Determination of Vs30 for Seismic Ground Classification in the Ljubljana Area, Slovenia

Ljubljana, the capital of Slovenia, is located in the central region of the country in a shallow sedimentary basin filled with Quaternary deposits. It is one of the most seismically active regions in Slovenia, with some important historical earthquakes that have caused damage and economic losses. The soft sediments are some of the most important factors responsible for the amplification of seismic ground motions in the area. The existing microzonation studies in the Ljubljana basin are inadequate, since there is a lack of borehole and geophysical data. Thus, investigations of the seismic shear-wave velocity for the purposes of supplementing the existing seismic microzonation of Ljubljana were carried out. The average shear-wave velocity for the topmost 30 m of sediments at 30 sites was determined using the joint modelling of three-component, microtremor, horizontal-to-vertical spectral ratio (HVSR) data and dispersion curves obtained through microtremor array measurements analyzed by the Extended Spatial Autocorrelation (ESAC) and Refraction Microtremor (ReMi) methods. The phase-velocity dispersion curves obtained from the ESAC and ReMi analysis of the microtremor record showed very good agreement. The estimated Vs30 values are higher in the northern and north-eastern parts of the study area (300–600 m/s), where the Pleistocene and Holocene glaciofluvial deposits of the Sava River, mostly gravel and sand, are dominant. In the southern part of the city (140–300 m/s) the lowest values of Vs30 correspond to the lacustrine and fluvial deposits of the Ljubljana Moor. The results are in good agreement with the distribution of the sedimentary units, while in comparison with the existing EC8 based microzonation, the investigated area can be classified, generally, as being one class better in terms of the EC8 classification. The obtained spatial distribution of Vs30 in Ljubljana therefore provides valuable information to supplement the existing microzonation.