1851 Bulletin of the Seismological Society of America, 91, 6, pp. 1851–1866, December 2001 Edge-Diffracted 1-Sec Surface Waves Observed in a Small-Size Intramountain Basin (Colﬁorito, Central Italy) by Antonio Rovelli, Laura Scognamiglio, Fabrizio Marra, and Arrigo Caserta Abstract During the Umbria-Marche, central Italy seismic sequence a small- aperture (200 m), four-station array was operating in the Colﬁorito plain, a few kilometers away from the epicenters of the M L 5.6 and 5.8 mainshocks of 26 Sep- tember 1997. The array was deployed approximately 500 m from the eastern edge of the basin. We analyze the three-component seismograms of 12 aftershocks, in a magnitude range of 2.5 to 4.1. Amplitudes of the horizontal components are system- atically higher than those of the vertical component, with an average horizontal-to- vertical spectral ratio of about 3 at 1 Hz. In this frequency band, earthquake-induced ground shaking is highly coherent across the array. A 1-sec running-window zero- lag cross-correlation algorithm is used to compute apparent velocity and backazimuth of coherent wave trains in the frequency band 0.5 to 2 Hz. Apparent velocity and backazimuth show a different behavior in the ﬁrst part of the seismograms compared to the late coda. The largest amplitude waves, that is, S waves and early coda, are characterized by low apparent velocities, mostly between 400 and 1200 m/sec. This suggests that, near the rock edge, the most signiﬁcant part of seismic energy prop- agates horizontally in the basin. Backazimuth of these low-frequency, coherent wave- trains never coincides with the array-to-source direction. The predominant backazi- muth is peaked around N110°, corresponding to the nearest, steep outcrop of the basin edge. The observed 1-sec coherent wave trains are interpreted as locally gen- erated surface waves that are persistently diffracted from the nearby basin edge as long as a signiﬁcant level of seismic radiation is incident to the bedrock. When the bedrock excitation decreases a much larger variability of both apparent velocity and backazimuth is observed, suggesting that, in the coda, randomly scattered waves within the basin and late arrivals of deeper origin become more important. Multi- pathing from the source to the site as well as multipathing within the basin are therefore interpreted as the main causes of the observed long-duration, coherent low- frequency basin shaking. Introduction On 26 September 1997 two moderate (M L 5.6 and 5.8 at 00:33 and 9:40 GMT, respectively) earthquakes occurred a few kilometers from the Colﬁorito village (Fig. 1), close to the border between Umbria and Marche districts, central Italy (see Amato et al., 1998). The distribution of damage was very irregular, suggesting a large variability of building vulnerability in the area. However, one common feature within many affected villages was the high destruction level at soft sediment sites close to basin edges (see Tertulliani, 1999). In these situations houses built on unconsolidated sediments often had serious damage while a few tens of me- ters away the level of damage was signiﬁcantly lower for similar buildings at rock sites. Concentration of damage at soft sites close to basin edges has been already documented (see Weischet, 1963; Yuan et al., 1992; Stewart et al., 1995; Tertulliani and Riguzzi, 1995; among many others). In many cases numerical modeling provided a satisfactory explana- tion for mechanisms of spatial variations due to basin-edge effects (Kawase and Aki, 1989; Moczo and Bard, 1993; Rovelli et al., 1994, 1995; Kawase, 1996; Graves et al., 1998; Pitarka et al., 1998). In the Colﬁorito plain a SMA-1 strong-motion acceler- ograph of the Italian accelerometric network was triggered by 50 earthquakes during the seismic sequence through mid- 1998. The accelerograph is located approximately 600 m away from the limestone outcrop of the eastern basin edge (Fig. 1). Recordings of ground motion show long-duration, low-frequency (1 Hz) wave trains even for the smallest