GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 41 Unit 4 is about 25-cm-thick and is affected down-slope by a small antithetic fault. Moreover, it is slightly displaced (just a few centimetres) by a secondary splay that is linked downwards to the major fault plane, and, in turn, is sealed by unit 3. The radiocarbon dating of a bulk sample taken in unit 4 (SD 65 in fig. 3B; see Tab. 1) provided a calibrated age of 3640-3480 BC. The faulted colluvial wedge dated back to 3640-3480 BC implies the occurrence of two surface faulting events, one slightly older than this age, and the other younger. The age of both events may be constrained better by the radiocarbon dating of unit 3. Indeed, just above unit 4, this tiny colluvial layer (unit 3) - that partly constitutes the matrix of the bottom level of the overlaying gravels (bottom of unit 2) - is also involved in the deformation across the main, northern splays, being displaced by at least 15 centimeters (Fig. 3). Its calibrated age (2760- 2470 BC, Tab. 1) is the terminus post quem for the most recent surface faulting event that was recorded by the sequence exposed in the San Demetrio trench. Moreover, as unit 3 seals the antithetic splay cutting through the colluvial wedge of unit 4, it represents also the terminus ante quem for the penultimate event. As far as unit 2, this is made by colluvial gravels overlaying unit 3 (Fig. 3). Laterally, it can be correlated with layers containing pottery shards that can be attributable at least to the Roman Imperial period (less than two millennia). Therefore, as the bottom of unit 2 is faulted together with unit 3, the age of the last visible faulting event is surely successive to the Roman period, being sealed only by the present soil. Other two previous surface faulting events can be inferred observing the bottom part of the sequence exposed in the eastern trench wall. Here, a ~50 cm thick colluvial wedge, which is made of coarse gravels (unit 10 in Fig. 3B) is clearly visible. It is characterised by a very steep attitude of the stratification and it is also affected by a secondary synthetic splay which jois, at depth, the major fault plane. This splay does not cut layers younger than unit 9, the upper part was of which was dated back to 7030-6570 BC (Tab. 1). This circumstance, as in the previous case, should indicate the occurrence of two older events. The presence of an antithetic fault located a few meters downslope the fault zone (see Fig. 3), indicates the formation of a “gravity graben” during the older surface faulting events. The gravels of the upper part of unit 9 are clearly dragged by this antithetic fault (Fig. 3), such as the gravels of unit 12. The formation of the gravity graben prevents to calculate the amount of the coseismic offset related to the formation of colluvial wedge 10, and also the mag- nitude of the triggering event (e.g., from Wells and Coppersmith, 1994) cannot be evaluated. Nevertheless, such features, and in particular the presence of such a wide and deep “gravity gra- ben”, suggests a magnitude value considerably larger than both the 2009 earthquake (Mw=6.3) and the younger events seen in the upper part of the excavation. Acknowledgments. The Authors wish to thank Valeria Eulilli, Fernando Ferri and Luca M. Puzzilli for providing the geophysical investigations, and Leonello Serva for his valuable suggestions in the paleoseismic interpretation of the stratigraphic log. References Bagnaia R., D’EpifanioA. and Sylos Labini S.; 1992: Aquila and subaequan basins: an example of Quaternary evolu- tion in Central Apennines, Italy. ����������� ����� Quaternaria Nova, II , 187-209 (preprint spec. number 1, 1-23- 1989). Bertini T. and Bosi C.; 1993; La tettonica quaternaria nella conca di Fossa (L’Aquila) . Il Quaternario, 6 , 293-314. Blumetti A.M.; 1995: Neotectonic investigations and evidence of paleoseismicity in the epicentral area of the Ja- nuary-February 1703 Central Italy earthquakes , in Perspectives in Paleoseismology Leonello Serva (Editor), Bulletin of the American Association of Engineering Geologists, Special Volume n. 6 , Boston, 83-100. Blumetti A.M. and Guerrieri L.; 2007: Fault-generated mountain fronts and the identification of fault segments: im- plications for seismic hazard assessment , Boll. Soc. Geol. It. (Ital.J.Geosci.), 126 (2) 307-322. Blumetti, A.M., Guerrieri, L. and Vittori, E.; 2013: The primary role of the Paganica-San Demetrio fault system in the seismic landscape of the Middle Aterno Valley basin (central Apennines). Quaternary International, http://dx.doi . org/10.1016/j.quaint.2012.04.040. Boncio P., Lavecchia G., and B. Pace; 2004: Defining a model of 3D seismogenic sources for Seismic Hazard Assess- ment applications: the case of central Apennines (Italy) , Journal of Seismology, 8 407-425.