H/V spectral ratios and array techniques applied to ambient noise recorded in Colfiorito basin, Central Italy.

The Colfiorito Basin is a small intramountain depression in the southern
section of the Northern Apennine chain that is filled with Quaternary alluvial deposits.
The presence of soft alluvial deposits has significantly influenced the level of
local damage that was caused by two major earthquakes (ML 5.6 and 5.8) belonging
to the swarm that started in September 1997. To verify the effects of the basin
structure on the predominant frequency of seismic motion, ambient noise measurements
were carried out in the Colfiorito Basin during two experiments in May and
July of 2002. The horizontal-to-vertical spectral ratios (HVSRs) were calculated for
data collected at four profiles in the basin. Array techniques were applied to determine
the wave types that composed the noise, to estimate their apparent velocity and
azimuth of propagation, and to calculate a velocity-dispersion curve from which a
velocity-depth structure was derived. The data analysis shows a high amplification
in the HVSR at low frequency. This feature is common to most of the sites, including
the reference site, and it is interpreted as being due to weather disturbances. The
peak frequencies of the spectral ratio calculated at the sites located in the center of
the basin coincide with the theoretically estimated resonance frequencies. The arrayaveraged
HVSR calculated for the array located in the middle of the plain has a
pronounced peak at 0.9 Hz. This corresponds to the peak of the amplification function
calculated on the basis of the velocity model deduced from the dispersion analysis.
The HVSR method is instead unsuitable for the prediction of the resonance frequencies
of sediments in the sites where strong lateral variations of basement topography
are present. We measured apparent velocities in the range of 0.3–0.8 km/sec by
applying f-k methods to array recordings. These values are compatible with the predominance
of surface waves in the noise, as also confirmed by polarization analysis.
Both Rayleigh and Love waves are present in the background seismic noise. The
results obtained by applying the spatial autocorrelation method to the vertical component
of the ground motion recorded at a 240-m-wide circular array deployed in
the middle of the basin revealed the presence of Rayleigh waves, and f-k methods
combined with polarization techniques revealed the presence of polarized Love
waves. The wave-field analysis indicates two main propagation directions: the first
is around N100 E in the frequency band of 1.0–2.0 Hz; this radiation can be interpreted
as being generated at the east-southeast step borders of the basin. The second
main direction is around N300 E in the frequency band of 2.0–3.0 Hz; its source
may be a 180-m-deep depression located at the southwest corner of the basin.

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