SESS 2_16022011_TPR3.pub Il Quaternario Italian Journal of Quaternary Sciences 24, (Abstract AIQUA, Roma 02/2011), Congresso AIQUA Il Quaternario Italiano: conoscenze e prospettive Roma 24 e 25 febbraio 2011 THE PRIMARY ROLE OF THE PAGANICA-SAN DEMETRIO FAULT IN THE SEISMIC LANDSCAPE OF THE MIDDLE ATERNO VALLEY BASIN Anna Maria Blumetti, Luca Guerrieri & Eutizio Vittori ISPRA – Servizio Geologico d’Italia, Roma Corresponding author: A. Blumetti ABSTRACT: Blumetti A. M., Guerrieri L. & Vittori E., The primary role of the Paganica-San Demetrio fault in the seismic landscape of the Middle Aterno Valley basin. (IT ISSN 0394-3356, 2011) The Paganica-San Demetrio fault, which caused the April 9, 2009 earthquake, has played a key role in the evolution of the Middle Aterno Valley basin since the Middle Pleistocene. Its minimum offset, marked by the elevation of the rem- nant paleosurface of the Anzano Plateau, is in excess of 700 m. In our interpretation, the Assergi and Campo Impera- tore faults with their assumed listric geometry would merge into the Paganica-San Demetrio master fault. RIASSUNTO: Blumetti A. M., Guerrieri L. & Vittori E., Il ruolo primario della faglia Paganica-San Demetrio nel paesag- gio sismico del bacino della media Valle dell’Aterno. (IT ISSN 0394-3356, 2011) La faglia Paganica-San Demetrio, cui è legato il terremoto del 9 Aprile 2009, ha giocato un ruolo primario nell’evoluzione del bacino della media Valle dell’Aterno almeno a partire dal Pleistocene Medio. Il suo rigetto minimo, calcolato sulla base della differenza in quota tra la media Valle dell’Aterno e lembi dislocati di una paleosuperficie, su- pera i 700 m. Nella nostra interpretazione essa è da considerare la master fault del blocco tettonico del Gran Sasso alla quale le faglie di Assergi e Campo Imperatore, ad andamento listrico, si raccordano in profondità. Key words: Block faulting, morphotectonics, Central Apennines. Parole chiave: Tettonica a blocchi, morfotettonica, Appennino centrale. 1. INTRODUCTION On April 6 2009 a Mw 6.3 earthquake rocked the Middle Aterno Valley basin (Abruzzo, Central Italy) causing about 300 casualties and strong damage to the L’Aquila city and its surroundings. The Paganica-San Demetrio fault, which bounds the Middle Aterno Valley basin to the N-E, has been considered the causative fault of this event. This paper aims at better pointing out the long- term role of this fault in the Quaternary evolution of the Middle Aterno Valley basin. 2. REGIONAL GEOLOGICAL FRAMEWORK The characteristic landscape of Central Apennines, marked by fault-bounded basins and ranges, has been interpreted as the long-term effect of block- faulting and fast uplift starting approximately 0.8 Myr ago (BLUMETTI & GUERRIERI, 2007 and refer- ences therein). NE-SW extension across the Apen- nines is still active, as suggested by geodetic data that indicate rates ranging from 3 to 5 mm/yr (respectively, FAURE WALKER et al., 2010; D’AGOSTINO et al., 2008). Moreover, the size of these tectonic basins and of the associated fault-generated mountain fronts has been considered a peculiar feature, indicative of the local seismic potential (concept of seismic landscape, e.g. MICHETTI et al., 2005). The Middle Aterno Valley basin (L’Aquila) is a sec- tor of Central Apennines (Fig. 1) located between the Gran Sasso and the Monti d’Ocre morphotec- tonic units. In this region, surface geology and geo- physical data reveal a composite structural setting given by several over-thrust tectonic units belong- ing to the transitional domain between the Lazio- Abruzzi carbonate shelf platform and the Umbria- Marche pelagic basin, dissected by a network of Quaternary normal faults, in places located close to the uppermost traces of older thrust planes (APAT, 2006). Stratigraphic and morphological evidence suggest ongoing activity along several of these faults (Fig. 1; BAGNAIA et al., 1992; BLUMETTI, 1995; GALLI et al., 2008) as confirmed by historical seismicity (three Imcs ≥ IX events hit this same area in 1461, 1703 and 1762 (Gruppo di Lavoro CPTI, 2004). 3. PAGANICA-SAN DEMETRIO FAULT AND QUATERNARY EVOLUTION OF THE MIDDLE ATERNO VALLEY BASIN The Quaternary evolution of the L’Aquila region can be synthesized in two phases. The Lower Pleistocene landscape was characterized by sev- eral lacustrine basins controlled by normal faults. Nevertheless, the low uplift rates allowed to form a low energy relief landscape with wide pediments mainly formed by areal erosional processes (DEMANGEOT, 1965). After the dramatic increase of uplift rates and faults 84 - 86 85 thrusts at depth (D’AGOSTINO et al., 1997). The region at the footwall of the Paganica-San De- metrio fault (i.e. the Gran Sasso Chain together with Anzano and Aragno Plateau zone) in our view is a single first order morphotectonic unit. This block has been displaced in several tectonic units of lower rank by very long and important faults, like the Assergi and Campo Imperatore faults (slip rates 0.67-1 mm/yr; GIRAUDI & FREZZOTTI, 1995; GALLI et al., 2002) as well as by a set of secondary very shallow listric faults (e.g., the Ruzza Mt. and S. Stefano di Sessanio faults) that have an arc shape in plant view and are mostly gravity driven (BAGNAIA et al., 1992). Based on their likely listric geometry, the Assergi and Campo Imperatore faults should join at depth the Paganica - San De- metrio high angle normal fault. Gravity-driven movements may have contributed to the total slip activity since the end of the Lower Pleistocene (DEMANGEOT, 1965; AMBROSETTI et al., 1982) these pediments have been displaced and are now pre- served as remnants at different elevations in the present landscape, that is deeply dissected by lin- ear erosion. We interpret that during this period (about 800 kyr) the Middle Aterno Valley unit (with an average sur- face at about 700 m a.s.l.) has been downthrown by the Paganica - San Demetrio normal fault with respect to the 1500 m a.s.l. high Anzano and Aragno Plateau summit paleosurface area. At the same time, this zone was downthrown with respect to the Gran Sasso chain (highest elevation in the Apennines, close to 3000 m a.s.l.) by the WNW- ESE trending south-dipping Assergi and Campo Imperatore normal faults, which have a listric ge- ometry, possibly representing reactivations of old Fig. 1, 3D view (based on a 20 m DTM) of the L'Aquila region with reported the net of capable faults, defining a seismic landscape. The major faults (1) are signed by prominent fault escarpments and bound first order tectonic blocks. Minor faults within the major blocks, sometimes with an evident listric geometry, can be ranked in two categories: (2) high slip rate faults at the base of large fault escarpments generally representing reactivations of old thrusts; these bound sec- ond order blocks; (3) faults that produce lower fault scarps and bound third order blocks. Remnants of low relief land- scape (4) can be used as throw markers. Immagine 3D (basata su un DTM a 20 m) della regione aquilana, con riportate le faglie capaci che definiscono il pae- saggio sismico. Le faglie principali (1) sono caratterizzate da versanti di faglia che bordano blocchi tettonici di primo ordine. Faglie minori all’interno dei blocchi principali, talvolta con evidente andamento listrico, possono essere gerar- chizzate in due categorie: (2) faglie alla base di grandi versanti di faglia, con elevato slip rate e che generalmente inver- tono piani di sovrascorrimento; queste definiscono blocchi di secondo ordine; (3) faglie che producono scarpate di fa- glia minori e bordano blocchi di terzo ordine. Lembi di un paesaggio a bassa energia di rilievo (4) possono essere utiliz- zati come riferimento nel calcolo dei rigetti post Pleistocene Medio  The primary role of the Paganica-San Demetrio– ... 86 Ms. received: Testo ricevuto il Geol., It. 126 (2), 307-322. BLUMETTI A.M. 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(2009) - Quaternary tectonic evolution of the area affected by the Paganica Fault (2009 L'Aquila earthquake). In: Atti 28° Convegno Nazionale GNGTS, Trieste, 47-50. MICHETTI A.M., AUDEMARD F. & MARCO S. (2005) - Fu- ture trends in paleoseismology: Integrated study of the seismic landscape as a vital tool in seismic hazard analyses. In: Paleoseismology, integrated study of the Quaternary geological record for earthquake deforma- tion and faulting, A.M. MICHETTI, F. AUDEMARD & MARCO S. (Eds), Special Issue, Tectonophysics, 408 (1 -4) , 3-21. NIJMAN W. (1971) - Neotectonics of the Velino-Sirente area, Abruzzi, Central Italy. Koninkl. Nederl. Akademie van Wetenshappen, Amsterdam, Proceedings, 74 (2), 156-184. observed on the higher listric faults (NIJMAN, 1971; BLUMETTI, 1995; DRAMIS & BLUMETTI, 2005) and this would account for their very high slip rate. In this model, the Paganica - San Demetrio would be the master fault of the whole Gran Sasso - Mid- dle Aterno Valley fault system, despite the lack of a clear fault-generated mountain front related to it. Actually, the Middle Aterno Valley border lacks the prominent fault-generated mountain front typical of other basins. Instead, at the footwall of the bound- ing fault, the extension is accommodated along several NW-SE trending normal faults which cause an horsts and graben landscape. The elevation of about 1500 m of the Anzano Plateau is gradually reached through a set of narrow ridges and small elongated basins. Moreover, in the area where surface faulting occurred in 2009, i.e. along the Paganica fault segment, the presence in the foot- wall of a paleo-drainage since at least the end of Lower Pleistocene (Paleo-Raiale, sensu MESSINA et al., 2009) contributed to smooth the fault escarp- ment. Relict forms of this paleo-landscape and dif- ferent orders of fluvial deposits terraced by the pro- gressive activity of the Paganica fault are still pre- served in its footwall. Volcanic deposits dated be- tween 560 and 360 kyr B.P., being displaced of about 150 m, constrain the Paganica fault long term slip rate to ca. 0.4 mm/yr (MESSINA et al., 2009). The primary role played by the Paganica-San De- metrio fault has important consequences in terms of seismic hazard, being now acknowledged as the main seismic source of the basin. If, as suggested here, the Campo Imperatore and Assergi faults move as its splays, their seismic potential would be directly controlled by that of the main fault. REFERENCES AMBROSETTI P., CARRARO F., DEIANA G. & DRAMIS, F. (1982) - Il sollevamento dell’Italia centrale tra il Pleisto- cene inferiore e il Pleistocene medio. Contributi conclu- sivi per la realizzazione della Carta Neotettonica d’Italia. CNR-PFG, 513, 219–223. APAT (2006). Foglio 359 L’Aquila. Carta Geologica d’Italia alla scala 1:50000. S.EL.CA., Firenze. BAGNAIA R., D’EPIFANIO A. & SYLOS LABINI S. (1992) - Aquilan and Subaequan basins: an example of Quater- nary evolution in central Apennines, Italy. Quaternaria Nova, 2, 187-209. BLUMETTI A.M. & GUERRIERI L. (2007) - Fault-generated mountain fronts and the identification of fault segments: implications for seismic hazard assessment, Boll. Soc. January 15, 2011 15 gennaio 2011 A. M. Blumetti, L. Guerrieri & E. Vittori