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

摘要

Various authors, analysing the set of accelerograms recorded at GubbioPiana (GBP) (central Italy), have demonstrated that strong amplification occurs atthis 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 valuespredicted by the attenuation relationships they derived for Europe. In this work, weanalyse and discuss some characteristics of the ground motion recorded at the GBPstation. We show that the ground motion parameters, such as peak-ground accelerationand peak-ground velocity, are strongly influenced by the presence of locallyinduced surface waves that produce both a lengthening of the significant shakingduration and an increase in the peak values with respect to a nearby bedrock site. Thebasin-induced surface waves are observed in the three components of motion andtheir 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 thefrequency band 0.4–0.8Hz for both the horizontal and vertical components. The horizontaland vertical Fourier amplitudes are also very similar, and this indicates thatthe H/V spectral ratio technique is not applicable to describing the site response dueto 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 maybe related to a complex propagation of Love and Rayleigh waves within the basin.