Site Effects Study in a Shallow Glaciofluvial Basin Using H/V Spectral Ratios From Ambient Noise and Earthquake Data: The Case of Bovec Basin (NW Slovenia)

Abstract

The Bovec basin, which is located in the alpine valley of the Soča river (NW Slovenia), was recently struck by two strong earthquakes (1998 and 2004) which caused extensive damage of maximum intensity VII-VIII EMS-98. Macroseismic data for both events showed large variations in damage to buildings within short distances and indicated strong effects of sediments on ground motion. A site effects study was therefore performed using H/V spectral ratios from earthquake data and from ambient noise, as well as standard spectral ratio technique using the reference station located on the edge of the basin. Following the July 12, 2004 (Mw = 5.2) earthquake, six strong motion seismic stations were deployed in a profile across the Bovec basin to record the aftershock sequence. Accelerograms of eight stronger aftershocks (ML = 2.5–3.6) and additional ambient noise measurements were used in the study. Spectral ratio analyses showed that ground motion amplification occurs mainly in a frequency range of 5–10 Hz, with corresponding amplitudes in the range of 6–11. The observed range of amplification cannot be related to the total thickness of Quaternary sediments, which is up to 100 m in the Bovec basin. The variability in the main peak frequencies and in their amplitudes is therefore explained by the complex geological structure of the basin, filled with heterogeneous glacial and fluvial sediments. Irregular layers of conglomerate within sand/gravel deposits and layers of tillite result in large impedance contrasts at several interfaces within Quaternary sediments. Spectral ratios from earthquake data are therefore quite complex and show a broad range of ground motion amplification. On the other hand, ambient noise data revealed only the first stronger impedance contrast which is related in the border areas to the flysch bedrock and in the central part of the basin to a shallow layer of conglomerate. Comparison of the two H/V analyses showed that the amplitude obtained from ambient noise data is always lower than the amplitude from earthquake data. The difference can be as much as a factor of two. These quantitative results apply to weak ground motion and because of nonlinearity cannot be directly extrapolated to damaging earthquake situation. Since one and two-story houses prevail in the Bovec basin, with the main building frequency in the range of 6–11 Hz, the danger of soil-structure resonance is considerable in the area. It was presumably one of the main reasons for the relatively high level of damage observed from both earthquakes.