Abstract
The city of Zagreb (Croatian capital) is situated in the contact area of three major regional tectonic units: the SE Alps, NW Dinarides, and Tisza Unit in the southwestern margin of the Pannonian Basin. The Zagreb seismic zone encompasses the Medvednica Mountains and the city of Zagreb with its surrounding areas, which was struck by the strongest instrumentally recorded earthquake (M5.5) on 22 March 2020. The objective of this contribution is the estimation of the high-frequency attenuation spectral parameter kappa (κ) and its local site-specific component for the Zagreb (Croatia) seismic stations to which we were particularly encouraged after the scale of the damage after the Zagreb 2020 earthquake. We tested linear dependence of κ with epicentral distance using traditional linear least square regression, linear regression for data with errors, and constrained model at close distances to estimate near-site attenuation (κ0). Regression-estimated site kappa values at zero-distance are within the range of the uncertainty (±1 standard deviation) with constrained κ0 value as well within the range of existing global κ0 and VS30 (shear wave velocity in the top 30 m) values. Spatial distribution of κ within the Zagreb seismic zone shows that κ is not isotropic and high-frequency attenuation anisotropy is probably affected by local and regional geological variability, regional active faults and a complex tectonic structure in each direction.
Highlights
Attenuation of seismic waves is one of the key factors in seismic hazard assessment for earthquake-prone regions as well as being important for the quantification of earthquakes and plays a significant role in studies of seismic source and crustal structure [1]
The total path attenuation of shear waves (S-waves) within the crust is separated into two attenuation parameters: the frequency-dependent quality factor Q(f ) and the near-surface attenuation parameter κ as shown in Equation (1) where A(f,t) is acceleration spectrum that contains the effects of source, distance and local site effects
Under the assumption that the local site effects are more pronounced in the horizontal components than in the vertical components of ground motion, we present the spatial distribution of horizontal κ values (Figure 7) to determine if there exist different trends in the high-frequency regional attenuation between different azimuthal area subsets
Summary
Attenuation of seismic waves is one of the key factors in seismic hazard assessment for earthquake-prone regions as well as being important for the quantification of earthquakes and plays a significant role in studies of seismic source and crustal structure [1]. The Fourier amplitude spectrum (FAS) of ground motion is influenced by source parameters, propagation path, and local site conditions as modelled by the Brune’s [2] theoretical omega-square source model. Sci. 2020, 10, 8974 limitation parameter (fmax ) was defined as the cut-off frequency at which the spectrum starts to decay very rapidly. Hanks [3] concluded that the fmax observed in acceleration FAS is controlled by the local site conditions, by the subsurface geological structures below and near the site. Empirical spectral decay parameter kappa (κ) was introduced by Anderson and Hough [5] to describe the difference between the observed acceleration spectrum of S-waves and Brune’s [2] omega-square source model at the high-frequency part of the FAS.
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