Site Amplification at High Spatial Resolution from Combined Ambient Noise and Earthquake Recordings in Sion, Switzerland

Abstract

Abstract Earthquake site effects have a major impact on the seismic hazard. However, evaluating the site response over a broad frequency range and with a high spatial resolution remains difficult. Therefore, there is a high demand from the engineering seismology community in having cost-effective and reliable approaches to evaluate the site response. Empirical site effect assessment has shown good reliability up to high frequencies but relies on earthquake recordings that require long station deployments. In contrast, seismic ambient noise can be rapidly recorded anywhere at any time. The hybrid standard spectral ratio (SSRh) combines both the spectral ratio from earthquake recordings at a few sites and the spectral ratio from ambient noise recordings at many sites from short duration deployments. The objective of our study is to evaluate the applicability of the SSRh to a large urban area. Our studied area is the Rhône valley around the city of Sion (Switzerland), which is prone to complex 2D and 3D site effects in a broad frequency range caused by the soft, thick, and variable sedimentary deposit of the Rhône river. We benefit from four permanent and five long-term seismic stations present in the area, and we performed 299 short ambient noise measurements on a regular 300 m side grid. At the permanent stations, we find good agreement between the SSRh and the empirical amplification function estimates using earthquake observations, and we observe a good stability of the SSRh over time, frequency, and space, despite the strong variability of the ambient noise. Based on that, we estimate the SSRh for every ambient noise recording point in the area to compute a high-resolution amplification model and subsequently a site-specific seismic hazard model for the Sion area. Our models are in good agreement with the geological information.