Paolo Bergamo

We present three-dimensional (3D) spectral-element simulations of seismic wave propagation in the Swiss Rhône Valley to quantify how realistic Alpine topography affects ground motion across frequency bands. The simulations incorporate a digital elevation model–based free surface topography representation, a 3D crustal velocity model, and geotechnically constrained near-surface layers. The simulations are performed for three local earthquakes (Mw 3.2–4.5) recorded in two different configurations: topographic and flat-surface with basin geometry preserved. Topo/flat spectral ratios reveal a systematic component asymmetry in the topographic response. At mountain sites sampled by a spatially uniform synthetic grid, horizontal motion is deamplified across the dominant low-to-intermediate frequency bands (median 0.80–0.97× at 0.05–0.50 Hz), whereas vertical motion is consistently amplified (1.16–2.32×). The real seismic network with preferential sites confirms the strong vertical amplification, whereas the horizontal regime yields weak deamplification (0.65–1.00×). Basin sites exhibit mixed horizontal response and strong vertical amplification, consistent with coupled topographic scattering and basin-wave interactions as well as excitation of the Rhône basin’s fundamental frequencies. The observed asymmetry is consistent with topography-induced mode conversion and constructive interference associated with Alpine ridge geometry. These results demonstrate that realistic surface topography can fundamentally redistribute seismic energy between motion components and that sparse observational networks may incompletely capture this behavior. Physics-based simulations with realistic relief and spatially unbiased receiver sampling therefore provide a complementary framework for seismic hazard assessment in mountainous terrain.

This study presents a geostatistical framework for lake-wide mapping of sediment undrained shear strength ( s _u ) on subaqueous slopes in Lake Lucerne using regression-kriging (RK). The spatial variability of the increment of s _u with depth from the lake floor, modelled through a power-law function defined by coefficients α and γ , was retrieved from a comprehensive set of Cone Penetration Tests with pore pressure measurements (CPTu). Multivariate linear regressions were developed to predict α and γ based on topographic and morphometric lake floor parameters, selected through correlation analysis. Regression residuals were analysed via empirical semi-variograms and interpolated using ordinary kriging to correct first-order predictions. The resulting maps of α and γ for lacustrine, glaciolacustrine and deltaic sediments enable site-specific estimation of s _u as a function of depth on lake slopes where these sediments are deposited. The final s _u maps are consistent with sedimentological and geomorphological features of the lake. While artifacts might appear in under-represented settings, the RK framework proves robust and offers a transferable methodology for geotechnical mapping in other sedimentary basins with similar data availability. These results may support future applications such as slope stability assessments and subaqueous hazard analysis in Lake Lucerne and similar lacustrine or marine settings.

We present the development of a nationwide seismic soil class map for Switzerland that implements the revised soil classification introduced in the new Eurocode 8 (EC8). In contrast to the current fragmented cantonal products, which are largely based on geological criteria and not directly compatible with EC8, the new scheme prioritizes geophysical descriptors of the subsurface, in particular average shear-wave velocity of the near surface (Vs30, vs,H) and the depth to engineering bedrock (H800). The resulting map will also support the updated national seismic hazard model. To enable a consistent national study, we assembled large-scale datasets. These include shear-wave profiles estimated at seismic stations and other sites, horizontal-to-vertical spectral ratios (HVSR) analyses, standard- and cone penetration test datasets, and complementary geological information such as lithology, digital bedrock models, and borehole data. Together, these resources would allow mapping of shear-wave velocity structure and sediment thickness, which form the basis for the new EC8 classification. The project is currently in its initial stage. We design a workflow that integrates measured velocity and HVSR information, where available, and use lithological and geological classifications as proxies where coverage is sparse. Thereby shifting from a purely geological classification to one that privileges geophysical parameters prescribed by EC8. This contribution outlines the conceptual design, data resources, and preliminary implementation of this geophysics-driven national soil class map and highlights its relevance for seismic hazard assessment, engineering practice, and future updates as new data become available.

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.

Seismic hazard assessments in Switzerland rely on the Seismic Hazard Model 2015 developed by the Swiss Seismological Service. The standards for seismic impacts on new constructions or existing structures, as outlined in the official building code for Switzerland, consider seismic zones and local soil conditions. However, no updated hazard assessments currently exist for ETH Zurich or Paul Scherrer Institute (PSI) buildings. The last site-specific studies are now outdated. Existing cantonal soil classification maps for PSI are deficient, relying primarily on geological data, and no soil class map is available for dense urban areas, such as Zurich. Accurate assessments require additional geophysical and seismological data. This pilot project aims to develop detailed soil classification maps for the ETH and PSI areas.This project’s key steps include analyzing existing geophysical measurements, collecting and interpreting geological and geotechnical data, and conducting new Horizontal-to-Vertical Spectral Ratio (H/V) and array measurements to map soil resonance. Active seismic methods determine site-specific shear-wave velocity profiles and soil damping values. Existing seismic data undergo reinterpretation using advanced techniques, such as high-resolution beamforming and WaveDec, to compute dispersion and ellipticity curves to obtain subsurface velocity profiles with Bayesian inversions.Subsequently, with the new data, we can estimate local soil amplifications and their variability at investigated sites and classify them into soil classes to produce a soil classification map. Updated seismic hazard studies for two buildings and elastic response spectra accompany an evaluation of the feasibility and effort required for a microzonation study of the ETH and PSI areas.

Physics-based numerical simulations of seismic ground motion are crucial for advancing our understanding of regional earthquake hazard and risk. Complex geometries in sedimentary basins, coupled with strong surface topography, can cause significant variations in ground motion. To model these effects, a 3D representation of surface topography and available 3D models of the sedimentary basin velocity structure are used in numerical simulations. Using the spectral-element method, we synthesise displacement, velocity and acceleration waveforms in the Rhône valley, Switzerland. This region is characterized by complex topography, morphology, and significant seismic hazard.We perform spectral-element waveform simulations with a maximum resolvable frequency of 5 Hz to investigate the joint effects of 3D basin structure and surface topography on ground shaking. Moderate-magnitude earthquakes that have been recorded in the area are used as point sources. Additionally, we compute waveforms for scenario earthquakes taken from the disaggregation of the current Swiss hazard model, SUIhaz2015 (Wiemer et al., 2016).Our goal is to assess how these factors affect amplification patterns in different basin parts and topographic areas. We do so by comparing ground motion peak values at different altitudes (on mountains and valley floors with soft sediment conditions). Additionally, we calculate engineering-relevant ground motion parameters, such as cumulative velocity and significant duration up to the resolved frequencies, that help improve hazard estimations in the Rhône valley.With our study, we show that the joint effects of topography and basin structure lead to larger amplification variations within the basin, and in the surrounding reliefs. We conclude that physics-based simulations have the potential to provide an adequate alternative for input ground motion in seismic hazard analysis. This is particularly relevant for modelling hypothesized earthquakes, which are essential for the assessment of seismic hazard in areas facilitating crucial infrastructure.

Risk modelling is a key tool for mitigating the seismic risk associated with geo-energy applications and CO2 storage. Prior estimation of potential damages and mapping of affected communities enable, among other things, the feasibility assessment of geothermal projects in their planning phase and the allocation of appropriate resources for damage compensation. This promotes societal preparedness and acceptance towards such applications. With rising interest in geothermal energy in Switzerland, the Federal Office of Energy tasked the Swiss Seismological Service (SED) to extend the national Earthquake Risk Model ERM-CH23 (Papadopoulos et al., 2024) to include induced seismicity associated with geo-energy applications and CO2 injection projects. Since these activities typically trigger shallow earthquakes with low-to-moderate magnitudes, the need arose to extend the range of modelled intensity measures towards intermediate periods, which are more sensitive to potential damage from the expected scenario ruptures. As ERM-CH23 already covers PGV and PSA at 1, 0.6 and 0.3 s, it was decided to additionally integrate PSA(0.4s) and PSA(0.2s). From the perspective of soil amplification modelling, we first verified that the existing ERM-CH23 local response layers (Bergamo et al., 2023) are suitable for induced seismicity scenarios. We then applied the procedure of Bergamo et al. (2023), i.e., combining empirical amplification factors with site proxies via geostatistical interpolation, to generate the additional soil response layers for PSA(0.4, 0.2s). Leveraging an extended ground motion database and proven site condition indicators, the maps cover the whole of Switzerland while achieving a fine spatial resolution (250 m); the high quality of the input datasets contributes to keeping the associated (and mapped) epistemic uncertainty (ϕS2S) within reasonable limits.In addition to seismic risk modelling, the complete set of national soil amplification maps for PSA(0.2 - 1.0s) has also been incorporated into the GRID approach of the current revision of SED's Good-Practice Guide for managing induced seismicity in deep geothermal projects (Kraft et al., 2025). GRID (Geothermal Risk of Induced seismicity Diagnosis, Trutnevyte & Wiemer 2017) is a diagnostic tool developed to classify a project's induced seismicity risk. The PSA(0.2 - 1.0s) amplification maps have been collated to consistently chart soft soil areas (soil types D, E and F of the Swiss building code SIA 261) at the national scale, contributing to GRID’s “local amplification” indicator. ReferencesBergamo, P., et al. (2023). A site amplification model for Switzerland based on site-condition indicators and incorporating local response as measured at seismic stations. Bull Earthquake Eng 21, 5831–5865. https://doi.org/10.1007/s10518-023-01766-zPapadopoulos, A. N., et al. (2024). The Earthquake Risk Model of Switzerland, ERM-CH23, Nat. Hazards Earth Syst. Sci., 24, 3561–3578, https://doi.org/10.5194/nhess-24-3561-2024Kraft, T., et al. (2025). Good-Practice Guide for Managing Induced Seismicity in Deep Geothermal Energy Projects in Switzerland, Version 3, Report of the Swiss Seismological Service (SED) at ETH Zurich, pp. 80, https://10.3929/ethz-b-000714220Trutnevyte, E., & S. Wiemer (2017). Tailor-made risk governance for induced seismicity of geothermal energy projects: An application to Switzerland, Geothermics, 65, 295-312, https://doi.org/10.1016/j.geothermics.2016.10.006.

Deeply incised valleys or sedimentary basins often exhibit complex resonance patterns that diverge from the commonly assumed one-dimensional (1D) behaviour. In such cases, the soil resonance fundamental frequency f0 is not determined by the local depth-to-bedrock; instead, f0 is constant across the central portion of the basin section, reflecting the overall geometry and material properties of the sedimentary infill. These 2D (or even 3D) resonance regimes are challenging to identify and are generally overlooked in building codes. This study, funded by the Swiss Federal Office for the Environment, seeks to characterize 2D resonance phenomena across Switzerland by leveraging over 6000 ambient noise measurements and a large-scale morphometric dataset.The primary dataset comprises ~4000 ambient vibration measurements acquired across Switzerland since the late 1990s, archived in the Swiss Seismological Service (SED) site characterization database. The recordings were processed using the horizontal-to-vertical spectral ratio (H/V) technique and soil resonance frequencies were identified following the best practice criteria. This database has been further enhanced by recent high-resolution ambient noise campaigns conducted by SED in key sedimentary basins: the Swiss Rhône Valley, the Lucerne and Horw basins in Central Switzerland, and the High Rhine Valley near Basel. These campaigns, with spatial resolutions ranging from 100 to 400 m, contribute approximately 2000 additional measurements with their f0 for the areas of interest.This sizeable ambient noise database is paired with a collation of various geological/geophysical models: the backbone model by the Swiss Federal Office of Topography is complemented by regional models for the Alpine and High Rhine valleys, the Geneva Basin, the Grisons, and the Basel area. The collation of such models maps the depth of the sediments-to-bedrock interface over most of Switzerland. Based on this information, we performed morphometric analyses, which allowed extracting key geometrical parameters (shape, width, maximum depth) of the sedimentary infill along 4500 transects – spaced by 250 m and spanning all large sedimentary basins.Cross-referencing the soil resonance frequencies with the morphometric characteristics of the sedimentary basins, we observed patterns consistent with those predicted by numerical studies from the literature. Our analysis distinguishes valleys with 1D resonance behaviour from those with 2D resonance regimes. Furthermore, as a valley's shape ratio (half-width over maximum depth) increases, resonance frequencies converge towards specific 2D vibration modes, particularly fundamental SH- and SV-modes and their higher harmonics. We also examined whether these ambient vibration resonance modes reflect into the (directional) ground motion local response at seismic stations.The results of this study are synthesized into a national-scale map identifying basins and valley bottoms with 1D or 2D resonance behaviours and their corresponding resonance frequencies. Our study will contribute to the decision of whether the Swiss national building code should adopt tailored elastic response spectra for alpine valleys prone to 2D resonance patterns.

The significance of local site effects in seismic hazard assessment is well-recognised. However, nonlinear soil behaviour during high-strain conditions is often neglected or oversimplified, even for large return periods. This study aims to estimate the impact of nonlinearity and liquefaction on the local seismic hazard using the multistep approach by Janusz et al. (2024), in particular in low-to-moderate seismicity areas, where nonlinearity is not instrumentally observed and hence particularly difficult to assess. We focus on sites in Switzerland, where strong earthquakes are relatively rare, although documented in the past. An exemplary area is the subalpine sedimentary basin of Lucerne, which is vulnerable to nonlinearity and liquefaction because of soft alluvial deposits (VS30

The geometry and soil conditions at a site can strongly influence the ground shaking induced by an earthquake and an important part of seismic hazard assessment is therefore to characterize such site effects. At stiff sites and for weak ground motions, the soil response behaves linearly, however, for larger ground motions and softer sites the site response becomes nonlinear and thus more challenging to assess. Because recordings of such strong shaking are rare, nonlinear soil parameters are often defined from laboratory measurements and numerical simulations. However, the increase of available ground motion data, in particular from sites with recordings from both weak and strong motions, has increased the possibility of deriving nonlinear site parameters directly from empirical data. In this study, we take advantage of the comprehensive KiK-net network in Japan, consisting of stations with recording instruments at both surface and at depth. We use the surface-to-depth ratios of each event recorded by each station, to systematically capture the empirical effects of nonlinear soil response on the local response. Station-specific parameters for the degree of nonlinearity and PGA thresholds for the onset of nonlinear behaviour are then derived and the statistical correlation between these parameters and a selection of geotechnical and geological indicators are investigated. Our results show that - although finding site parameters suitable for predicting nonlinear site effects remains challenging as the nonlinear soil-behaviour is largely site-specific - proxies describing the depth to bedrock, sediment thickness and characterization of the shallowest part of the soil layer display a certain degree of potential for prediction.