Site effects and soil-structure resonance study in the Kobarid basin (NW Slovenia) using microtremors

The town of Kobarid is located in one of three areas with the highest seismic hazard in Slovenia. It was hit by several 1976-1977 Friuli sequence earthquakes and recently by the 1998 and 2004 Krn Mountains earthquakes which caused damage of intensity up to VII EMS-98 scale. The town is located in a small basin filled with heterogeneous glaciofluvial Quaternary sediments in which site effects due to soft sediments are expected. The existing microzonation which is based on surface geological data only is inadequate, and no borehole or geophysical data are available in the basin that would allow a modelling approach of site effects assessment. The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied in order to assess the fundamental frequency of the sediments. Investigations were performed on a 100x100 m dense grid and 106 free-field measurements acquired. Clear HVSR peaks were obtained in the majority of the surveyed area. The eastern part of the basin is characterized by two well separated peaks which indicate distinct shallow and deep impedance contrasts. The iso-frequency map of sediments shows a distribution in a broad range of 1.8-22.2 Hz. The observed frequencies can be related to the total thickness of Quaternary sediments (sand, gravel) in the western part of the basin only. They are deposited over bedrock built of Cretaceous flysch. In the eastern part the obtained fundamental frequencies are influenced by the presence of a shallow conglomerate layer inside sandy gravel or lacustrine chalk. The extent of these layers was not known before. Microtremor measurements were also performed inside 19 characteristic buildings of various heights (from two to four stories), and longitudinal and transverse fundamental frequencies determined from amplitude spectra. A potential of soil-structure resonance was assessed by comparing building frequencies with the free-field sediments frequencies derived from the iso-frequency map. For two surveyed buildings a high danger of soil-structure resonance was assessed and for three buildings the danger was of medium level. The building resonant frequency of two- and three-story houses, which prevail in the area, spans the range 4-11 Hz, with an average value of 7.7 Hz. The danger of soil-structure resonance should be therefore sought in this frequency range. Since the majority of Kobarid area is characterized by lower (W part) or higher (E part) frequencies, the danger exist mainly in a relatively narrow transition zone.