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An Integrated Approach to the Seismotectonics of the Inner Cottian Alps (Western Alps) G

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An Integrated Approach to the Seismotectonics of the Inner Cottian Alps (Western Alps) G GNGTS 2009 SESSIONE 1.1 prior to the beginning of the earthquake activity with a hydraulic diffusion equal to D = 0.1 m2/s. Moreover assuming that the earthquake migration reflects paths along which fluid flow and that the propagation speed of a fluid pressure pulse is similar to the velocity of earthquake migration, first order hydraulic conductivity and permeability are estimated. In the study area we find a hydraulic conductivity equal to 1.15 10-5 m/s and a permeability equal to 7 10-13 m?, which are in good agree- ment with other observations in the region. Fluid circulation at depth and the relatively high perme- ability indicate a fractured medium and attest the complexity and heterogeneity of the western Corinth Rift, a fact corroborated by the complex structure of the seismically active zone analyzed, the heterogeneous orientations of the fault planes, and the observed high b-values. AN INTEGRATED APPROACH TO THE SEISMOTECTONICS OF THE INNER COTTIAN ALPS (WESTERN ALPS) G. Perrone 1, E. Eva 2, G. Balestro 1,3, P. Cadoppi 1,3, S. Solarino 2, S. Tallone 3 1 Dipartimento di Scienze della Terra, Università di Torino 2 Istituto Nazionale di Geofisica e Vulcanologia, CNT, c/o Dip.Te.Ris., Genova 3 CNR-Istituto di Geoscienze e Georisorse, Unità di Torino The inner Cottian Alps represent an area of a low- to moderate- magnitude seismicity (ML<5; Eva et al., 1990; Capponi et al., 1981) even though characterized in the past by some historical earthquakes (Fig.1) causing both material and social damages (VIII-IX degree of the Mercalli scale; Camassi & Stucchi, 1997). Although the frame of seismicity is quite well known, the relation be- tween faults and earthquake sources is still under debate due to the fair knowledge of the fault net- work affecting this area. Moreover, the low deformation rates and the occurrence of sever- al glacial-interglacial cycles during the Plio- Pleistocene partly masked the geomor- phological evidences of the recent tectonic ac- tivity. Recent studies (Balestro et al. 2009, Fig. 1 – Map of the historical and instrumental seismicity (location error minor than 10 km) with the main faults of the Northern Cottian Alps. CF: Chisone Fault; CL: Canavese Line; LTZ: Lis-Trana Deformation Zone; PGFS: Pinasca- Gran Dubbione Fault system; PTF: Padan Thrust Front; SFS: Sangone Fault system. Instrumental seismicity from the database of the Seismic network of the Western Italy (RSNI). Historical seismicity from the Catalogue of the strong earthquakes in Italy (NT4.1). 114 GNGTS 2009 SESSIONE 1.1 Perrone et al. 2009) however showed that the inner Cott- ian Alps are dissected by some regional-scale faults; a more comprehensive map- ping of the area has been car- ried out improving the knowledge of size and exten- sion of these tectonic align- ments. In this paper we com- bine the results of these nov- el studies and updated seis- mological data with the aim to investigate the relations between mapped faults and seismic activity. In the ana- lyzed area both continental crust and oceanic tectonic units, belonging to the Pen- ninic Domain of the Western Alps, crop out. The main brittle tectonic feature of this area is represented by the Lis-Trana Deformation Zone (LTZ; Balestro et al. 2009), a N-S striking, steep structure that extends for about 35 km from the Lower Lanzo Val- ley to the Lower Sangone Valley (Fig.1). The occur- rence of steep faults displac- ing the metamorphic base- Fig. 2 - Map of the instrumental seismicity (location error of seismic events ment, showed in seismic sec- minor than 3 km) with the focal mechanisms of the Northern Cottian Alps. tions carried out for oil ex- Instrumental seismicity from the RSNI. Abbreviations as in Fig. 1. Focal ploration (Mosca 2006), sug- mechanisms calculated in this work are represented with black compressional gest that the LTZ may be quadrants. prolonged Southward be- neath the Plio-Quaternary deposits of the Po Plain. West of the LTZ some other minor E-W (San- gone Fault system; SFS and CF) and N-S faults (Pinasca-Gran Dubbione Fault system; PGFS) are also present (Perrone et al., 2009). Zircon and apatite fission tracks indicate that the activity of these faults started since the Oligocene. Two main faulting stages characterize the post-metamorphic structural evolution of this area: the earlier (faulting stage A; Oligocene?-Early Miocene?) is asso- ciated to right-lateral movements along the LTZ and sinistral movements along E-W faults; the sub- sequent faulting stage (faulting stage B; post-Early Miocene) is related to transtensive/extensional movements along the LTZ and the development of minor sub-parallel N-S faults. This kinematic evolution fits in a model of dextral-transtension at regional scale, as already proposed by Sue and Tricart (2003).The more recent activity of the LTZ may have caused the development of Pleistocene lacustrine basin, several hundred metres thick, in the Lower Chisone and Pellice valleys, which did not hosted glacial tongues. To the Plio-Quaternary activity of the N-S faults may be also attributed the origin of two inselbergs (Madonna di Mombruno and Rocca di Cavour; Biancotti 1979) of the 115 GNGTS 2009 SESSIONE 1.1 Dora-Maira Unit in the westernmost Po plain. With the aim to better understand the relations between the current seismic activity and faults, an analysis was carried out by selecting the best located earth- quakes (location error less than 3 kilometres) recorded by the seismic network of the North West- ern Italy (RSNI). The selection is made necessary by the relatively small size of the structures under in- vestigations in order to avoid fake attributions. In addition to get qualitative information about the seismogenic source, the focal mechanisms of four earthquakes occurring along the mapped faults were calculated sorting out the best locatable events among those occurred in the area. The integration and comparison between geolog- ical and seismological data show that: - epicentres resemble the track of the N-S faults (LTZ and PGFS) within the error locations (Fig. 2); - in the cross-sections, perpendicular to the N-S faults, seismic events are aligned along steep sur- faces (Fig. 3); - focal mechanisms show strike-slip and transtensio- nal/extensional solutions western of the LTZ. Transpressive solutions predominate East of the Fig. 3 – Seismic cross-sections transversal to the LTZ (Po Plain); N-S structures. Traces of sections in Fig. 2. - most of the nodal planes of the focal mechanisms near the LTZ are steeply dipping and N-S to NNE- SSW striking; - P axes of focal mechanisms are subhorizontal (trending NE-SW to ENE-WSW) or steeply dip- ping; T axes are either subhorizontal (mostly trending WNW-ESE to NNW-SSE) or steeply dip- ping. The good geometric and kinematic agreement between structural and seismological data indi- cate a possible dependence of the seismicity of the inner Cottian Alps with the current tectonic activity of the LTZ and its associated minor structures. In conclusion, this study shows that, even in a region characterized by low-deformation rates and where evidence of recent tectonic deformations are poor, the integration between detailed field mapping and seismological analysis may contribute to understand the location and evolution of active deformations. References Balestro G., Cadoppi P., Perrone G., Tallone S. ; 2009 : Tectonic evolution along the Col del Lis-Trana deformation Zone (Internal Western Alps). Italian J Geosciences, 128, 331-339. Biancotti A.; 1979 Rapporti fra morfologia e tettonica nella pianura cuneese. Geogr. Fis. Dinam. Quat., 2, 51-56. Camassi R., Stucchi M.; 1997: NT4.1, a parametric catalogue of damaging earthquakes in the Italian area (Release NT4.1.1). GNDT, Gruppo Nazionale per la Difesa dai Terremoti, Milano. Internet, http://emidius.itim.mi.cnr.it/NT/home.html. Capponi G., Eva C., Merlanti, F.; 1981: Il terremoto del 5-1-1980 nel Pinerolese. Boll. Soc. Geol. It., 4, 497–501. Eva C., Augliera P., Cattaneo M., Pastore S., Tomaselli A.; 1990: Revisione di dati sismometrici in Italia Nord-Occidentale nel periodo 1971-1989. Atti del Convegno Gruppo Nazionale Difesa dai terremoti, Ed. Ambiente, Pisa, 1, 25-34. Mosca P.; 2006: Neogene basin evolution in the western Po Plain (NW Italy). PhD Thesis, CNR –University of Amsterdam, 190 pp. Perrone G., Morelli M., Cadoppi P., Tallone S., Giardino M.; 2009: A multidisciplinary approach for the study of the fault network in the Northern sector of the Dora-Maira Unit (Western Alps). Italian J Geosciences, 128, 541-549. Sue C., Tricart P.; 2003: Neogene to current normal faulting in the inner western Alps: A major evolution of the late alpine tectonics. Tectonics, 22 (5):1050, doi:10.1029/2002TC001426, 2003. 116.
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