Characteristics of strong ground motion data recorded in the Gubbio sedimentary basin (Central Italy)

Various authors, analysing the set of accelerograms recorded at Gubbio
Piana (GBP) (central Italy), have demonstrated that strong amplification occurs at
this accelerometric station, which is installed within an alluvial basin. In particular,
Ambraseys et al. [(2005a), Bull Earthq Eng 3:1–53; (2005b), Bull Earth Eng 3:55–73]
observed that the strong motion peaks at GBP greatly exceed the median values
predicted by the attenuation relationships they derived for Europe. In this work, we
analyse and discuss some characteristics of the ground motion recorded at the GBP
station. We show that the ground motion parameters, such as peak-ground acceleration
and peak-ground velocity, are strongly influenced by the presence of locally
induced surface waves that produce both a lengthening of the significant shaking
duration and an increase in the peak values with respect to a nearby bedrock site. The
basin-induced surface waves are observed in the three components of motion and
their effects on the peak values are particularly evident in the vertical component.
In the frequency domain, the energy of the surface waves is mostly restricted to the
frequency band 0.4–0.8Hz for both the horizontal and vertical components. The horizontal
and vertical Fourier amplitudes are also very similar, and this indicates that
the H/V spectral ratio technique is not applicable to describing the site response due
to the propagation of seismic wave in a complex 2D/3D geological structure. Finally,
a preliminary polarization analysis shows that the directions of polarization, as well as the degree of elliptical polarization, exhibit a strong variability with time, that may
be related to a complex propagation of Love and Rayleigh waves within the basin.