Single-station geophysical and seismological investigations towards revising seismic microzonation of the Basel region

Seismic microzonation is essential for urban planning and earthquake risk mitigation by delineating areas with varying seismic hazards. In 2009, a comprehensive microzonation map was developed for the canton of Basel-Stadt and parts of Basel-Landschaft and Solothurn. This map supported the Swiss standard SIA 261 by identifying site-specific earthquake hazards. Since then, new geophysical, geotechnical, and seismological datasets have been collected within various projects, and advancements in data analysis methods have been made. Together with the updates to the SIA 261 standard (2020) and the national seismic hazard model (Wiemer et al., 2016), this necessitates a revision of the 2009 microzonation.

To support this revision and refine the understanding of local seismic response, we complement the existing dataset with new single-station ambient vibration measurements and deployment of temporary seismic stations (to evaluate seismic amplification in the areas of interest) and utilize advanced methodologies to analyze geophysical and seismological data.

All available single-station geophysical data allow for resolving the areas with variable subsurface structure and properties. In particular, this data is used to retrieve the horizontal-to-vertical spectral ratio (HVSR) and fundamental frequencies of resonance (f₀) across the study area. Additionally, the HVSR and f₀ are used for cluster analysis to support the definition of the boundaries between microzones for revised microzonation maps.

The data recorded by the network of existing and previously available seismic stations and six new temporary stations are used to obtain refined estimates of empirical amplification functions (EAFs) using Empirical Spectral Modeling (ESM, Edwards et al. 2013) and Standard Spectral Ratio (SSR, Borcherdt, 1970) methods. These EAFs are also used to validate the 2009 amplification models (Shynkarenko et al. 2024) and cross-check fundamental resonance frequencies retrieved from the HVSR. To retrieve ground motion amplification in regions lacking seismic station observations, the Canonical Correlation method will be applied to HVSR data (Panzera et al., 2021; Imtiaz et al., 2024).

The outcomes of this study will allow for the integration of ground motion amplification data with seismic hazard models on rock and updating uniform hazard spectra, thus enhancing the microzonation's contribution to risk mitigation and urban planning.

References:

Borcherdt, R.D. (1970). Effects of local geology on ground motion near San Francisco Bay, Bull. Seismol. Soc. Am. 60(1), 29-61.

Edwards, B., Michel, C., Poggi, V., Fäh, D. (2013). Determination of site amplification from regional seismicity: application to the Swiss National seismic Networks, Seismol. Res. Lett. 84(4), 611-621.

Wiemer, S. et al. (2016). Seismic Hazard Model 2015 for Switzerland (SUIhaz2015), http://www.seismo.ethz.ch/export/sites/sedsite/knowledge/.galleries/pdf_knowledge/SUIhaz2015_final-report_16072016_2.pdf_2063069299.pdf.

Panzera, F., Bergamo, P., Fäh, D. (2021). Canonical correlation analysis based on site-response proxies to predict site-specific amplification functions in Switzerland, Bull. Seismol. Soc. Am. 111(4), 1905‑1920.

Imtiaz, A., Panzera, F., Fäh, D. (2024). Performance of canonical correlation in developing a high-resolution site amplification map in Basel. Proceedings of the 18th World Conference on Earthquake Engineering (18WCEE), 9 pages, Milan, Italy.

Shynkarenko, A., Bergamo, P. Imtiaz, A., Chieppa, D., Fäh, D. (2024). Report on the Common Task 1 of Basel Landschaft and Basel Stadt Microzonation Project: Verification of the amplification functions used in 2009, Report, Swiss Seismological Service.