The High-Frequency Decay Slope of Spectra (Kappa) for M≥3.5 Earthquakes on Rock Sites in Eastern and Western Canada

Abstract

Spectral decay of ground-motion amplitudes at high frequencies is primarily influenced by two parameters: site-related kappa (κ0) and regional Q (quality factor, inversely proportional to anelastic attenuation). We examine kappa and apparent Q-values (Qa) for M≥3.5 earthquakes recorded at seismograph stations on rock sites in eastern and western Canada. Our database contains 20 earthquakes recorded on nine stations in eastern Canada and 404 earthquakes recorded on eight stations in western Canada, resulting in 105 and 865 Fourier amplitude spectra, respectively. We apply two different methods: (1) a modified version of the classical S-wave acceleration method; and (2) a new stacking method that is consistent with the use of kappa in ground-motion modeling. The results are robust with respect to the method used and also with respect to the frequency band selected, which ranges from 9 to 38 Hz depending on the region, event, and method. Kappa values obtained from the classical method are consistent with those of the stacked method, but the stacked method provides a lower uncertainty. A general observation is that kappa is usually larger, and apparent Q is smaller, for the horizontal component in comparison to the vertical component. We determine an average regional κ0=7 ms (horizontal) and 0 ms (vertical) for rock sites in eastern Canada; we obtain κ0=19 ms (horizontal) and 14 ms (vertical) for rock sites in western Canada. We note that kappa measurements are quite sensitive to details of data selection criteria and methodology, and may be significantly influenced by site effects, resulting in large site-to-site variability.

Estimation of the High-Frequency Attenuation Parameter Kappa for the Zagreb (Croatia) Seismic Stations

“…However, large data scatter in κ and R e is not only sensitive to differences in methods and data, and to the local and regional characteristics of study area (the effect of distance and local site effects), but also to complex Q structure, different ray paths across faulting structure for different earthquakes [36,37] and anisotropic attenuation properties [1]. Therefore, we choose an area within about 20 km of the epicentre as near-field attenuation [38] to constrain κ 0 value to κ value at shortest R e (Figure 6) using nearby records (hockey-stick model proposed by [36]).…”

“…This enabled us to determine site kappa value affected primarily by upper local site condition that would not be influenced by changes in slope of κ R related to Q; in order to minimize influence of regional effects (deeper local site effect + source and path effects, [32]) on κ 0 . Additionally, by restricting the distance range for kappa we ensure that local site attenuation at zero distance is overwhelmed by the path component at larger distances [36,37]. Figure 6 shows comparison of results from different methods to estimate κ 0 for the PTJ and ZAG stations.…”

“…Compared to [1], κ 0 values for Zagreb area are higher than those for Kvarner region stations (Brijuni: 0.0249-0.0362 s, Rijeka: 0.0212-0.0239 s and Novalja: 0.0225-0.0235 s). The Kvarner stations sites and surrounding area are composed mainly of carbonates and the lowest κ values are regionally stabilized for hard rocks [11,32,37].…”

“…According to the model illustrated by [32], surrounding areas of Hrvatsko Zagorje and Sava Basins, with sediments depth up to 2 km (Figure 1), have influence on the increase of the high frequency attenuation and local site attenuation due to the addition of the deeper site attenuation. The older and more competent carbonate rocks in the top few kilometres of the crust are expected to have a lower kappa [37] as the situation is in the Kvarner region [1], than younger and less competent Miocene softer rocks in the uppermost crust surrounding the Medvednica Mountains.…”

“…and Kalnik Mt., to the south by Žumberak Mt., and in its centre by the Medvednica Mountains, that are distributed predominantly within regional active faults ([19], Figure 1) and main seismic zones (Figure 2). We limit spatial distribution of κ (Figure 7) within distance of 80-100 km from Zagreb area to avoid additional path complications, since ground motion ray paths from source to station show a wider range of Q (Figure 6, [37]), and to derive conclusions within seismic zones Novo Mesto-Krško, Karlovac-Metlika and Pokupsko-Petrinja (Figure 2), that influence seismic hazard of the Zagreb city area.…”