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Recent Measurements of Ambient Vibrations in Free-Field and in Buildings in Croatia

Recent measurements of ambient vibrations in Croatia include free-field measurements in Zagreb and Ston, as well as those done within buildings. Free-field data at both localities are consistent with the properties of shallow geological structures there. In Zagreb, horizontal-to-vertical spectral ratios (HVSR) of microtremors point to existence of thick alluvial cover (over 100 m) that gradually gets thinner as it reaches the southern slopes of the Mt. Medvednica. In Ston HVSR profiles reveal several tens of meters thick sedimentary cover over the bedrock which gets exposed at the Stoviš hill. Over 100 measurements of ambient vibrations in buildings that have recently been performed across Croatia, were analysed by a suite of Matlab routines yielding frequencies and damping for the fundamental and higher modes of oscillation. Applications to real buildings proved that analyses of shaking induced by ambient vibrations in most cases lead to well constrained, reliable, and time independent estimates of frequencies and damping of the building vibrational modes. The measurements done so far form the initial nucleus of the Croatian building inventory, a collection of observed fundamental periods, damping, and spectral shapes, which can prove important in documenting building structural integrity and in assessing the degree of possible damage in future earthquakes.

building frequency, soil frequency, soil-structure resonance, height-period relationship

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