sessione 2.3 - Ogs

GNGTS 2009 SESSIONE 2.3
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.
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