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

Abstract

[1] 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 M 0 = 1.2 Â 10 25 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 M 0 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.

Evidence for ground motion polarization on fault zones of Mount Etna volcano

“…[33] The former (results 1 and 2) of the above listed issues are not new: the occurrence of directional ground motion amplifications in fault zones is well documented in the literature [e.g., Peng et al, 2002;Cultrera et al, 2003]. In contrast, very few papers dealt with microtremors in fault zones, after the pioneering work by Irikura and Kawanaka [1980], who first inferred that vertical discontinuities can affect ambient noise characteristics.…”

“…In the present study, we apply the same algorithm used by Cultrera et al [2003]. The horizontal plane is divided into a set of directions spaced by 10°bins from 0°(north) to 180°(south), and the pair of horizontal components is rotated by those angles.…”

“…[18] Rotated H/H ref spectral ratios are a powerful tool to recognize directional amplifications [Spudich et al, 1996;Cultrera et al, 2003]. However, this approach provides a relative estimate and may thus be biased by the denominator behavior.…”

Geotechnical Site Characterisation in the Umbria-Marche Area and Evaluation of Earthquake Site-Response

“…Although the S-wave TF of stations CLF and NCR2 are correctly estimated, it is worthy of mention that they are affected by complex wave-propagation phenomena such as edge-diffracted waves (CULTRERA et al, 2003;DI GIULIO et al, 2003;ROVELLI et al, 2001). In the case of NCR2 the amplification is dependent on the earthquake azimuth and the two horizontal components display a remarkable difference in the level of amplification due to 2-D effects on a fault gauge zone.…”

“…A number of microzonation studies were encouraged by the local government to promote safer building reconstruction and retrofit (PERGALANI et al, 1999;MARCELLINI et al, 2001). The most significant site-effect cases have been investigated by several authors (SANO`et al, 1998;GAFFET et al, 2000;ROVELLI et al, 2001;FRANCESCHINA et al, 2001;CULTRERA et al, 2003;CASTRO et al, 2004).…”