Large amplification of ground motion at rock sites within a fault zone in Nocera Umbra (central Italy)

Abstract

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 expected on the basis of the usual regressions with magnitude and distance. A broad-band amplification up to a factor of 10 was consistently estimated in previous papers, using both weak and strong motion data recorded at the accelerograph site during local moderate earthquakes. To study the cause of this amplification we deployed six seismologic stations across the tectonic contact between the Ceno-Mesozoic limestone and the Mesozoic marly sandstone where the accelerograph is installed. Seismograms of 21 shallow aftershocks in the magnitude range from 2.2 to 4.0 and a subcrustal Mw = 5.3 event are analysed. Regardless of epicentre location, waveforms show a large complexity in an approximately 200 m wide band adjacent to the tectonic contact. This is interpreted as the effect of trapped waves in the highly fractured, lower velocity materials within the fault zone.