Seismological Research Letters Volume 82, Number 3 May/June 2011 413 doi: 10.1785/gssrl.82.3.413 Evidence of Topographic Effects through the Analysis of Ambient Noise Measurements F. Panzera, G. Lombardo, and R. Rigano University of Catania, Catania, Italy INTRODUCTION Ampliication of the ground motion in a topographic irregular- ity is generally linked to the focalization of seismic waves at its efects at the topmost part of a hill are frequency-dependent so that resonance phenomena occur when the wavelength of the incident wave is comparable to the horizontal dimension of the hill. In addition, the inluence of the topography on ground shows increasing resonant motion with respect to the whole of the structure. his ground-motion ampliication mechanism at ridge crests is in principle similar to the well-known efects in the seismic design of buildings, which appears to apply at a larger scale to mountains as well (Buech 2010). Moreover, the ridge, are oten observed (Spudich 1996). Several analytical and numerical methods have been devel- oped to study incoming seismic waves when crossing a hill- shaped morphology ( , LeBrun instrumental records are relatively few, and only in recent times have data from small arrays started to be commonly used ( , Buech 2010). In a review - greement between observed and predicted ampliication on a topographic feature oten occurs. Several studies (Tucker and Barker 1996) have shown that this diference derives from the di iculty of inding a reference station not signiicantly inluenced by the topography itself. To resolve such di iculty in evaluating topographic site efects, the studies of Spudich use of a nonreference technique such as the earthquake hor- and Boatwright (1991). Generally, the use of earthquake data has been shown to be a successful tool for the evaluation of noise records, processed with the horizontal-to-vertical noise - ful alternative seismic input, as several studies support ( , response in the area of downtown Syracuse (Siracusa), Sicily, and to test the reliability of ambient noise recordings, pro- efects. Ambient noise has in recent years been widely used for site ampliication studies. Its use signiicantly reduces ield data acquisition time and costs. he evaluation of site response only one mobile seismic station with no additional measure- ments at rock sites for comparison. It also does not require the long and simultaneous deployment of several instruments to collect a useful set of earthquake data. he basic hypothesis for using ambient noise is that the resonance of a sot layer cor- responds to the fundamental mode of Rayleigh waves, which is associated with an inversion of the direction of Rayleigh waves hus, the ratio between the horizontal and vertical spectral components of motion can reveal the fundamental resonance frequency of the site. he reliability of such an approach has been asserted by many authors ( local seismic response estimates. It is commonly accepted that, although the single components of ambient noise can show large spectral variations as a function of natural and cultural disturbances, the / spectral ratio tends to remain invariant, adopted for investigating topographic efects, producing rather - deposits, the basic composition of the wave ield is the same for earthquake and noise ground motion. here is, indeed, no rea- son to have Rayleigh waves due to the topography in the wave ield of the noise and it has to be remembered that the topo- graphic efect is much higher in the horizontal component of motion rather than in the vertical one (LeBrun 1999). GEOLOGY OF THE STUDY AREA he study area (Ortigia peninsula) is located along the south- east coast of Sicily (southern Italy), in downtown Syracuse. It