Natural Hazards 33: 365–378, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 365 Spectral Amplification Effects at Sellano, Central Italy, for the 1997–98 Umbria Seismic Sequence MADDALENA NATALE ⋆ and CONCETTINA NUNZIATA Dipartimento di Geofisica e Vulcanologia, Università “Federico II” Largo S. Marcellino 10, 80138 Napoli, Italy (Received: 2 December 2002; accepted: 15 December 2003) Abstract. The seismic events recorded at two accelerographs installed at Sellano (central Italy) during the 1997–1998 Umbria seismic sequence, one on detritic material, at the historical centre, and the other one on rock, about 200 m distant, were analysed in terms of spectral amplification of the historical centre site. Epicentres were mainly concentrated in the north and south-east directions of Sellano area. The SH wave component average amplifications were evaluated from the smoothed Fourier spectral ratios of the recordings on soil and rock sites, along the two main epicentral lines. Similar amplifications resulted, with two main peaks in the frequency range of 3–5 Hz, corresponding to the eigenfrequencies of the damaged buildings. Shear velocities of the shallowest 30 m of soils were obtained by FTAN measurements along refraction seismic spreadings, and utilized to compute spectral amplification of soil station to rock station along the geological cross sections. A good agreement was found between observed Fourier spectral ratios and the computed 2D amplification modelling, which explains the damage level of the historical buildings beside the degraded conditions of brick masonry. Key words: Spectral amplification, Umbria seismic sequence, Sellano 1. Introduction The study of the historical and recent earthquakes shows that Sellano has been severely damaged by two earthquakes (Camassi and Stucchi, 1997), that are the February 14, 1838 earthquake (I = VIII on Mercalli–Cancani–Sieberg, MCS, scale, estimated M s = 5.5) and September 19, 1979 (I = VII on MCS scale, M s = 5.5). The 1997 Umbria (central Italy) seismic sequence started on September 4 (M L = 4.7) with the epicenter at the Colfiorito area. Two main events happened on September 26, the first at 0:33 (M L = 5.6) and the second at 9:40 (M L = 5.9), and they were very close (Figure 1). Cattaneo et al. (2000) inferred focal mechanisms of normal faulting on NW-SE, with SW dipping planes, in agreement with the structural setting of the central Apennines. The seismic activity was character- ized by other smaller events with epicenters migrating from the Colfiorito area to the South, toward Sellano. It culminated with the October 14, 1997 earthquake (M L = 5.6), located at about 2.5 km North of Sellano, which produced extensive ⋆ Author for correspondence. E-mail: Conunzia@unina.it