Site effect in the city of Gemona

The high-quality data recorded by the digital instruments, with a large frequency response and a high dynamic range, installed in the Friuli Accelerometric Network permit to obtain important pieces of information at low frequencies (< 1 Hz) also from records of low-magnitude earthquakes. In fact, the waveforms recorded by the network from 1993 to 1998 have been used for different seismological and engineering applications (Fäh et al., 1994; Costa et al., 1996; Costa et al., 1998).

The area of Gemona is characterised by terrigenous sediments (Flysch), which are covered locally by a thin Quaternary layer, forming a sedimentary basin. The centre of the city of Gemona (Friuli) is located at the margin of this sedimentary basin on an alluvial fan. The response spectra of accelerometric stations GEPF (reference station on bedrock), GESC (in the lower part of the alluvial fan) and GETM (in the sedimentary basin) clearly show the important site effects on the alluvial fan and on the sedimentary basin. In particular, we show, in the figure below, the response spectra, computed with a damping of 5%, of the records obtained during the 1998 Bovec earthquake (epicentral distance about 50 km) in the city of Gemona.

DEM of Gemona del Friuli area showing the response spectra, computed with a damping of 5%, of the records obtained during the 1998 Bovec earthquake and the recent Carnia event (14/02/02).