sessione 2.3 - Ogs

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GNGTS 2009 SESSIONE <strong>2.3</strong> Fig. 2 - Comparison between recorded (black) and synthetic (grey) seismograms computed with the low frequency GRFLT12S code. Velocity time histories and Fourier spectra at GENL station are plotted: NS (a), EW (b) and UD (c) components. analysis for the region [for details see Lai et al., 2009]. Parametric analyses have been performed, studying the influence of the hypocentre location and the rake angle, to identify the combination of parameters which would lead to the most severe ground shaking. For brevity only the results related to the Western Liguria fault are reported herein. From the parametric analyses it turns out that a Fig. 3 - Numerical results in terms of velocity time histories and Fourier spectra for a Mw 6.5 Western Liguria event, assuming the nucleation point represented by the star and a rake angle equals to 135 degree: NS (a), EW (b) and UD (c) components computed by GRFLT12S (black), d) horizontal average component computed EXSIM (grey), whose Fourier spectra is also overlapped on the two low frequency horizontal components in (a) and (b). 488
GNGTS 2009 SESSIONE <strong>2.3</strong> Fig. 4 - Comparison between the results obtained by the low and high frequency simulations in terms of PGA and PGV assuming different hypocentre locations for each fault: a) Western Liguria; b) Western Alps; c) Monferrato. kinematic rupture with a rake angle of 135 degree and the hypocentre represented by a star in Fig. 3 yields the worst shaking scenario illustrated in Fig. 3. The Fourier spectra obtained by the two procedures are complementary, with a range of superimposition between 0.2 and 0.6 Hz. After this range, GRFLT12S results start to decay significantly. The S-waves contributions computed by the two methods are in agreement, both in terms of wave arrival and peak amplitude. In terms of time histories, while in the GRFLT12S traces the arrivals of P and S waves can be clearly distinguished, EXSIM reproduces the S waves only, as expected. A total of 144 low frequency and 50 high frequency simulations have been computed to define the deterministic worst ground shaking scenarios. In Fig. 4 the numerical results are summarised in terms of horizontal Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV) relative to the worst scenarios computed for each fault, assuming different hypocentre locations. As expected, high frequency simulations predict larger values of ground shaking if compared with the low frequency results. The difference of the ground motion parameters estimated by the two methods increases by increasing the magnitude of the event. GRFLT12S correctly simulates the directivity effects, ignored by EXSIM. The Western Liguria event generates the largest PGA and PGV values of 0.06 g and 0.065 m/s respectively. Similar values of maximum PGV of ~0.02 m/s and PGA of ~0.02 g have been calculated for both the Western Alps and Monferrato events. Acknowledgements. The present contribution has been developed in the framework the research contract undersigned with the Administration of Vicoforte Sanctuary for the monitoring and survey of the “Monte Regalis Basilica”, with the support of the Fondazione Cassa di Risparmio di Cuneo. The authors would like to extend their gratitude to Prof. Mario Alberto Chiorino, Coordinator of the Program (Politecnico di Torino) and to the Administration of the Sanctuary. References. Hisada Y. & Bielak J. [2003], A theoretical method for computing near fault ground motion in a layered half-spaces considering static offset due a surface faulting, with a physical interpretation of fling step and rupture directivity, Bull. Seism. Soc. Am,. 93 (3), 1154-1168. Lai C.G, Corigliano M., Sánchez H., Scandella L. [2009], Definition of seismic input at the “Regina Montis Regalis” Basilica of Vicoforte, Northern Italy, in press by IUSS press Motazedian D. and Atkinson, G. [2005], Stochastic Finite-Fault Modeling Based on a Dynamic Corner Frequency, Bulletin of the Seismological Society of America, 995-1010. 489
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