Assessment of Seismic Site Effects in Aachen (Germany) to Support Seismic Microzonation: Geophysical Observations and Numerical Modeling

Accurate assessment of seismic hazard in urban areas requires a detailed characterization of local site effects controlled by near-surface geology, stratigraphy, and tectonic structures. This study presents a comprehensive assessment of seismic site conditions in the central part of Aachen (Germany), conducted to support future seismic microzonation, and covering an area of approximately 5 × 5 km², with particular focus on the previously underexplored southeastern, southwestern, and northeastern districts.
The investigation integrates a large multidisciplinary dataset, including 450 ambient noise horizontal-to-vertical spectral ratio (HVSR) measurements, six microtremor array measurements (MAM) for shear-wave velocity (Vs) profiling, six electrical resistivity tomography (ERT) profiles for stratigraphic validation, and information from 175 geotechnical boreholes. The primary objectives were to characterize the spatial distribution of fundamental resonance frequency (f₀), site amplification effects, and Vs₃₀ values, which represent key input parameters for seismic hazard assessment and seismic microzonation studies.
Special emphasis was placed on the joint use of Rayleigh- and Love-wave dispersion curves and Rayleigh-wave ellipticity inversion to improve subsurface resolution at low frequencies (<1 Hz), allowing a more reliable estimation of sediment thickness and deep impedance contrasts. Along a southwest–northeast-oriented cross-section (A–A′), intersecting major tectonic features such as the Laurensberg Fault, 30 additional HVSR measurements reveal a strong correlation between f₀ variations, sedimentary geometry, and dynamic soil properties.
To investigate seismic wave propagation effects, two 2D numerical dynamic models were developed along cross-section A–A′. Profile 1 explicitly incorporates the Laurensberg Fault, constrained by HVSR results and geological data, whereas Profile 2 neglects fault structures to isolate their influence on seismic ground motion. Model results were validated using a 2D standard spectral ratio (SSR) analysis and systematically compared with HVSR observations along the same profile. This comparison enables the identification of peak ground acceleration (PGA) patterns, amplification zones, and fault-controlled energy redistribution effects.
The results demonstrate that local site response in Aachen is strongly influenced by both sedimentary structure and fault geometry, emphasizing the importance of accounting for tectonic features in site-effect studies, even in regions of moderate seismicity. The outcomes of this study provide a robust geophysical basis to support seismic microzonation efforts, future 3D numerical simulations, and seismic-informed urban planning in the Aachen urban area.