Long-duration asynchronous ground motions in the Colfiorito plain, central Italy, observed on a two-dimensional dense array

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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