Does VS30 reflect seismic amplification? Observations from the West Bohemia Seismic Network

Abstract

The present study addresses verification of average seismic shear-wave velocity from the surface to a depth of 30 m (V(S30)) as a suitable proxy for a seismic amplification. For this purpose, we used instrumentally homogeneous and spatially dense seismic network WEBNET (West Bohemia Seismic Network), designed to monitor an intraplate earthquake swarm activity in the West Bohemia/Vogtland region (Czech Republic/Germany). Using a Multichannel analysis of surface waves (MASW) shear-wave velocity models and parameters V(S30) and H(800) (depth of V(s) > 800 m/s) were obtained at 17 WEBNET sites. V(S30) were compared with (i) H(800) and (ii) determined bedrock densities. To understand the relation between V(S30) and site amplification, V(S30) values were correlated with amplitudes of two earthquakes: (1) Mw 6.4 Petrinja, Croatia (12/2020) and (2) Mw 4.2 SE of Vienna, Austria (03/2021) both recorded by the WEBNET. The correlation analysis examined four categories of seismic waves in nine frequency windows and described the relation between amplification and V(S30) using newly defined regression model. The results show that for the regression model, the frequency window with the highest correlation is in the 1-3 Hz range, and this dependence is statistically best observed in the full wave record. The amplification generally decreases with increasing V(S30). However, a large scatter in amplification within Eurocode 8 category B is observed. Based on the observations a new general approach is put forward to finely indicate the relation between amplification and V(S30) and the use of other site proxies is discussed.