F. Marra et al., Large amplification of ground motion at rock sites within a fault zone in Nocera Umbra (central Italy), J SEISMOL, 4(4), 2000, pp. 543-554
During the two mainshocks of September 26, 1997 in the Umbria-Marche border
a strong-motion accelerograph recorded peak ground accelerations as large
as 0.6 g, approximately, in the town of Nocera Umbra, at distances of 10 to
15 km from the epicentres. This value is significantly larger than expecte
d on the basis of the usual regressions with magnitude and distance. A broa
d-band amplification up to a factor of 10 was consistently estimated in pre
vious papers, using both weak and strong motion data recorded at the accele
rograph site during local moderate earthquakes. To study the cause of this
amplification we deployed six seismologic stations across the tectonic cont
act between the Ceno-Mesozoic limestone and the Mesozoic marly sandstone wh
ere the accelerograph is installed. Seismograms of 21 shallow aftershocks i
n the magnitude range from 2.2 to 4.0 and a subcrustal M-w = 5.3 event are
analysed. Regardless of epicentre location, waveforms show a large complexi
ty in an approximately 200 m wide band adjacent to the tectonic contact. Th
is is interpreted as the effect of trapped waves in the highly fractured, l
ower velocity materials within the fault zone.