engleski

Estimation of Seismic Site Effects from Ambient Noise Measurements using HVSR Method: Varaždin city, Croatia

Varaždin is an old baroque city located in the northern part of Croatia. It is situated on shallow Quaternary alluvial sediments (clays, sands, gravel) in the Drava River basin. Local site effects due to alluvial soft sediments can play a crucial role in the amplification of seismic wave ground motions. Microtremor Horizontal-to-Vertical Spectral Ratio (HVSR) method was used to determine seismic ground response of the area, expressed by fundamental/natural frequency (f0) and site amplification (A0) of the sediments. Iso-maps showed variations of soil fundamental frequencies (0.91-5.19 Hz), site HVSR amplifications (1.92- 3.94) and sediments cover thickness (ca. 15-85 m) in the city area. Results are found in good agreement with the geological map, thickness of the sediments from boreholes and known depth of Varaždin aquifer. It was observed that directional HVSR analysis and the polarization angles of microtremors are affected by horizontal polarization of the ground motion at the measurement points near, along and across the faults and by earthquake mechanism of the fault- plane solutions from the past earthquakes in the area. Microtremor ambient noise measurements HVSR method in Varaždin city proved to be a useful tool for estimation of seismic site effects in the area with low seismicity. Varaždin is growing urban city, and this study is important for future aseismic urban planning and seismic historical preservation of monumental heritage.

Local site effects ; HVSR ; microtremor ; fundamental frequency ; site amplification ; directional HVSR

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