GNGTS 2015 - Atti del 34° Convegno Nazionale

SSRs at Mount Ocre show spectral behavior similar to the one discussed previously for the basic geometries, with multiple alternation of amplification and deamplification patterns. In proximity of the crest where CADO site is located, the 2-D simulation shows ground motion amplification at high frequency ( f > 3 Hz) and the amplitude is smaller than a factor of 2 around 1 Hz. Although the modelling is very simple (2-D and uniform rock), the numerical predictions suggest that the peculiar high-amplitude wave train observed at 1 Hz cannot be reproduced by lateral variations of the topography. This is true also considering the effect of nearby topographic peaks assuming a longer profile in the x direction, or reducing the velocity values by a factor of 2. SSRs computed on synthetics still indicate that no realistic velocities give amplifications comparable to the observations when uniform rock models are adopted. Avallone et al. (2014) commented the amplification mechanism of CADO as related to wave- guide amplification phenomena of trapped waves within a low-velocity fault zone (LVFZ). The horizontal extension of the fault zone in proximity of CADO was estimated by these authors around 650 m, and the velocity reduction within the LVFZ was around 40% with respect to the surrounding quarter-spaces. Conclusion. Topographic effects have been investigated from spectral-element P-SV simulations considering three basic geometries (Fig. 1). The geometrical irregularities, in case of vertically incident plane waves, generate lateral propagating surface pulses and distinct patterns of amplification and deamplification, with an amplitude level mostly below 2 for SSR or below 4.5 for HVSR (Fig. 2). The pattern of amplification/deamplification is different for the horizontal and vertical components, depending on the shapes of the modeled irregularities and on polarization of the seismic input. The output synthetics allow also to provide insights on the topographic aggravation factor ( TAF ), which is computed as the ratio of peak ground displacements (PGD) using an ideal reference site. The TAF pattern clearly illustrates the alternate position of amplified and deamplified values along the profile (Fig. 3). The numerical simulations show that the effect on the PGD distribution spatially extends before and after the position of the basic geometries, for a length of the same order of the half-width of the topographical irregularities (Fig. 3). Further in case of the geometrical slope, the largest amplitudes of late pulse are found in the part of the model characterized by higher elevation. Assuming the real topography of Mount Ocre, the polarized 1 Hz amplification cannot reproduced by simple models with homogeneous velocities and taking into account only the topography. As discussed in Avallone et al. (2014), the observed amplification is likely related to a fault-zone effect, indicating that more advanced simulations are necessary in understating topographic effects on rock sites. Acknowledgments. Antonio Rovelli and Antonio Avallone are gratefully acknowledged. Their constructive comments and suggestions helped to improve this work. References Avallone A., Rovelli A., Di Giulio G., Improta L., Ben-Zion Y., Milana G. and Cara F.; 2014: Waveguide effects in very high rate GPS record of the 6 April 2009, Mw 6.1 L’Aquila, central Italy earthquake . J. Geophys. Res.: Solid Earth, 119(1), 490-501. Burjánek J., Moore J.R., Yugsi Molina F.X. and Fäh D.; 2012a: Instrumental evidence of normal mode rock slope vibration . Geophys. J. Int., 188 (2), 559-569. Burjánek J., Cauzzi C., Fäh D., Bard P.Y., Cornou C., Pitilakis K., Massa M., Theodulidis N. and Bertrand E.; 2012b: Toward reliable characterization of sites with pronounced topography and related effects on ground motion . 15 WCEE (World Conference Earthquake Engineering), Lisboa, Portugal. Caserta A.; 1998: A time domain finite-difference technique for oblique incidence of antiplane waves in heterogeneous dissipative media . Annals of Geophysics, 41 (4), 617-631. Caserta A., Ruggiero V. and Lanucara P.; 2002: Numerical modelling of dynamical interaction between seismic radiation and near-surface geological structures: a parallel approach . Computers & geosciences, 28 (9), 1069- 1077. CEN (Comité Européen de Normalisation) Eurocode 8; 2008: Design of structures for earthquake resistance. Part 1: General rules, seismic actions and rules for buildings. 92 GNGTS 2015 S essione 2.2