L'uso delle misure di microtremore per investigare le strutture tettoniche sepolte: il caso di studio dell'anticlinale di Mirandola

The Mirandola Anticline represents a buried fault-propagation fold which has been growing during Quaternary due to the seismogenic activity of a blind segment belonging to the broader Ferrara Arc. The last reactivation occurred during the May 2012 Emilia sequence. In correspondence with this structure the thickness of the marine and continental deposits of the Po Plain foredeep is particularly reduced. In order to better define the shallow geometry of this tectonic structure, and hence its recent activity, a depth range which is intermediate between the surficial morphological observations and seismic profiles information was investigated. In particular, numerous passive seismic measurements (single station microtremor) for obtaining the horizontal to vertical spectral ratio (HVSR) were carried out. For each site the amplitude of the peak value of the HVSR curve, A, and the corresponding frequency, f0 (commonly referred to as natural frequency), have been considered. The distribution of both parameters has been further elaborated by creating a colour-shaded map. The results of the geophysical campaign and the gridding clearly document the presence of areas characterized by resonance phenomena, locally very important ones, and allow to map their distribution. In particular a first map evidences the occurrence of a narrow zone (2.5-3.5 km-wide), trending ESE-WNW and characterized by A values of the HVSR curves greater than 2.5. Local maxima occur, from west to east, along the central sector. A similar pattern could be also observed in a second map, where the natural frequency f0 has been interpolated with the same procedure described above. In this case, the selected discriminant value is ca. 1 Hz and the gridding emphasizes an elongated ESE-WNW trending area characterized by natural frequencies up to 2.0 Hz. Assuming as a first approximation laterally uniform (or smoothly variable) seismic waves velocities within the uppermost sedimentary units, say the first 100150 m, the mapped distribution of the natural frequencies is certainly due to a strongly variable depth of the surface producing the resonance (i.e. characterized by an impedance contrast). The areas emphasized in the two maps basically coincide and are both characterized by marked gradients north and south and a progressive fading ESE-wards. Position and dimensions of the overlapping area as well as the corresponding values of the two mapped parameters are due to laterally changing impedance contrast associated with the variable stratigraphic succession developed during Pliocene-Quaternary on top of the Mirandola anticline. In order to further constrain and validate the subsoil model here proposed, HVSR measurements were also carried out in correspondence of two boreholes cored by Regione Emilia-Romagna down to a depth of 101 and 127 m, respectively. Accordingly, at these two sites the detailed stratigraphic succession has been reconstructed showing the occurrence of the Pliocene top, the so called seismic pseudo-bedrock interface of the area (i.e. vs ≥ 600 m/s) at ca. 96 and 113 m, respectively. Moreover, at both sites a second borehole was drilled to perform a crosshole investigation for measuring the velocity distribution at depth. Based on a simplified inversion approach I succeeded to reproduce measured HVSR curves and particularly the major and meaningful peaks down to the bedrock interface separating the continental Middle Quaternary from the marine Early Quaternary-Late Pliocene deposits. The results of a combined analysis of the peak frequency and its amplitude nicely fit also the available geological information derived from boreholes and seismic reflection profiles carried out for hydrocarbon purposes, suggesting that this low-cost geophysical technique could be successfully applied in other sectors of wide morphologically flat alluvial plains to investigate blind and completely buried potential seismogenic structures. One of the major results of the present reasearch consists in the reconstruction of the natural amplification distribution associated with, and caused by, the presence of impedance contrast in the subsoil. As above mentioned, this information has been obtained either in terms of frequency and amplitude of the HVSR. This study also shows how the systematic application of the HVSR method, a low-cost non-invasive geophysical technique, generating a relatively dense grid of measurements over a wide area and strictly following as a standard the well tested SESAME criteria, could allow to laterally correlate specific amplification peaks and hence to interpolate with confidence the same impedance contrast surface. According to the instrumentation used and the stratigraphic setting of the investigated area, such a surface that represents the real target of the whole procedure could be up to 150-200 m-deep. The capacity to model it in some detail could enable to map large-scale deformations, which are too subtle to be reconstructed on the basis of morphological analyses (due to the lack of cumulative effects) and generally too shallow to be taken into account during seismic reflection surveys devoted to hydrocarbon exploration. Finally, the application of the above procedure to the Mirandola case study allowed to emphasize the persisting and recent growth of a major fault-propagation anticline, where both the causative thrust and the associated fold are completely buried by the Middle-Upper Pleistocene to Holocene continental deposits (e.g. Martelli and Molinari 2008; Bonini et al. 2014). At this regard, the obtained results clearly and independently document the presence of a folded surface in the shallow Mirandola subsoil; the crest is oriented ESE-WNW with a culmination towards the west and a periclinal setting eastwards in perfect agreement with the deeper tectonic structure reconstructed on the basis of seismic reflection profiles. The results of this methodological approach are quite encouraging and could be easily applied to other morphologically flat regions affected by blind faulting and folding. Seismic amplification is influenced by the stiffness of the soil, and especially by the impedance contrast among shallow seismic units. Accordingly, maps of natural frequency are of utmost importance because they allow to recognize areas characterized by a high impedance contrast where a greater amplification in ground motion is expected to occur in case of seismic shaking. Indeed, if the amplified frequency at a site is close to that of a standing or planned building, a resonance effect may occur and therefore the risk for the building to suffer structural damage greatly increases (e.g. Mucciarelli et al. 2001; Castellaro et al. 2014). At this regard, amplification maps are crucial for urban planners in defining the height of buidings (viz. the number of floors) characterized by a resonance coincident with the natural one and enabling engineers to improve the antiseismic behaviour of new constructions or to retrofit old ones. Seismic amplification is in fact considered the first cause of damage and collapse during an earthquake (e.g. Mucciarelli et al. 2001; Gallipoli et al. 2004).

L'anticlinale di Mirandola rappresenta una piega per propagazione di faglia sepolta che si è accresciuta nel Quaternario a seguito della attività sismogenica di un segmento cieco dell'Arco Ferrarese. L'ultima riattivazione si è verificata durante la sequenza sismica dell'Emilia del maggio 2012. In corrispondenza di tale struttura lo spessore dei depositi marini e continentali della avanfossa padana è particolarmente ridotto. Al fine di meglio definire la geometria superficiale di questa struttura tettonica, e quindi la sua recente attività, è stato studiato un intervallo di profondità, che è intermedio tra le osservazioni morfologiche superficiali e le informazioni ricavate dai profili sismici. In particolare, sono state effettuate numerose misure di sismica passiva (microtremori a stazione singola) per ottenere il rapporto spettrale tra le componenti orizzontali e quella verticale (HVSR). I risultati di un'analisi combinata della frequenza di picco e della sua ampiezza ben si accordano alle informazioni geologiche disponibili suggerendo che questa tecnica geofisica a basso costo può essere applicata con successo in altri settori di ampie piane alluvionali morfologicamente piatte per investigare potenziali strutture sismogeniche cieche e completamente sepolte.