Study of the effects of local geological structure on seismic ground motion and building vulnerability with microtremors

Examples of several earthquakes in last decades have shown that the influenceoflocalgeological structure (site effects) on seismic ground motion was often underestimated; the effects of the earthquake were therefore greater than expected. Especially dangerous is the situation when both the fundamental frequency of sediments and of structures are similar and a resonance occurs. Among the other methods for assessing the effects of local geology the use of microtremors achieved a recognition in the last decade. The method is based on measurements of ambient vibration of natural and artificial origin with three-component sensor. Spectral ratio between the records on horizontal and vertical component yield fundamental frequency of soft sediments deposited over hard bedrock. By measurements inside buildings the main building frequency in longitudinal and transverse direction and consequently the range of increased vulnerability is determined. By comparison of thefundamental frequency map of sediments and measurements inside buildings, we can perform quantitative microzonation of urbane areas and identify areas where the danger of soil-structure resonance exists. In Slovenia microtremors investigations are carried out in the frame of a NATO Science for peace project. By measurements in a very dense grid of 200 m x 200 m a new microzonation of Ljubljana will be performed. In the Bovec basin, where the site effects were very prominent during 1998 and 2004 damaging earthquakes, we found out that relative high damage to some buildings were caused by soil-structure resonance effects. In Ilirska Bistrica, which is one of the most seismically active regions in Slovenia, we also established that most of the buildings damaged during stronger earthquakesare located in areas characterised by the fundamental frequency of soil which coincide with the range of the main building frequencies.