Ground Shaking Scenarios at the Town of Vicoforte, Italy
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Soil Dynamics and Earthquake Engineering 31 (2011) 757–772 Contents lists available at ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn Ground shaking scenarios at the town of Vicoforte, Italy L. Scandella a,n, C.G. Lai a, D. Spallarossa b,1, M. Corigliano a a European Centre for Earthquake Engineering (EUCENTRE), via Ferrata 1, Pavia, Italy b Dipartimento per lo Studio del Territorio e delle sue Risorse (DIPTERIS), University of Genoa, Viale Benedetto XV 5, 16132 Genoa, Italy article info abstract Article history: Vicoforte is a small town in Northern Italy, which hosts a Cathedral with the world’s largest elliptical Received 11 February 2010 dome. The name of the Basilica is ‘‘Regina Montis Regalis’’ and it is of extraordinary architectural and Received in revised form structural importance. The main objective of this study is the definition of the seismic hazard at the site of 25 November 2010 Vicoforte following a deterministic approach. Although Vicoforte is located in an area of moderate Accepted 1 December 2010 seismicity, the calculation of the most unfavourable seismic ground shaking scenarios is of great interest due to the importance of the Basilica and its vulnerability to even a moderate seismic excitation. The closest active faults to Vicoforte were identified in order to simulate the potentially most severe ground shaking scenarios compatibly with the tectonic and seismic setting of the region. Subsequently, numerical simulations were conducted through finite faults numerical models using two different approaches: the extended kinematic source model of Hisada and Bielak [24] and the stochastic method of Motazedian and Atkinson [38]. They, respectively, simulate the low and high frequency ranges of predicted ground motion. The numerical models used for the simulations were calibrated by a comparison between synthetic results and recorded data. A parametric study was finally carried out to identify the most critical fault rupture mechanisms. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Crack patterns due to the foundation settlements and to the structural configurations of the Cathedral are currently present on This paper illustrates the steps that were undertaken to estimate the the dome-drum system, but they seem stabilized. From the historical worst rock shaking scenarios at the site of Vicoforte, a municipality of records at the Sanctuary, it is known that cracking phenomena began to the city of Cuneo in Northern Italy, where the ‘‘Regina Montis Regalis’’ occur in the early stages of the construction, in particular in the zone Basilica sits. Although Vicoforte is located in an area of moderate between the drum windows and at the base of the buttresses. In 1985 seismicity, it was selected as case study due to the presence of the such severe cracking prompted the decision to undertake monitoring Cathedral with the world’s largest elliptical dome (Fig. 1). and strengthening works. A system of 56 active tie-bars, slightly The internal axes of the dome of the Sanctuary are 37.15 m  stressed by jacks, was installed in 1987 within the masonry at the top of 24.8 m, making it the fifth largest dome in the world (after the the drum along 14 tangents around the perimeter, and a complex Pantheon and Saint Peter in Rome, S. Maria del Fiore in Florence, monitoring system was set up to measure movements of the structure Italy and Gol Gumbaz Mausoleum in India) and by far the largest and propagation of cracks, as well as stresses in the reinforcing tie-bars. elliptical structure. In recent years a new project was started for a thorough renovation of The Basilica was first conceived by Duke Charles Emanuel I of Savoy the monitoring system [15] and the updating of the structural models as the mausoleum of the family. The original architectural composition adopted to explore the static configuration and integrity of the Basilica was an idea of engineer Ercole Negro di Centallo, Cont of Sanfrount, but and to define the characteristics of the strengthening system [13]. architect Ascanio Vitozzi implemented the project. The construction The present study was developed in the framework of a more wide- started in 1596 and since the early beginning the building was affected ranging research aimed to define the seismic input for dynamic by differential settlements due to inhomogeneity of foundation soil. analyses of the Basilica. The study was carried out in two phases: Due to the large settlements (of the order of 25–30 cm) in 1615 the the first part concerned with site-specific Probabilistic Seismic Hazard project was abandoned for more than a century, till 1692 when the Analysis (PSHA, [28]), while the second part focused on a Deterministic construction works were entrusted to the architect Francesco Gallo. Seismic Hazard Analysis (DSHA) at the site, both under the assumption The dome was completed in 1731. of stiff ground and level topographic condition. The choice of whether using PSHA or DSHA for an adequate analysis constitutes a vivid debate in the scientific community. Fiercely n discussions whether PSHA or DSHA or a possible combination of Corresponding author. Tel.: +39 0382 516925; fax: +39 0382 529131. E-mail address: [email protected] (L. Scandella). the two should be used show how debated this topic is [9,26,27]. 1 Tel.: +39 010 3538 038; fax: +39 010 352 169. Nevertheless, both methods present advantages and limitations, 0267-7261/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.soildyn.2010.12.004 758 L. Scandella et al. / Soil Dynamics and Earthquake Engineering 31 (2011) 757–772 Fig. 1. ‘‘Regina Montis Regalis’’ Basilica of Vicoforte, Northern Italy and detail of the world’s largest elliptical dome. as summarized by McGuire [34]. The PSHA study is performed to determine the seismic hazard at the site as resulting from the historical seismicity. On the other hand, a deterministic approach is more suitable when the hazard comes from a single fault which is dominating the overall hazard. As a consequence, DSHA illustrated in the present paper was carried out to evaluate ground shaking scenarios, compatibly with the regional seismotectonic setting. As a first phase of the DSHA, the closest active faults to Vicoforte were identified in order to simulate the potentially most severe ground shaking scenarios. Numerical analyses were performed using two different approaches which simulate the low and high frequency ranges, respectively. The low frequency analyses allowed identifying the most critical fault rupture mechanisms, while the high frequency simulations were performed with the aim to obtain a valid result of comparison with the PSHA in terms of Peak Ground Acceleration (PGA), which is associated to the high frequencies. Calibration of synthetic seismograms was performed through a comparison with recorded data of recent seismic events. Finally, a parametric study was carried out to identify the potential most critical rupture scenarios, thus the most unfavourable ground Fig. 2. Map of the area of interest with the surface projections of the planes of the shaking scenarios, which represented the input for the evaluation main faults identified in the region: Monferrato, Western Alps and Western Liguria faults. of local site effects. This aspect was investigated in [28] through 1D stochastic and 2D deterministic approaches. On the basis of historical and recent events which struck Piedmont and the surrounding regions, the seismic activity in North-Western Italy was identified in three main regions: Western Liguria, Western 2. Tectonic and seismological setting of Vicoforte area Alps and Monferrato (Fig. 2). Three potential seismogenic sources were associated to these areas, characterized by the seismological para- The definition of ground-shaking scenarios in North-Western meters listed in Table 1. Given the uncertainty of the magnitude and the Italy clashes with the strong uncertainty on geometry and kine- insufficient knowledge of the seismic potential of each source due to matics of the sources of earthquakes occurred in the past. Due to lack of geologic data, a range of possible maximum magnitudes was the large return periods of earthquakes on individual seismogenic specified instead of a single value. faults in Italy (often greater than 1000 years), it is not usually possible to identify such sources on the basis of recent seismolo- 2.1. Western Liguria gical data, thus data from pre-instrumental earthquakes based on macroseismic databases (e.g., DBMI04 database, [47]) and seismo- The Ligurian Sea originated from the counter-clockwise rotation tectonic interpretations are generally used. of the Corsican-Sardinian block relative to the European plate, due From a detail study of the seismotechtonic setting in the region, to the convergence of the European and African plates. The the main active faults of interest for the site could be identified. In continental slope is characterized by shallow normal faults parallel France the only region characterized by the presence of active faults to the coast which intersect a more recent fault system NW–SE is in Provence [31], at a distance of more than 100 km from oriented [14]. The geological and tectonic features would seem to Vicoforte and characterized by a very low seismic potential. confirm the extensional origin of the basin, but recent seismolo- The seismotectonic setting of North-western Italy, associated to gical observations depict a present-day compression. Major recent the complex geodynamic process of the Western Alps evolution earthquakes in the Ligurian Sea, indeed, show compressive focal (e.g., [8,43]) was studied in detail to identify the main seismogenic mechanisms (Fig. 3) with NW–SE principal horizontal stress sources, which may potentially affect Vicoforte. vectors [20,29].Be´thoux et al. [6] suggest that the Ligurian Sea is L. Scandella et al. / Soil Dynamics and Earthquake Engineering 31 (2011) 757–772 759 Table 1 Main parameters of the identified seismic sources.