A Geophysical Transect Across the Central Sector of the Ferrara Arc: Detailed Gravimetric Survey – Part I F

GNGTS 2015 SESSIONE 1.2 A GEOPHYSICAL TRANSECT ACROSS THE CENTRAL SECTOR OF THE FERRARA ARC: DETAILED GRAVIMETRIC SURvey – pART I F. Palmieri1, A. Mantovani2, G. Santarato2 1 OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale) Trieste, Italy 2 Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Italy Introduction. The investigated area for this study is the eastern sector of the Po Plain that represents the foredeep basin of both Southern Alps and northern Apennines. We focused our attention on the Ferrara Arc (Fig. 1a), which is one of the major arcs, consisting of blind, north- verging thrusts and folds, which represent the external northern Apennines front (Pieri and Groppi, 1981; Bigi et al., 1982; Boccaletti et al., 2004). The recent tectonic activity of this area is well documented by the occurrence of moderate earthquakes, such as the 1570 Ferrara, and 1624 Argenta earthquakes (Guidoboni et al., 2007; Rovida et al., 2011) and recently in May 2012, when two moderate (Mw 6.1 and 5.9; e.g. Pondrelli et al., 2012) earthquakes affected the western Ferrara province. The bending of the topographic surface and the consequent uplift of the broader epicentral area are among the major coseismic effects due to the reactivation of reverse blind faults as, for example, in the case of the northern Apennines underlying the Po Plain: in fact, as a consequence of the fault geometry and kinematics, the rock volume above the co-seismic rupture tip is characterised by a typical fault-propagation folding process (Okada, 1985). Depending on the seismotectonic parameters of the underlying seismogenic source, the uplifted area has an 127 GNGTS 2015 SESSIONE 1.2 Fig. 1 – a) Simplified tectonic map of the buried northern Apennines showing the studied area (black boxes ESE of Ferrara. Modified from CNR-PFG, 1991). b) Location of the measured sites (blue dots) along investigated profile (black line). elliptical shape that is characterized, in correspondence of the epicentral area, by a maximum vertical displacement of some tens of centimeters. The application of satellite interferometry (DinSAR technique) and high-precision levelling (Bignami et al., 2012; Salvi et al., 2012; Caputo et al., 2015) to the Emilia seismic sequence clearly documented the occurrence of this phenomenon; in particular, the main shocks of May 20 and 29 produced two uplifted areas, characterized by a maximum vertical displacement of about 25 cm, partly overlapping and with a cumulative length of about 50 km in an E-W direction. The recurrence of similar “areal morphogenic earthquakes” (Caputo, 2005) and the “competition” with the high subsidence and depositional rates that characterize the Po Plain, have progressively modified the geomorphology and stratigraphy of the region. In these conditions, the hydrographic network has proven to be particularly sensitive to vertical deformations, so even small altimetric and gradient changes led to river avulsions and diversions, highlighted by the presence of several drainage anomalies (Burrato et al., 2003, 2012). Consequently, the alluvial plain is actually crossed by numerous abandoned river channels, some of which are still well preserved (Castiglioni et al., 1999). Obviously, the presence of active tectonic structures responsible for the local uplifts and even for the complex interactions with the hydrographic network has influenced not only the distribution of the sediments on the surface, but also in the subsoil (down to some tens of meters) producing important stratigraphic variations and therefore also changes in the geophysical properties of the materials. Therefore, determining the uplift spatial distribution is crucial for reconstructing the recent tectonic evolution of the region as well as for understanding where active faults are located, and what is their possible seismogenic potential. With this premise, we planned a geophysical survey across some major tectonic structures affecting the subsoil of the eastern Po Plain, ��that possibly represent the seismogenic volumes of the above mentioned historical earthquakes��. Our investigation consisted in a gravity survey carried out along a transect (Fig. 1b), ca. 170 km2 and oriented SSW-NNE, i.e. almost perpendicular to the regional trend of the buried structures belonging to the central sector of the Ferrara Arc. Along a profile centered around the studied transect, in order to investigate the shallow subsurface (say, down to ca. 150-200 m), several passive seismic measurements of ambient noise were also performed, whose results are described and discussed in a companion paper (see Mantovani et al., 2015). 128 GNGTS 2015 SESSIONE 1.2 Method and data acquisition. Gravity prospecting is a geophysical method used to infer the subsurface density distribution measuring changes, of the order of a few parts per million or lower, in the Earth’s gravitational field caused by densities’ lateral variations. A gravity survey was carried out on a rectangular area (6 x 28 km) that runs between Traghetto (near Molinella) and Formignana, in the eastern Ferrara Province (Fig. 1b); ��according to local topography and roads’ pattern, the average spacing between contiguous measurement points was roughly 800 m. ��To carry out the gravity survey a LaCoste&Romberg mod. D, equipped with a ZLS feedback, whose range is about 10 mGal, was used and a whole of 274 stations, as much as homogeneously distributed, were acquired. Since the gravity meters measure gravity differences from place to place, a First Order Gravity Net (FOGN) has been established in the surveyed area; and to define the gravity datum, 2 stations belonging to FOGN have been linked to an eccentric point of the absolute station situated at the Radio Astronomical Station of Medicina (Bo) (Cerruti et al., 1992). The FOGN gravity data have been adjusted by means of the least square method after removing the instrumental drift whose systematic variations, which are common for all measured stations, have been modelled by means of a third order polynomial curve. The rms of the FOGN gravity station values has been ±0.0025 mGal. Starting from FOGN’s stations, 24 gravity loops have been organized according to a sequence data acquisition allowing the instrumental drift checking and loops’s closure error detection. The loops’ gravity data have been adjusted in two steps: firstly, the instrumental drift has been linearly distributed, in function of the time, among the loop’s stations, then the error closure has been distributed in function of the number of ties of the loop. The instrumental drift spans between -0.004 and 0.003 mGal/h and the closure errors between -0.003 and 0.005 mGal. The elevation and position of the measuring points were estimated from the CTR of the Regione Emilia Romagna at scale 1:5000; the estimated altimetric and planimetric errors were ±0.15 m and ±5.00 m, respectively. The observed gravity values, gobs, are then compared with the theoretical values gth, computed on the a homogeneous earth at the ellipsoid surface at the coordinates of the measuring point, corrected also for the height, Faye (CF) and Bouguer (CB) corrections, and terrain effects (CT). Therefore, the so called Bouguer anomaly (ggeol) could be written as: ggeol gobs – (gth – CF CB) CT. It means that the Bouguer anomaly is the difference between the gravity acceleration measured, on the true Earth, and the theoretical gravity acceleration at the same point computed on a homogeneous Earth, whose density could change vertically but not horizontally. Therefore, the pattern of the Bouguer anomalies reflect horizontal variations of densities inside the Earth. In the data processing, the theoretical gravity values (gth) were computed according to the GRS80 ellipsoid formula (Moritz, 1980); the Faye correction (CF), according to the GRS80 ellipsoid and considering also the second order term of the height (Hinze et al., 2005); the Bouguer correction (CB) considering also the Earth’s curvature, i.e. Bouguer slab and Bullard B term, which reduces the infinite Bouguer slab to that of a spherical cap, (LaFehr, 1991a, 1991b). The terrain effects (CT) were computed in two different steps: from 0.001 up to 0.010 km by means of the sloping wedge technique (Olivier et al., 1981; Barrows et al., 1991), due to the mostly flat surveyed area this technique was applied only in three points; from 0.010 to 15,000 km by means of an algorithm based on vertical right parallelepiped (Banerjee et al., 1977), in particular from 0.010 to 1,000 km it was utilized the Digital Elevation Model of the Regione Emilia-Romagna, opportunely resampled with a grid resolution of 0.005 km, instead of the original 0.010 km; and from 1,000 to 15,000 km the SRTM database resampled with a grid resolution of 0.050 km, instead of the original 3”x3”, roughly 0.090x0.065 km. Starting from 10 km it was also considered the Earth’s curvature; it is worth notice that the spherical approach in CT calculation introduces negative contribution due the masses located above the 129 GNGTS 2015 SESSIONE 1.2 Fig. 2 – a) Bouguer anomaly over the transect and b) along the investigated profile. c) Residual anomaly over the transect and d) along the investigated profile. spherical plate, but that are below the top of planar Bouguer plate. The estimated density value for CB and CT corrections was 2.00 g/cm3, bearing in mind that the elevations range from -2.30 to 14.20 m asl and the shallow subsurface is composed of loose, fine sediments. The cumulative error of the gravity anomalies is equal to the square root of the estimated errors of the individual components involved in the calculations above briefly described; the estimated error is of the order of ±0.035 mGal. As well known, the observed Bouguer anomaly is the summation of the gravity effects related to deep anomalous bodies with shallower ones: the different effects can be recognized by the different curvatures, weaker for deeper bodies, steeper for shallow ones.

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