sessione 2.3 - Ogs

GNGTS 2009 SESSIONE 2.3
Fig. 1 - Area of interest with the surface
projections of the main fault planes.
have been calculated by means of numerical finite-source ground-motion simulations. A parametric
study has been performed to identify the most critical rupture mechanisms for the ground response
at Vicoforte.
Numerical modelling has been carried out following two different approaches:
• the deterministic method by Hisada and Bielak [2003] implemented by the GRFLT12S code for
the low frequency range simulations (from 0 to 1÷2 Hz);
• the stochastic method implemented in EXSIM code [Motazedian and Atkinson, 2005] for the high
frequency range simulation (higher than 0.5÷1 Hz).
The deterministic Hisada-Bielak method simulates the complete 3D wave propagation field
induced by an extended kinematic source based on the static and dynamic Green functions. The high
frequency EXSIM approach simulates the finite fault as a plane divided into a series of sub-faults,
each one modelled as a stochastic point source, using a Brune (w 2 ) source spectrum. This program
allows to simulate only the S-wave field. Prior to the numerical simulation of future ground shaking
scenarios due to the activation of the identified faults, synthetic seismograms have been generated
with reference to a well-documented historical earthquake. The Asti event of August 21, 2000
of M w 4.86 was used as a reference. Although this earthquake is a low magnitude event, it has been
selected since it is located on the plane of the Monferrato fault. Furthermore, numerous recordings
on rock stations are available for this event. The GENL rock station of the Regional Seismic Network
of North-western Italy (http://www.dipteris.unige.it/geofisica/) has been chosen for the calibration,
being the nearest one to the fault and with an epicentral distance of about 65 km, close to
the distance between this fault and Vicoforte site. Fig. 2 shows the satisfactory agreement achieved
between recorded data and GRFLT12S simulations.
To identify the worst potential ground shaking scenarios at Vicoforte site, three events have been
selected, representing the most critical situation with reference to the expected average ranges of
magnitude for the faults listed in Table 1:
• Monferrato fault: event of M w =5.5, corresponding to a return period of about 690 years;
• Western Alps fault: event of M w =5.7, corresponding to a return period of about 475 years;
• Western Liguria fault: event of M w =6.5, corresponding to a return period of about 2475 years.
The return periods have been evaluated in the framework of a probabilistic seismic hazard
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