaging earthquakes – Belice 1968 (Maw=6.12) erg) released in each province by applying the energy- and Catanese 1818 (Maw=6.0) – are associated with the magnitude relation of GUTENBERG-RICHTER (1956) (Log Mainland Sicily Province. A catastrophic event which E=11.4+1.5M). For uniformity, in this calculation we struck central Sicily around the year 361 earthquake always used magnitude values expressed as Maw, with (Maw=6.6) may be also associated with this province the only exception of two minor instrumental events for (JENNY et alii, 2006). which Maw was not available. Consequently, in the case of seismic sequences, such as the Belice one for example, we used only the available Maw, estimated for maximum 6. THE ENERGY RELEASE magnitude event. The computed yearly energy release (ER/∆TC) is Usually, several difficulties have to be dealt with ~1E+19 erg/y for the ABT and ~5E+18 erg/y for the before evaluating the level of seismic activity of a region: SBT. In order to better compare the level of seismic the seismological catalogues should contain homoge- activity among the four sub-provinces, we computed neous magnitude determinations, declustering should be the yearly energy released by each of them and nor- performed, and the completeness of the record should be malised this value to the sub-province areal extent (A). determined. Then, in order to compute the level of seis- We obtained similar ER/∆DTC/A values in the case of mic activity in the two study provinces in terms of energy the two deep sub-provinces (~8E+14 erg/y/km2 for the release, we first determined, through application of the ABT and ~5E+14 erg/y/km2 for the SBT) and different MULARGIA’s et alii (1987) method, the completeness of values for the two shallow sub-provinces, the energy the here compiled seismological dataset, which consists released by the shallow ABT being significantly higher of the seismic events with magnitude moment ≥4.5 than that by the shallow SBT (~8E+14 erg/y/km2 for occurred in the time interval 217 B.C.-2005 A.D. above the ABT and ~5E+13 erg/y/km2 for the SBT) (see tab. the 0-to-30 km ABT and SBT depth contour lines. We 3). If we subtract the area occupied by the central Agri- verified that the Sicilian dataset may be considered com- gento-Licata zone (dotted area in fig. 5b) from the total plete since 1600 A.D.±200 for Maw≥5.5, since 1680 areal extent of the SBT shallow sub-province, the 2 A.D.±100 for 5.0≤Maw<5.5 and since 1820 A.D.±60 for yearly ER increases up to ~9E+13 erg/y/km , but 4.5≤Maw<5.0. The Adriatic dataset may be considered remains significantly lower than that of the shallow complete since 1640 A.D.±200 for Maw≥5.5, since 1680 ABT (tab. 3). A.D.±80 for 5.0≤Maw<5.5 and since 1860 A.D.±20 for In summary, the total level of energy released within 4.5≤Maw<5.0. We excluded from the dataset the events the ABT Province is higher than that within the SBT linked to the Etna volcanic activity and to the intra- Province, but once we normalise the ER value to the Apennine and northern Sicily extensional deformation areal extent of the sub-provinces almost all the differ- plus those with uncertain seismotectonic assignation. ence is absorbed by the shallow sub-provinces, whereas Among the remaining events (labelled SC, shallow com- the deep ones are very similar. If an earthquake with pressional, and DC, deep compressional within the list of magnitude about 6 would occur within the AL zone, tabs. 2a and 2b), we only consider those which occurred which has been silent in historical times, then also the within the time range of the completeness catalogue for level of activity of the shallow sub-provinces would be the different magnitude classes. Starting from the events almost the same. 06 133-20 06(209-222) 25-06-2007 14:24 Pagina 220

220 G. LAVECCHIA ET ALII

7. CONCLUSIONS BARATTA M. (1897) - Materiali per un catalogo dei fenomeni sismici avvenuti in Italia (1800-1872). Mem. Soc. Geog. It., 7, 81-164. Taking into account data from the previous literature BARCHI M.R., DE FEYTER A., MAGNANI M.B., MINELLI G., PIALLI G. and the seismological evidence discussed here, we feel & SOTERA B.M. (1998) - The structural style of the Umbria-Mar- confident enough to state that the ABT and SBT are che fold and thrust belt. Mem. Soc. Geol. It., 52, 557-578. BELLO M., FRANCHINO A. & MERLINI S. (2000) - Structural model of active segments of the outer front of the Apennine- Eastern Sicily. Mem. Soc. Geol. It., 55, 61-70. Maghrebian fold-and-thrust belt system. They are both BEN AVRAHAM Z., BOCCALETTI M., CELLO G., GRASSO M., LENTINI first-order crustal-scale seismogenic structures which F., TORELLI L. & TORTORICI L. (1990) - Principali domini strut- strongly control reverse, reverse-oblique and strike-slip turali originatisi dalla collisione Neogenico-Quaternaria nel Medi- seismic activity. Relatively deep (10-to-30 km) seismicity terraneo. Mem. Soc. Geol. It., 45, 453-462. occurring in the pede-Apennine region and in mainland BOLLETTINO MACROSISMICO - Istituto Nazionale di Geofisica, Roma. Sicily may be associated with brittle shearing of the mid- http://www.ingv.it/~roma/attivita/pererischio/macrosismica/macros/ dle and lower crust thrusts segments, whereas the shal- bollettino/elenco.html. low seismicity (<~10 km) close to the ABT and SBT sur- BOLLETTINO SISMICO ITALIANO. http://www.ingv.it/~roma/reti/rms/ bollettino/ face tip line is associated with reverse, oblique and BONCIO P., LAVECCHIA G. & PACE B. (2004) - Defining a model of 3D strike-slip shearing of the two upper crust thrust seg- seismogenic sources for Seismic Hazard Assessment applications: ments and of their frontal and lateral splays. The case of central Apennines (Italy). Journal of Seismology, 8, Although there is knowledge of a significant number 407-425. of moderate to relatively large earthquakes which struck BOSCHI E., GUIDOBONI E., FERRARI G., GASPERINI P., MARIOTTI both the ABT and SBT seismogenic compressional D. & VALENSISE G. (2000) - Catalogo dei forti terremoti in Ita- lia dal 461 a.C. al 1997. Annali di Geofisica, 43, 843-868 and provinces (42 events with 4.5≤Maw ~6.0 in Sicily and 76 in the Adriatic within the completeness interval), the seismic CD-ROM. BOUSQUET J.C. & LANZAFAME G. (1986) - Deformations compressives hazard of Sicily is commonly underestimated (WORKING quaternaires au bord sud de l’Etna. C.R. Acad. Sc. Paris, 303, GROUP MPS, 2004 and references therein). In this paper, 235-240. we point out not only a close geometric and seismotec- BUTLER R.W.H., GRASSO M. & LA MANNA F. (1992) - Origin and tonic similarity between the ABT and SBT (figs. 2 and 5), deformation of the Neogene-Recent Maghrebian foredeep at the but also an impressive, and up to now unnoticed, equiva- Gela Nappe, SE Sicily. Journal of Geological Society of London, lence between the yearly energetic release per unit area of 149, 547-556. the «Pede-Apennine Province» and of the «Mainland CACCAMO D., NERI G., SARAO A. & WYSS M. (1996) - Estimates of Sicily Province» (tab. 3). The highlighted differences in stress directions by inversion of earthquake fault plane solutions in Sicily. Geophys. J. Int., 125, 857-868. seismic activity between the «Coastal-Adriatic Province» and the «Southern Sicily Province» also appears CALAMITA F., CELLO G., CENTAMORE E., DEIANA G., MICARELLI A., PALTRINIERI W. & RIDOLFI M. (1991) - Stile deformativo e crono- extremely intriguing, as they are similar in terms of active logia della deformazione lungo tre sezioni bilanciate dall’appenni- tectonics and seismogenic sources. This difference may no Umbro-Marchigiano alla costa Adriatica. Studi Geologici Ca- be ascribed to a more ductile mechanical behaviour of merti, Vol. speciale 1991/1, 295-314. the shallow SBT, under different rheological constraints, CALAMITA F., COLTORTI M., PIERUCCINI P. & PIZZI A. (1999) - Evolu- and/or to longer recurrence time intervals. Our research zione strutturale e morfogenesi plio-quaternaria dell’Appennino certainly needs further development, but we hope it will umbro-marchigiano tra il preappennino umbro e la costa adriatica. Boll. Soc. Geol. It., 118, 125-139. help to stimulate a revaluation of the seismic potential of CALAMITA F., SCISCIANI V., MONTEFALCONE R., PALTRINIERI W. & Sicily in terms of seismic hazard assessments. PIZZI A. 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Received 14 December 2006; revised version accepted 19 March 2007.