Study on the applicability of microtremor HVSR method to support seismic microzonation in the town of Idrija (W Slovenia)
Other literature type
(issn: 1684-9981, eissn: 1684-9981)
The Idrija town is located in area with increased seismic hazard in W Slovenia and is partly built on alluvial sediments or artificial mining and smelting deposits which can amplify seismic ground motion. There is a need to prepare a comprehensive seismic microzonation in the near future to support seismic hazard and risk assessment. To study the applicability of microtremor Horizontal-to-Vertical Spectral Ratio (HVSR) method for this purpose, 70 free-field microtremor measurements were performed in 0.8 km<sup>2</sup> large town area with 50–200 m spacing between points. HVSR analysis has shown that it is possible to derive sediments resonance frequency at 48 point, whereas at remaining one third of measurements nearly flat HVSR curves were obtained indicating small or no impedance contrast with the seismological bedrock. Iso-frequency (range 2.5–19.5 Hz) and HVSR peak amplitude (range 3–6, with few larger values) maps were prepared by using natural neighbour interpolation algorithm and compared with the geological map and map of artificial deposits. Surprisingly no clear correlation was found between distribution of resonance frequencies or peak amplitudes and the known extent of supposed <q>soft</q> sediments or deposits. This can be explained by relatively well compacted and rather stiff deposits and complex geometry of sedimentary bodies. However, at several individual locations it was possible to correlate the shape and amplitude of the HVSR curve with the known geological structure and prominent site effects were established in different places. On the other hand, in given conditions (very limited free space and high level of noise) it would be difficult to perform active seismic refraction or MASW measurements to investigate the S-waves velocity profiles and thickness of sediments in details, which would be representative enough for microzonation purposes. The importance of microtremor method is therefore even greater, because it enables direct estimation of the resonance frequency without knowing the internal structure and physical properties of the shallow subsurface. The results of this study can be used directly in analyses of possible occurrence of soil-structure resonance of individual buildings, including important cultural heritage mining and other structures protected by UNESCO. Second application of the derived free-field iso-frequency map is to support soil classification according to the recent trends in building codes.