Active Normal Faulting and Large-Scale Mass Wasting in Urban Areas: The San Gregorio Village Case Study (L’Aquila, Central Italy). Methodological Insight for Seismic Microzonation Studies

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Active Normal Faulting and Large-Scale Mass Wasting in Urban Areas: The San Gregorio Village Case Study (L’Aquila, Central Italy). Methodological Insight for Seismic Microzonation Studies

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  Active normal faulting and large-scale mass wasting in urban areas: the San Gregorio village case study (L’Aquila, central Italy). Methodological insight for seismic microzonation studies. Gori Stefano 1 , Falcucci Emanuela 1 , Di Giulio Giuseppe 1 , Moro Marco 1 , Saroli Michele 2,1 , Vassallo Maurizio 1 , Ciampaglia Andrea 3 , Di Marcantonio Paolo 3 , Trotta Domenico 3   1  Istituto Nazionale di Geofisica e Vulcanologia, Italy. 2  DiMSAT, Università di Cassino, Italy. 3  Geologist The 2009 L’Aquila earthquake determined ground cracks in the area of San Gre-gorio, along a normal fault branch NW-SE trending and SW dipping. We dug two paleoseismological trenches across the fault to investigate its Late Quaternary ac-tivity and to verify whether the co-seismic ground ruptures could be effectively re-lated to the fault activation. The trenches showed that, in the past few millennia, the fault branch was responsible for surface displacement much larger than that determined by the 2009 earthquake. Moreover, geological/geomorphological field survey de-fined that the central portion of the structure is utilised in its shallowest portion as sliding plane of a large-scale gravitational mass movement. In particular, one of the paleoseismological trenches revealed that the large scale mass wasting is probably characterised by both continuous displacement and abrupt events of movement. Seismological investigations defined small amplification on rock site along the investigated fault, to be likely related to the joint and fracture condition deter-mined by the fault activity. Keywords: L’Aquila earthquake, Large scale mass wasting; Seismic microzonation 1.   Introduction The 2009 L’Aquila earthquake (Mw 6.1) ruptured a 12-13 km long (e.g. Gori et al. 2012), NW-SE striking and SW dipping normal fault (Fig.1a, b), the Paganica fault. Surface faulting occurred along the tectonic structure, and it appeared as sets of aligned ground fractures, that showed 10-15 cm vertical displacement along the  central part of the fault (e.g. Emergeo 2010). The southern segment of the Paganica fault affects the area of the San Gregorio village (Fig.1c). Tens-of-metres-long ground cracks were seen along the northern tip of the San Gregorio fault branch after the 2009 event, along a limestone fault scarp crossing the Aterno River plain (Fig. 1c); in the southernmost part of the branch, ground cracks occurred along NW-SE and WNW-ESE fault planes crossing San Gregorio (Falcucci et al. 2009) (Fig. 1c). Except for the above described co-seismic ground cracks, no evidence of the Quaternary activity of the San Gregorio fault segment were previously identified. We thus performed geological-geomorphological field investigations and paleoseismological analysis along the tectonic structure aimed at investigating its recent kinematic behaviour. 2.   Geological/geomorphological field survey Aerial photographs analysis and field survey defined that the central portion of the San Gregorio fault segment coincides with the uphill flank of a NW-SE trending narrow valley, the “Valle degli Asini”, located on top of Mt. Manicola, a carbonate relief located just N-NW of San Gregorio (Fig.1c). The geomorphic features of the area suggest that “Valle degli Asini” represents a trench located on the upper portion of a large scale gravitational mass movement that affects part of Mt. Manicola (Fig.1c). The gravitationally unstable rock mass is laterally bounded by stream incisions (Fig.1c) having curvilinear pattern (in plan view). The incision bounding the rock mass towards SE feeds an alluvial fan that opens on the San Gregorio area. The fact that the up-slope flank of “Valle degli Asini” coincides with the damage zone of the fault segment suggests that the structural feature is partly used in its shallowest portion as preferred sliding plane to lead the gravitational movement. Moreover, the Mt. Manicola large scale mass wasting affects the carbonate formations “Calcari a Briozoi e Litotamni” and “Calcareniti a Macroforaminiferi” (MS–AQ Working Group 2010; geological map, Macroarea 3) – mapped as “Scaglia Cinerea Formation” by Foglio CARG (2009) – that gently dip downslope (roughly 10°-30°), and that are overlain by marly-clayey lithotypes of the “Unità argilloso-marnosa” Formation, outcropping at the base of the relief. This setting allows hypothesising that the rock mass slides over the marly-clayey portion of the “Calcareniti a Macroforaminiferi” Formation (which was found at the bottom of the “Valle degli Asini” valley). Although low angle bedding and the low local relief would not intuitively render the slope prone to landsliding, reactivations of large-scale mass movements that slid over the low gradient (about 20°) clayey marly lithotypes of the “Scaglia Cinerea” Formation (that are similar to those of Mt. Manicola) have been observed (Moro et al. 2011).    3.   Paleoseismological investigations The recent activity of the San Gregorio fault segment was investigated by digging two excavations across the tectonic structure, one across “Valle degli Asini” (trench 1) and one in the northern portion of the fault (trench 2), across the bedrock fault scarp along which ground cracks occurred after the 2009 earthquake (Fig. 1c). The former trench uncovered a sequence of slope deposits overlaying the carbonate bedrock. Most of the sedimentary succession was dragged and warped against a shear plane occurring at the base of the uphill side of the valley, i.e. coinciding with the San Gregorio fault segment scarp (Fig.2a). In the deepest portion of the trench, a fracture that affected the lowermost colluvial units was seen. Moreover, a paleosol occurring within the colluvial sequence, radiocarbon dated at 3772-3649 B.C. (cal., 2 σ ), was warped against the main fault plane and it was Fig 1. a) simplified structural scheme of the central Apennines; study area, black dashed rectangle. b) DTM of the study area; the Paganica fault segments, white lines; the study area, black dashed rectangle. c) Perspective view of the San Gregorio sector; the San Gregorio fault segment, white line; the “Valle degli Asini” gravitational trench, black dotted line; co-seismic ground fracture, black dots; paleoseismological trenches location, white rectangles; stream incisions bounding the Mt. Manicola mass wasting, white dashes arrows; topographic profile (the trace, black line) across Mt. Manicola, inset.  affected by antithetic shear planes (Fig.2a). Warping suggests the occurrence of a continuous and progressive deformation of the paleosol. Indeed, taking into account the scheme proposed by Lettis and Associates, Inc. (1999, page A-131), the observed deformation features suggest that part of the deformation took place as a continuous creep during episodic deposition (testified by the paleosol). Such a progressive and continuous deformation may be ascribed to the gravitational movement. On the other hand, the observed shear planes and the above described fracture affecting the colluvial units likely testify to brittle and abrupt displacements, probably related to fault segment movements. The other excavation was performed across the 1-2-cm large ground fractures (no more visible when the trench was dug) seen after the 2009 earthquake (Fig.1c). The trench exposed colluvial deposits displaced along secondary fault planes, synthetic to the main fault scarp. The westernmost of these structures displaced by about 40 cm (minimum value) the lowermost colluvial unit exposed by the trench (Fig.2b). This offset was probably determined by a single faulting event, as no evidence of interposed angular unconformity or scarp-derived deposit (colluvial wedge) were seen. This fault plane was sealed by a colluvial deposit that, in turn, was displaced by an eastern shear plane by about 10 cm (Fig.2c). A few-cm large fissure coincided with fault plane; it affected the carbonate bedrock found at the base of the trench and it was filled by the overlaying displaced unit (Fig.2c). Radiocarbon dating of a charcoal found within this deposits gave an age of 1785-1509 B.C. (cal., 2 σ ). Both the described fault planes were sealed by two levels of frequentation containing abundant pottery fragments related to the late Bronze Age; a charcoal contained within the frequentation levels was radiocarbon dated at 1885-1743 B.C. (cal., 2 σ ) corroborating the archaeological age determination. No clear evidence related to faint ground fissures induced by the 2009 earthquake were found along the trench walls. Nevertheless, their location was coinciding with the upward prolongation of the above described fault planes, specifically with that responsible for the earliest faulting event. 4.   Discussion and concluding remarks The trench dug across “Valle degli Asini” confirmed the presence of a shear zone along the uphill side of the valley, that coincides with the San Gregorio fault segment. We also perfomed ambient seismic noise measurements across the trough on rock sites, and they revealed a small NE-SW oriented ground motion amplification in the 4-8 Hz frequency band, i.e. roughly perpendicular to the valley and to the fault segment (Di Giulio et al. this issue). This can be reasonably due to the jointing condition of the carbonate rocks induced by the fault movements, that is consistent with the results of studies on seismic amplification on rock sites (e.g. Martino et al. 2006; Marzorati et al. 2011). This cause may be partly also invoked to explain the NE-SW oriented ground amplification that is also recorded within  San Gregorio, along the southern tip of the fault branch. Paleoseismological analysis suggests that the deformation affecting the probably Late Quaternary sedimentary sequence exposed by the “Valle degli Asini” excavation may be partly due to the fault activity and partly related to the large-scale gravitational mass movement affecting Mt. Manicola. The trench dug along the northernmost portion of the fault branch revealed the occurrence of two faulting events prior to the 2009 one. Also, the excavation seems to confirm that the ground fissures seen after the seismic event was determined by the reactivation – although subtle – of the fault segment. Moreover, one of the previous events determined a much larger displacement than that of the 2009 earthquake, consistently with the observed variable coseismic slip seen in the central part of the Paganica fault (Moro et al. 2013, and references therein). Finally, the obtained results highlight the importance of geological and geomorphologic analyses for active faulting investigation and mapping. Moreover, the present study provides insights on the importance of investigating large scale gravitational deformations, as they are able to determine permanent ground deformation that can exceed or sum to the surface displacement due to faulting. For this reason, these phenomena cannot be neglected in seismic microzonation studies. This is particularly true in the mountainous parts of Italy, where the number of the identified large scale mass movements is increasing with time. Fig. 2. a) Northern wall of trench 1, dug across “Valle degli Asini”; shear planes, white lines (a detail, inset); warping of the paleosol, black dashed line. b) Southern wall of trench 2, dug across the northern part of the fault branch; secondary fault plane, black line; stratigraphic contact between units displaced by the fault, white dashed line. c) further shear plane exposed by the excavation, white line; fissure affecting the carbonate rock, white dashed lines; contact between the carbonate bedrock and the overlaying colluvial unit, black dashed lines.
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