Paleoseismological investigations along the San Demetrio ne Vestini fault (AQ) A.M. Blumetti, P. Di Manna, E. Vittori, V. Comerci, L. Guerrieri ISPRA, Geological Survey of Italy Introduction and geological framework. In the months following the April 6 th, 2009, Mw 6.3 L Aquila earthquake, in the framework of a seismic microzonation, the Geological Survey of Italy (ISPRA) carried out a detailed geological and geomorphological survey of the San Demetrio ne Vestini territorial municipality. This village, severely damaged by the 2009 event, is located in the south-eastern part of the L Aquila basin, a complex tectonic basin characterized by a system of northwest-southeasttrending tectonic depressions (total length about 30 kilometres) developed inside the inner sector of the Meso-Cenozoic orogenic belt of the Apennines, between the Gran Sasso and the Monti d Ocre morphotectonic units (Fig. 1). Active crustal extension in this zone is demonstrated by GPS observations, which reveal velocities across the Central Apennines in the order of 3 mm/yr (D Agostino et al., 2011), and confirmed by the large historical and instrumental seismicity occurred in the region. The geological evidence of active faulting is also widely recognised (Bagnaia et al., 1992; Blumetti, 1995; Boncio et al., 2004; Galadini and Galli, 2000; Roberts and Michetti, 2004; Blumetti and Guerrieri 2007; Galli et al., 2010; Giaccio et al., 2012; Blumetti et al., 2013). The San Demetrio area is located at the southern tip of the L Aquila basin fault zone, which is characterized by the presence of en echelon and sub-parallel NW-SE trending fault segments. This is a typical structural setting at the transition between two primary fault zones (in this case between the L Aquila Basin and Subequano Basin primary fault zones). Moreover in this area extension and faulting progressively migrated toward the inner part of the basin (Giaccio et al., 2012; Blumetti et al., 2013). The territory of the San Demetrio municipality is crossed by 4 NW-SE trending fault segments that have displaced also Quaternary deposits (Fig. 1B; Bosi and Bertini, 1993). One of them, crossing the historical settlement of the village, was pointed out as a capable fault by Working Group MS AQ, (2010). This fault (named San Demetrio fault) already identified as a possibly active fault by Bagnaia et al. (1992), does not crop out, but is inferred at the base of an up to 25 meters high fault scarp that displaces the flat surface of a recent alluvial terrace (Figs. 1 and 2). This note summarizes the first results of paleoseismological investigations carried out by ISPRA along the San Demetrio fault, with the aim to i) characterize its capability in terms of seismic and surface faulting potential and ii) map the fault, and an appropriate setback area along it, where introduce specific land use restrictions, as requested by the Mayor of the San Demetrio ne Vestini. Paleoseismological analysis. In order to better locating the most suitable trenching site, a geophysical survey was performed, consisting of an ERT (Electrical Resistivity Tomography) profile and a seismic refraction tomography line. A previous ERT profile (Working Group MS AQ, 2010), crossing the fault not far from the trench site, is reproduced in Fig. 2C. The site selected for trenching is located north-west of the historical centre of the village (Fig. 1), where morphotectonic observations and geophysical data were consistently indicating the occurrence of a fault cutting up to the surface. The detailed analysis of the stratigraphy exposed in the trench walls confirmed that the exposed fault is capable of producing surface ruptures (Fig. 3). In fact, a major fault zone, 5 meters wide, characterized by four main (e.g. offsets larger than the trench wall height) synthetic faults and an antithetic structure, was found in the trench walls. Analysing the fault zone exposed in the eastern wall, at least two colluvial wedges were identified, i.e. Levels 4 and 10 in Fig. 3B. Level 4 is about 20 cm thick and is bounded 29
downslope by a small antithetic fault. Moreover it is slightly displaced (just a few centimetres) by a tiny fracture that downward is linked to a major fault. This interpretation would imply two surface faulting events. Even if radiocarbon and OSL dating are still in progress, it is possible to refer these paleoearthquakes to the Holocene. In fact, the gravel layer sealing the faults (level 2 in Fig. 3B) appears to be very young, most likely historical due to its correlation with a nearby layer containing roman pottery (imperial age). Fig. 1 (A) Oblique view (based on a 20 m DTM) of the L Aquila region with the net of capable faults. Legend: 1) primary fault; 2) secondary fault. (B) geological map of S. Demetrio ne Vestini. Legend: 1) colluvial and debris deposits (Holocene); 2) alluvial fan gravels and sands (upper part of Middle Pleistocene); 3) sands (upper part of Middle Pleistocene); 4) fan delta conglomerates (Lower to Middle Pleistocene); 5) whitish silty lacustrine deposits (Lower to Middle Pleistocene); 6) capable fault (not outcropping); 7) fault scarp edge; 8) location of the cross section shown in Fig. 2B; 9) trace of the ERT profile shown in Fig. 2C; 10) trace of the paleoseismological trench. (modified after Working Group MS AQ, 2010). 30
Level 3 is also involved in the deformation, and displaced of few centimetres only by the most recent event. Level 10 is an about 50 cm thick colluvial wedge and is downthrown by a small synthetic fault linked in depth to a major fault. This small fault does not cut layers younger than level 10, so that it can be argued that it moved soon after the deposition of the colluvial wedge 10. This should indicate the occurrence of two additional older events, but occurred in a very short time, possibly in the same seismic sequence. The presence of an antithetic fault located a few metres downslope the fault zone (not drawn in Fig. 3B, but visible in Fig. 3A) indicates the occurrence of a gravity graben during such older surface faulting events. Consequently, the amount of the coseismic offset related to the formation of colluvial wedge 10, and also the magnitude of the triggering event (using Wells and Coppersmith, 1994) cannot be evaluated. Nevertheless, such features, and in particular the Fig. 2 (A) Panoramic view of the fault scarp related to the Demetrio fault. The orange net marks the trench site. (B) Geological profile across the fault (location in Fig. 1). Legend: 1) fan delta conglomerates (Lower to Middle Pleistocene); 2) gravels and sands (upper part of Middle Pleistocene) (modified, after Working Group MS AQ, 2010). (C) ERT profile across the San Demetrio fault (location in Fig. 1; after Working Group MS AQ, 2010). 31
presence of such a wide and deep gravity graben, suggest a magnitude value much larger than the 2009 earthquake (Mw=6.3). Acknowledgements. 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 Epifanio A. and Sylos Labini S.; 1992: Aquila and subaequan basins: an example of Quaternary evolution in Central Apennines, Italy. Quaternaria Nova, II, 187-209 (preprint spec. number 1, 1-23- 989). Bertini T. and Bosi C.; 1993; La tettonica quaternaria nella conca di Fossa (L Aquila). Il Quaternario, 6, 293-314. Fig. 3 (A) View of the fault zone exposed in the eastern wall. (B) Stratigraphic sketch of the fault zone exposed in the eastern wall of the San Demetrio trench. Levels 4 and 10 are colluvial wedges. The dashed box locates Fig. 3C. (C) Close-up view of the upper colluvial wedges exposed in the eastern wall of the San Demetrio trench. 32
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