Abstract
The city of Ivanec is located between valley of the Bednja River and Mt. Ivanščica and this area can be prone to significant seismic site amplification due to local site characteristics. This study presents the first assessment of seismic site amplification for the city of Ivanec by the microtremor horizontal-to-vertical-spectral-ratio (HVSR) method and the equivalent-linear (EQL) site response analysis. Based on microtremor measurements and HVSR analysis, fundamental soil frequency and HVSR peak amplitude maps indicate potentially seismic danger zones. The 1-D EQL site response analysis was performed using multiple suites of earthquake ground motions scaled to the 95- and 475-year return periods of peak ground accelerations. Site amplification maps at the predominant peak frequency and ground surface indicate two microzones, one with high amplification in the central part of the city due to soft soil characteristics, and the other with small amplification in the transitional zone from alluvial basin towards the foothills of Mt. Ivanščica. HVSR peak amplitudes and site response peak amplifications showed similar spatial distributions with similar predominant peak frequencies but with different amplitude levels. Site amplification maps provided significant information about potential resonance effects for structures of certain heights that can be correlated with the local ground shaking characteristics.
Highlights
The city of Ivanec (46.224◦ N, 16.124◦ E) is located in northwestern Croatia and is part of the County of Varaždin with a population of approx. 14,000 inhabitants
The main questions is whether the estimated HVSR peak amplitudes represent qualitatively an indication of local site amplification? The underlying principle of the HVSR methodology is that ambient noise measurements are performed without an earthquake influence [11,12,13,14,15,16,17,18,19,20,21]; the fundamental soil frequency and HVSR peak amplitude only represent a site response in the natural state and not within ground motion amplification [21,22,24,73]
The change from site response of natural state (HVSR) and the site response of the local soil when subject to an earthquake influence (PGAROCK =0.09 g and 0.19 g) is clearly observed in terms of site amplification changes, in the alluvial basin where amplification factor (AF) decreases under a higher input of ground motion levels (Figures 13 and 14)
Summary
The city of Ivanec (46.224◦ N, 16.124◦ E) is located in northwestern Croatia and is part of the County of Varaždin with a population of approx. 14,000 inhabitants. The city of Ivanec (46.224◦ N, 16.124◦ E) is located in northwestern Croatia and is part of the County of Varaždin with a population of approx. The area of the city of Ivanec belongs to the Varaždin-Ivanščica-Kozjansko epicentral area where moderate to strong earthquake events have occurred [2]. In the last 50 years, the proximate area of Ivanec (extending some 20 kilometres) has experienced more than ten ML ≥ 3.5 earthquakes and reported small to moderate damage to some buildings in different parts of the city area. The city of Ivanec is situated in a transitional zone between the Bednja River alluvial and coarse-grained clastic sediments toward carbonate rocks of Mount Ivanščica (Figure 1a). According to the spatial master plan of the city of Ivanec dated from 2012 [3] and amendments from 2016, the maximum allowed construction height of the buildings is 12–15 m (four or five floors) in the urban residential area in the central part of the city (Figure 1b, shadow grey area) and a maximum permitted height of 8 m for industrial buildings in the industrial area (Figure 1b, shadow red area)
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