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Authors: Di Giulio, G.* 
Rovelli, A.* 
Cara, F.* 
Azzara, R. M.* 
Marra, F.* 
Basili, R.* 
Title: Long-duration asynchronous ground motions in the Colfiorito plain, central Italy, observed on a two-dimensional dense array
Issue Date: 2003
Series/Report no.: B10/108(2003)
DOI: 10.1029/2002JB002367
Keywords: site effects
3-D basin resonance
seismic array
Colfiorito Basin
Subject Classification04. Solid Earth::04.06. Seismology::04.06.04. Ground motion 
Abstract: In winter 1998 a small-aperture (~200 m), four-station array was operating in the middle of the Colfiorito plain. Waveforms of seven small magnitude (2.1 < M < 3.7) local earthquakes are analyzed in this study. The peculiarity of the array seismograms is a 2-min long duration of the horizontal ground motion. The predominant frequency in the plain is around 1 Hz. In this frequency band, earthquake-induced ground shaking is synchronous for a few seconds during direct S waves; after this time window, wave trains show an increasingly chaotic behavior within the array. The energy variation of the horizontal ground shaking between the array and a nearby rock outcrop exceeds a factor of 500 at 1 Hz. In order to help understanding of observations and construct models for future numerical simulations, the buried structure of the Colfiorito plain has been investigated through seismic refraction profiles and geoelectric measurements. A three-dimensional reconstruction of the basin structure reveals an extremely complex bedrock topography, which is probably a recurrent feature for intermontane basins in tectonically active regions. Apparent velocities and backazimuths in the frequency band 0.5–1.5 Hz indicate that a 180-m deep depression NW of the array is responsible for the generation of edge-diffracted surface (Love) waves. Their arrival to the array breaks the synchronism of vertically reverberating waves which predominate in the first seconds of the S-wave window
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