Nuovi dati paleosismologici dal settore orientale del bacino del Fucino (Italia centrale)

L’analisi di depositi del Pleistocene superiore-Olocene in due cave a ridosso della faglia San Benedetto dei Marsi-Gioia dei Marsi (bacino del Fucino, Appennino centrale) ha consentito di ottenere informazioni paleosismologiche che integrano quanto già pubblicato in materia alla fine del secolo scorso. In entrambi gli affioramenti sono stati osservati sedimenti colluviali, alluvionali e lacustri deposti successivamente al Tardiglaciale (14-15 Ka BP). Vincoli cronologici per le successioni individuate vengono da una datazione radiocarbonio (1466-1372 B.C., età calibrata a 2 sigma), da numerose determinazioni archeologiche su materiali ceramici attribuiti all’Eneolitico e all’Età del Bronzo e dal rinvenimento di una fossa di cottura inquadrabile nello stesso ambito cronologico. Le unità stratigrafiche identificate sono in contatto per faglia con il substrato carbonatico. Il trascinamento dei sedimenti lungo i piani di faglia principali e le dislocazioni osservate lungo piani di taglio secondari definiscono l’attività olocenica di questo settore della faglia San Benedetto dei Marsi-Gioia dei Marsi. L’analisi del primo fronte di scavo ha fornito l’evidenza di tre eventi di dislocazione di cui il più recente avvenuto successivamente alla deposizione dell’unità colluviale con frammenti ceramici preistorici. L’analisi del secondo fronte di scavo ha ugualmente permesso di identificare tre eventi di dislocazione di cui il più recente interessa l’unità nella quale è stata scavata la fossa di cottura. Due dei tre eventi identificati potrebbero corrispondere a due dei tre più antichi eventi dell’Olocene identificati nei precedenti lavori e avvenuti all’incirca tra 5 e 10 Ka BP. L’evento più recente potrebbe invece essere associato a quello già riferito all’intervallo 1500-1300 a.C., e/o ai due eventi successivi, vale a dire quello riferibile alla Tarda Antichità-Alto Medioevo (426–782 d.C., età 14C cal. in Galadini e Galli, 1999) e quello avvenuto nel 1915. Infine, l’entità del rigetto verticale ha permesso di valutare un rateo minimo di movimento dell’ordine di 0.14-0.35 mm/a.

The Fucino Plain represents the area with the best Italian paleoseismological record. This is mainly due to the fact that, differently from other Apennine cases, Late Pleistocene-Holocene faults border and affect a lacustrine basin which has been persistent throughout the Late Pleistocene-Holocene until the 19th century. Thanks to this almost continuous stratigraphical record, Galadini and Galli (1999) described the complete displacement history of the Fucino fault, which finally activated in 1915, causing a Mw=7.0 earthquake. These authors also identified six previous Holocene events (with recurrence intervals of 1,400-2,600 years), and three others occurred in the Upper Late Pleistocene. On the whole, the Fucino paleoseismological record consists of ten events (including the 1915 one) in the past 32,500 years. Human works, especially those related to the excavation of foundations, extraction from quarries and cleaning of the drainage network, gave the opportunity to study new outcrops of paleoseismological interest. The recent analysis of two quarry walls located along the western flank of the basin (i.e. where the surficial expression of one of the main faults is located) has provided data which corroborate the previous studies. Alluvial gravels deposited along the shoreline of the lake and colluvial deposits outcrop in both quarries. Unit 1 of quarry-wall FS1 is made of matrix supported gravel, with pottery shards attributed to 3000-1500 B.C., whereas the radiocarbon dating of the organic matrix gave 1466-1372 cal B.C. (2 sigma). Unit 2, whose origin is uncertain (alluvial deposits accumulated and reworked along the lacustrine shoreline?), is made of gravel included in silty-sandy matrix. Apart from sub-unit 3b, Unit 3 is prevalently made of gravel and coarse sand due to alluvial deposition close to the lacustrine shoreline. Based on the available literature (Giraudi, 1988, 1999a), it deposited at the end of the Late Pleistocene. Sub-unit 3b is particularly significant for the paleoseismological analysis, since it has been interpreted as a colluvial wedge originated from the erosion of a coseismic fault scarp. Unit 4, made of clast-supported gravel, probably represents a shoreline deposit. The relationship between the fault planes and the described succession suggests that at least three events of displacement occurred after the deposition of sub-unit 3d, the oldest one being related to the colluvial wedge (unit 3b) and the youngest being represented by the displacement of the entire succession (including unit 1 containing the pottery shards). The other quarry wall (FS2) shows archaeological remains (inside unit 2) below the present soil (unit 1). These remains are basically represented by a pit-fire containing fragments of pottery shards attributed to the Eneolithic and the Bronze Age (3000-1500 B.C.). The characteristics of the Unit 3 gravel indicate deposition along the shoreline, while unit 4 (gravel in sandy matrix) has been interpreted as a colluvial wedge deposited from a coseismic scarp. Finally, unit 5 represents the result of the re-organisation of alluvial gravel by the lacustrine shoreline water. From a chronological point of view, this unit can be compared to unit 3 of FS1, i.e. it deposited at the end of the Late Pleistocene. The analysis of the relationship between the fault planes and the stratigraphic units defines at least three displacement events, the intermediate being represented by the deposition of the colluvial wedge and the ultimate(s) by the displacement of unit 3 within which the pit-fire has been excavated. On the whole, the oldest and the intermediate events may represent two of the three oldest Holocene events identified by Galadini and Galli (1999), which occurred between 5 and 10 ka BP.
Indeed, the oldest displaced units, based on the available literature (Giraudi, 1988; 1999a), deposited after 14-15 ka BP. The most recent event(s), besides the 1915 rupture, may be associated to the event dated at 1500-1300 B.C. by Galadini and Galli (1999) and/or, considering the displacement affecting unit 1 (FS1), to the penultimate Holocene event dated at the Late Antiquity/early Middle Ages (426 – 782 A.D., cal. age 14C in Galadini and Galli, 1999). Finally, the amount of displacement and the chronological constraints permit us to define vertical displacement rates not exceeding 0.35 mm/yr for the main fault of FS1 and 0.14 mm/yr for the main fault of FS2.