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Seeking for Dependencies of the High-Frequency “Kappa” Parameter of Earthquake Spectrum on Weather/climate Conditions

Description: 

Nowadays the influence of a site’s surface geological conditions on earthquake ground motion is well known. Several techniques have been proposed and applied at many sites around the world evaluating their so-called Site Amplification Factor (SAF), as well as their high frequency decay empirical factor, kappa-zero (κ₀). SAF and κ₀ control the ground motion and its simulation at a site, and they are considered significant for reliable seismic hazard assessment. Toward understanding the factors that can shape the high-frequency part of the earthquake spectrum, we investigate the existence of seasonal patterns in the variation of the Standard Spectral Ratio (SSR) and delta kappa (Δκ~Δκ₀) of the CK0 surface station of the ARGONET vertical array of accelerometers (Cephalonia Island, Greece), installed on top of sedimentary layers, with respect to the CK83 borehole station, installed in the bedrock at ~83m depth. The SSR and Δκ factors have been computed based on acceleration records of 964 earthquakes that have been recorded by the ARGONET array over a period of ~7.5 years. We detect and quantify the seasonal variation of these two factors, which clearly reflect the similar variation of the corresponding SAF and κ₀ factors of the CK0 station, reasonably considering that there is no weather effect at the depth of the reference station. Based on the SSRs, seasonal variation is clearly observed in the horizontal motion of CK0, but only in the high frequency range of 17-34Hz. A seasonal pattern is also observed in the vertical component of CK0, albeit in the even higher frequency range of about 40-55Hz. Δκ is subsequently determined at 4 different frequency ranges (f_e to f_end), for f_e=6 and 10 Hz and f_end=25 and 35Hz. As expected, Δκ bears the seasonal variation observed in the SSRs. Our results confirm the well-known dependence of κ on the frequency range used for its measurement and suggest that one of the reasons for this dependence, at least in soft soils, is the seasonal variation of their response associated with changes in their water content and possibly other weather/climate related factors.

Session: From Earthquake Recordings to Empirical Ground-Motion Modelling - I

Type: Oral

Date: 5/2/2024

Presentation Time: 08:15 AM (local time)

Presenting Author: Ioannis

Student Presenter: No

Invited Presentation: Yes

Authors