SOIL-STRUCTURE RESONANCE STUDY IN THE BOVEC BASIN USING THE MICROTREMOR METHOD

Abstract

Local geological structure has a major influence on seismic ground motion. The amplitude of seismic waves increases when they pass from hard bedrock to soft sediments at the surface. Amplification is highest at the fundamental frequency of sediments, which depends on their thickness and on shear-wave velocity. Even without knowing these two parameters, the fundamental frequency can be efficiently derived by analysis of small ground vibrations known as microtremors. By measurements taken inside buildings, the main building frequencies in two principal directions and, consequently, the range of increased vulnerability, are determined. Comparison of frequencies allows identification of the danger of soil-structure resonance, which is a frequent reason for damage enhancement. In the Bovec basin, which has been struck by earthquakes in 1998 and 2004, the resonance effects were studied by 124 free-field measurements in an area of 7 km2 and by measurements inside 20 buildings. The prevalent frequency range of increased vulnerability for buildings is 7-11 Hz. This range overlaps with fundamental frequency range of 6-12 Hz, which prevails in the map of free-field measurements. This indicates that the danger of soil[1]structure resonance is considerable in the area and has enhanced the damage to buildings. The observed great variability of the fundamental frequency is caused by very heterogeneous glacial and fluvial deposits in the basin.