Azimuth-dependent amplification of weak and strong ground motions within a fault zone (Nocera Umbra, central Italy)

During three moderate-magnitude earthquakes occurred in September–October 1997
in the central Apennines, Italy, accelerations larger than 0.5 g were recorded in the town of
Nocera Umbra, 10 to 15 km N-NW of the epicenters. The accelerograph is sited in a
fault zone, close to a N30 E tectonic contact. Six temporary seismological stations
installed across the fault recorded 82 aftershocks occurred in two seismogenic zones: the
Colfiorito-Sellano area, S-SE of the array, and the Gualdo Tadino area, to the north. The
array data reveal large variations in terms of both peak ground motions and spectral
amplitudes. Within the fault zone, amplifications show a strong dependence on the source
azimuth. At the accelerograph site, the effects are particularly large for events from S-SE:
peak ground motions are a factor of 14 larger than those of a reference site and
conventional spectral ratios attain amplitudes as large as 50 at 7 Hz along the N30 E
direction of motion, parallel to the strike of the fault. Nineteen strong motion
accelerograms were then used to compare ground motion properties between weak and
strong events up to M0 = 1.2 1025 dyn cm. A particle motion analysis shows that the
directional effect is also present in the strongest motions, even though the amplification of
peak ground motion decreases when M0 increases. Results from stochastic simulations
indicate that such a behavior is not due to nonlinearity: applying the empirical weak
motion transfer functions in a purely linear model the observed peak ground motions of
the largest events are fit satisfactorily.