GNGTS Sessione Amatrice

Transcription

1 On the significance of the Mt. Vettore surface ruptures A.M. Blumetti, V. Comerci, P. Di Manna, F. Fumanti, G. Leoni, L. Guerrieri, E. Vittori ISPRA Istituto Superiore per la Protezione e la Ricerca Ambientale, Roma, Italy The August 24, 2016, central Italy earthquake (Mw 6.0; ) was associated to well evident seismically-induced ground ruptures along the Cordone del Vettore fault trace and along the Mt. Vettoretto fault, which is its supposed southern continuation, throughout a right step (Fig. 1). The Cordone del Vettore fault runs in the upper part of the southwest-facing Mt. Vettore fault escarpment (Blumetti, 2011; Pierantoni et al., 2013), marked by a clear tectonic contact between Mesozoic limestones and slope deposits. In addition, minor and much less continuous reactivations were observed along other fault strands of the Vettore fault system located downslope, e.g. mid-slope of the Mt. Vettore and south of the Vettoretto rupture. Instead, the supposed main fault located at the foot of the slope bounding the Castelluccio Basin did not display any evidence of coseimic reactivation. The NW-SE trending surface ruptures along the Cordone del Vettore fault were surveyed in large detail, with significant continuity for a total length of about 3 km. Most commonly, the ruptures have affected colluvial and debris deposits overhanging the bedrock fault plane, but somewhere even the very fault mirror. The observed displacements were ranging from 5 to 15 cm in its northern part and from 5 to 25 cm southward, reaching 30 cm only at one site at its southern tip; heave generally ranged from 5 to 25 cm (see also EMERGEO Working Group, 2016). The set of ground ruptures observed along the Mt. Vettoretto fault were mainly NNW-SSE trending, with a slight left-lateral component. They generally affected colluvium and soil, often 17

2 GNGTS 2016 Fig. 1 Left: Oblique view of the Castelluccio Basin bounded, to the east, by the Mt. Vettore fault escarpment. Capable faults (after ITHACA database - are shown in red, with the surface fracturing along the Cordone del Vettore and Mt. Vettoretto faults overprinted in yellow. Right (a): detail of the ground ruptures pattern observed along the Cordone del Vettore and Mt. Vettoretto faults. very close to the bedrock fault plane, but sometimes at a distance of several meters. In the first days, the observed offsets were from 2 to 25 cm, while heave reached 10 cm. These ruptures were followed northward from the road SP34 to the end of the Mt. Vettoretto western slope, where the ruptures were bending downslope to a ca. WNW direction (see Fig. 1). This portion of the rupture is almost continuous for about 1.7 km. Southward from SP34, NNW-SSE trending discontinuous ruptures were followed for about 0.8 km. The latter are quite well aligned with the major rupture, but generally not related to a bedrock fault plane and sometimes associated to evident gravity-driven slope movements. Along the Mt. Vettoretto fault, repeated field surveys have clearly pointed out a postseismic evolution of the slip, whose vertical separation is continuing to increase with time. The phenomenon is particularly evident and monitored where the fractures have affected the road SP34. Here, on August 24, i.e., a few hours after the earthquake, it was observed the widening of a pre-existing asphalt crack with about no vertical displacement and a very thin new oblique crack. On August 31, a downwarping to the west was observed together with the widening of the oblique crack (4 to 5 cm downthrow) and the appearing of new cracks in the ground on both sides of the road. One of the issues debated by the scientific community is whether the Cordone del Vettore and Mt. Vettoretto ruptures are directly linked or not to the seismogenic source(s). Based on the observed phenomena and the literature data, we put forward here some considerations about the origin of the surface ruptures, discussing separately the two cases. According to the currently available seismological, geodetic and geologic data, the seismogenic structure of the Mw 6 (01:36 UTC) mainshock ruptured across a relay zone between two major NNW trending normal faults: the Vettore and Laga faults (total rupture length was km and rupture width ca. 9 km - Gruppo di Lavoro INGV sul terremoto di Amatrice, 2016). A second mainshock occurred at 02:33 UTC, with Mw 5.3. (Fig. 2 left) The Mt. Vettore rupture is likely connected to the first main shock (Fig. 2 left), i.e., the only one with a magnitude that can be reasonably associated to the observed surface faulting. The rupture of the second shock, despite much closer to the Vettore Fault, and very likely occurred on it, had a magnitude (5.3) and a depth (8 km) hardly able to reach the surface. 18

3 Fig. 2 Left: Surface ruptures, possibly linked to surface faulting, and InSAR-derived vertical coseismic surface deformation (Sentinel-1; Marinkovic and Larsen, 2016). The two mainshocks location and focal mechanisms (source are also shown; capable faults (after ITHACA database - are mapped for comparison. Right: Coseismic ground deformation map obtained from InSAR analysis of COSMO-Sky- Med images (ASI) acquired along a descending orbit on August 20, 2016 (before the event) and on August 28, 2016 (after the event). The arrow indicates the deformation along the slope of Vettore Mt, suggested to be due to slope instability ( The processing of high resolution satellite radar images (Alos 2, Sentinel and COSMO-Sky- Med) have allowed the drawing of the InSAR coseismic deformation maps shown in Fig. 2. Two major rupture patches have been modeled to have occurred along the same fault or along two distinct faults (ref. INGV CNR-IREA- ( k2&view=item&id=755:terremoto-di-amatrice). A secondary deformation along the western slope of the Mt. Vettore (indicated by an arrow) has been interpreted as a minor gravity-driven mass movement. This is the extreme end of a broad span of interpretations on the significance of the Cordone del Vettore surface rupture, that goes from primary surface faulting, i.e., directly related to the seismogenic fault, to pure sliding of the slope deposits. Here, we support the idea that the movement is basically due to deep-seated slip on a fault that is currently a splay of the main seismogenic fault, very likely locally emphasized by a nontrivial gravitational component. In fact, the analysis of the fractures surveyed in the field, very continuous and consistent as regard offsets and opening extent, together with the observation on the overall western slope of Mt Vettore, bring to exclude a pure slide of slope debris cover, because it would have been associated to some bulging at its base to be recognizable on the InSAR images. Instead, a collapse of the whole slope along a deep fault plane with a close relation with the seismogenic fault (see Fig. 3 below) is in good agreement with the analysis of the coseismic ground deformation from InSAR analysis of COSMO-Sky-Med images (Fig. 2 right), showing the deformation of the Mt. Vettore western slope in substantial continuity with the main coseismic ground deformation pattern shown by the InSAR image (cfr, COMET report- About the Mt. Vettoretto ruptures, the gravity-driven component is larger and more evident. According to the official Geological Map of Italy (1:100,000, Scarsella, 1941), the Mt. Vettoretto fault has an arc shaped, bending from the NNW-SSE trend in its southern part, to an about E-W direction, parallel to the Valle Santa (Fig. 1a). Actually, the 2016 surface fracturing followed the NNW-SSE trending section and the bending of the fault until the upper part of the Valle 19

4 Santa, but then stopped and no more fractures were observed further down along the valley. The Monte Vettoretto structure is presumably affected by a deep seated gravity deformation, that use the Sibillini thrust fault as sliding plane (Fig. 3 above). In fact, the Mt. Vettore normal fault system, in the area of Forca di Presta, crosses and displaces the Sibillini thrust fault for some hundred meters. According to some authors, normal faulting may have locally reutilized some steeper shallow planes of the thrust zone (cfr. tectonic inversion ; Cooper and Williams, 1989; Calamita et al., 1994; Pizzi and Galadini, 2009; Di Domenica et al., 2012 and bibliography therein). In this condition, the discontinuity represented by the limestone block thrusted onto the Laga flysch is a perfect surface for a gravitational decollement. Even the observed postseismic evolution of the Mt Vettoretto rupture, monitored along the SP34, but even more evident, for example, were the fractures reached the highest elevation across the trail from Forca di Presta to Cima del Redentore (arrow in Fig. 1a), might be evidence of some gravity-driven component of surface fracturing added to fault slip propagation from the deep-seated coseismic slip. So, what we want to stress here is that even in this case, where a sackung-type movement could be advocated, there must be a non-trivial tectonic component in the surface displacement. To explain our thought we use a figure from Serva et al. (2002), explaining the concept of seismic landscape (slightly modified in Dramis and Blumetti, 2005). There are represented the two end member geometry of faults splaying from the seismogenic fault with different dip and emerge at surface not far from the top of fault generated mountain fronts. These two end cases are a Deep Seated Gravitational Slope Deformations (in Fig. 3 indicated by 3 in the A seismic landscape model) and a secondary surface rupture with a very close relation with the seismogenic fault (in Fig. 3 indicated by 2 in the B seismic landscape model). In between, other fault types are possible between the two end cases. Obviously, it is very challenging to distinguish the tectonic and gravitative components of this deformation. In fact, normal fault motion and gravity collapse have the same slip vector, since gravity is the leading force of extensional tectonics (e.g., Doglioni et al., 2015a, 2015b). In this case, post-seismic monitoring of the deformation is essential also to calibrate 20 Fig. 3 To summarize in a sketch the geometry of the Cordone del Vettore and M. Vettoretto faults we use a scheme after Serva et al. (2002), explaining the concept of seismic landscape applied to two intermountain basins in Central Italy, different in size, showing also the typical occurrence of coseismic ground effects (after Serva et al., 2002 and Dramis and Blumetti, 2005). See text for details.

5 the results of future paleosesimological analyses. In fact, the displacements constrained by paleoseismological trenches will comprehend also the post-seismic slip along a structure that is between a normal fault and a deep seated gravitational deformation: therefore, the application of empirical relationships between surface faulting parameters and earthquake magnitude (e.g. Wells and Coppersmith, 1994) should consider this additional epistemic uncertainty. In conclusions, what has been observed during the August 24, 2016, central Italy earthquake has important consequences in conducting paleoseismological analysis. Based on empirical relationship (e.g., Wells and Coppersmith, 1994), 25 cm of offset in a trench wall would allow to estimate a magnitude 6.1 to 6.3 (depending if considering the maximum or average throw). Being 25 cm close to the maximum throw of fractures measured along the Cordone del Vettore fault, the Mw 6 earthquake still fit rather well. However, a future paleoseismological investigation on the Cordone del Vettore fault would lead to erroneously locate the epicenter in the Castelluccio basin, instead of the Accumoli- Amatrice area. References Blumetti, A.M. (1991). Evoluzione geomorfologica, attività tettonica quaternaria e paleosismicità in alcune depressioni tettoniche dell Appennino Centrale. Unpublished doctoral thesis, University of Camerino. Calamita, F., Coltorti, M., Farabollini, P., Pizzi, A. (1994). Le faglie normali quaternarie nella dorsale appenninica umbro-marchigiana: proposta di un modello di tettonica di inversione. Studi Geologici Camerti, Cooper, M.A., Williams, G.D. (1989). Inversion structures recognition and characteristics. In: Cooper, M.A., Williams, G.D. (Eds.), Inversion Tectonics. Geological Society of London, Special Publication, 44, pp Demangeot J. (1965) - Géomorphologie des Abruzzes Adriatiques. Mém. et Doc. du C.N.R.S., Paris, pp Di Domenica, A., Turtù, A., Satolli, S., Calamita, F. (2012). Relationships between thrusts and normal faults in curved belts: New insight in the inversion tectonics of the Central-Northern Apennines (Italy). Journal of Structural Geology, 42, Doglioni, C., Carminati, E., Petricca, P., Riguzzi, F. (2015a). Graviquakes. Abstracts Volume 6th International INQUA Meeting on Paleoseismology, Active Tectonics and Archaeoseismology, April 2015, Pescina, Fucino Basin, Italy. Miscellanea INGV Anno 2015_Numero 27 ISSN , Doglioni, C., Carminati, E., Petricca, P., Riguzzi, F. (2015b). Normal fault earthquakes or Graviquakes. com/scientificreports 5:12110 DOI: /srep12110 Dramis F., Blumetti A.M. (2005). Some Considerations Concerning Seismic Geomorphology And paleoseismology. In: MICHETTI A.M., AUDERMAN F. and MARCO S., Eds. Special Issue Paleoseismology: Integrated Study of the Quaternary Geological Record for Earthquake deformations and faulting. Tectonophysics, Vol. 408, EMERGEO Working Group (2016). Terremoto di Amatrice del 24 agosto 2016: Effetti Cosismici doi: / zenodo Gruppo di Lavoro INGV sul terremoto di Amatrice (2016). Primo rapporto di sintesi sul Terremoto di Amatrice Ml 6.0 del 24 Agosto 2016 (Italia Centrale), doi: /zenodo Pierantoni, P., Deiana, G., Galdenzi, S. (2013). Stratigraphic and structural features of the Sibillini Mountains (Umbria- Marche Apennines, Italy). Italian Journal of Geosciences, 132(3), Pizzi, A., Galadini, F. (2009). Pre-existing cross-structures and active fault segmentation in the northern-central Apennines (Italy). Tectonophysics, 476(1), Serva L. Blumetti A.M., Guerrieri L., Michetti A. M. (2002). The Apennine intermountain basins: the result of repeated strong earthquakes over a geological time interval. Proceedings Convegno Geological and geodynamic 21