Abstract
In this paper, we demonstrate how UHS-based seismic microzonation can be applied in low-to-medium seismicity areas with deep local soil and deep geological deposits under the local soil. The case study area surrounds the city of Osijek, Croatia, which is in the south–central region of the Pannonian Basin. New frequency-dependent scaling equations are derived, and the empirical response spectra are compared to the spectra of real strong motions in the surrounding region. Empirical calculations for deep soil atop deep geological strata show a 37% reduction in short-period spectral amplitudes when compared to rock locations. This demonstrates that local soil amplification is mitigated by energy dissipation in deep soils. For vibration periods longer than 0.3 s, spectral amplitudes are being amplified. This amplification goes up to 2.37 times for vibration periods around 0.5 s. UHS spectra for Osijek are computed using regional seismicity estimates, data on local soil and deeper geological surroundings, and newly created regional empirical equations for scaling various spectral amplitudes. UHS amplitudes for Osijek are also compared to the Eurocode 8 spectra for ground type C. The results show that ratios of the maximum UHS amplitudes to PGA values are up to 46% larger than the corresponding 2.5 factor that is recommended by Eurocode 8 for horizontal spectra. The UHS results might be viewed as preliminary for Osijek and regions with similar seismicity and local soil and deep geology conditions. When the number of regional strong-motion records grows many times beyond what it is currently, it will be feasible to properly calibrate the scaling equations, resulting in more reliable and long-term UHS estimations for the area under consideration.
Highlights
The amplification of ground motion generated by sedimentary basins has been investigated by many researchers using analytical and computational methods (e.g., [1,2,3,4,5,6])
In this paper, we demonstrate how UHS-based seismic microzonation can be applied in low-to-medium seismicity areas with deep local soil and deep geological deposits under the local soil
The case study area surrounds the city of Osijek, Croatia, which is in the south–central region of the Pannonian Basin
Summary
The amplification of ground motion generated by sedimentary basins has been investigated by many researchers using analytical and computational methods (e.g., [1,2,3,4,5,6]). Standard seismic hazard maps only consider shallow geology conditions (i.e., the local soil conditions for the first 30 m depth of the stratigraphic profile) This is because most empirical equations for scaling peak ground acceleration (hereinafter, PGA) values only include the effects of local soil on a tens-of-meters scale and disregard the impact of deeper geological site conditions [7]. For short vibration periods (less than 1 s), they found that the levels of response spectral acceleration were significantly below those of the 2012 Indonesian building Codes’ design response spectra, but approach or even exceed Code levels for longer periods [22] They discovered that available ground-motion prediction equations (hereinafter, GMPE) are incapable of capturing the impacts of basin geometry on seismic waves in a very deep basin filled with soft sediment [22]. Seismic hazard maps for several vibration periods and probabilities of exceedance are constructed
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