Despite the development of several empirical recipes, there is currently no consensus on how to determine site amplification for real earthquakes based on ambient noise characteristics. In fact, there is still no clear evidence that this is even possible. This uncertainty is closely linked to the fundamental differences between the seismic wavefields of earthquakes and ambient noise.
It is therefore important to better understand a) the earthquake ground motion in local surface sedimentary and topographic structures, b) seismic ambient noise in these structures, and c) characteristics of seismic ambient noise that could suggest, identify or even quantify a potential effect of surface geology on earthquake ground motion.
Numerical modelling can very much help to understand both earthquake ground motion and seismic ambient noise. Whereas more modelling was focused on the earthquake ground motion, only relatively limited work has been done on the numerical modelling of ambient noise: at both low frequencies (microseisms) and high frequencies (microtremor). As a result, we still lack a clear understanding of what quantitative information can be reliably extracted from noise measurements, and what processing techniques are most appropriate. This is especially true for frequency-dependent amplification in the linear domain.
The session will thus welcome contributions on site response and/or use of seismic ambient noise for characterizing site response. All kinds of approaches can be considered: instrumental measurements with sparse or dense arrays, advanced processing, theoretical or numerical modelling of earthquake ground motion and/or seismic ambient noise. A special interest will be brought to contributions linking earthquake site effects and seismic ambient noise.
Studies may concern sites with various dimensionalities (1D, 2D, 3D, with or without material heterogeneities) and various underground and surface geometry (including topography). Examples of apparent inconsistencies between noise measurements and instrumental earthquake site response are also welcome to gather as complete as possible picture of the issues which are still ahead.
In this study, we performed integrated geophysical surveys in the historical center of Messina, focusing on the area surrounding the Cathedral, where urban stratigraphy is strongly influenced by both natural and anthropogenic processes. The city of Messina, southern Italy, is characterized by high seismic hazard and complex near-surface conditions. Messina has been repeatedly struck by destructive earthquakes over the last centuries, most notably the M7.1 event of 1908, which caused near-total destruction. Reconstructions following these earthquakes generated thick and laterally non-uniform anthropogenic deposits (rubble and debris) that, combined with vertically heterogeneous stratigraphy, might pose significant challenges for accurate subsurface characterization and site response analysis. Ambient noise data were analyzed using the Horizontal-to-Vertical Spectral Ratio technique to estimate fundamental resonance frequencies and delineate major impedance contrasts. Results from HVSR measurements revealed a predominant fundamental resonance peak around 1.0 Hz, with amplitude factors between 4–6, consistent with the impedance contrast between alluvial sediments and the underlying metamorphic basement.
To examine the spatial distribution, the HVSR data were organized along five survey profiles. The corresponding two-dimensional cross-sections were generated by interpolating more than five HVSR measurements for each profile. In addition, high-frequency peaks (>30 Hz) were detected and mapped laterally for more than 40 m, strongly suggesting the presence of heterogeneous anthropogenic layers formed after the 1908 earthquake, highlighting the significant role of shallow debris deposits in conditioning site response.
Active and passive surface-wave methods, including Multichannel Analysis of Surface Waves (MASW), Extended Spatial Autocorrelation (ESAC) and (f-k) approaches, were employed to retrieve shear-wave velocity profiles at different depths. MASW results identify a 2–3 m low-Vs layer (150–200 m/s) overlying 350–450 m/s alluvial deposits, while array analyses (ESAC, f-k) extend the investigation depth to ~60 m. Joint HVSR–dispersion inversion constrains the main impedance contrast at 90–100 m, marking the transition to the metamorphic basement. Differences between MASW and joint inversion models highlight the importance of multi-method approaches when anthropogenic stratigraphy is present.
The joint analysis allowed the identification of key stratigraphic interfaces, the recognition of laterally variable anthropogenic fills and deposits, and the estimation of the main discontinuities within the uppermost layers. Importantly, the 2D HVSR cross-sections enabled mapping lateral variations in resonance frequencies, highlighting the spatial extent of post-1908 anthropogenic deposits. These findings demonstrate the effectiveness of a multi-method approach in resolving shallow subsurface complexity in highly urbanized areas.
How to cite: Adam Alddoum Adam, M., DAmico, S., De Domenico, D., Panzera, F., Presti, D., scolaro, S., and Totaro, C.: Combined Geophysical Approaches for Urban Subsurface Exploration: A Case Study from Messina, Italy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-398, https://doi.org/10.5194/egusphere-egu26-398, 2026.
The National Research Institute for Earth Science and Disaster Resilience (NIED) has created a shallow and deep integrated ground structure model (SD model) covering approximately one-third of Japan's land area for broadband strong ground motion evaluation. A portion of this model was released by Hedquater Eerthquake Reserch Promotion(HERP) and J-SHIS in March 2021. To create this ground model, over one million borehole data points have been collected and organized nationwide to date. Microtremor array observations and analysis have been conducted at approximately 60,000 locations spaced about 1 km apart (grid cells). Ground amplification coefficients based on AVS30, used to estimate ground amplification within Japan, are primarily calculated from PS logging results at seismic observation points operated by NIED, namely K-NET and KiK-net. However, K-NET has not conducted borehole surveys deeper than 20m in practice, and while KiK-net performs PS logging at depths exceeding 100m, the accuracy of near-surface stratigraphic data is not high. This study aims to verify the accuracy of AVS30-based amplification coefficients using the ground model and to achieve high-precision estimation of amplification coefficients without PS logging. To achieve this objective, microtremor array surveys (small-scale arrays) were conducted at approximately 2,500 sites distributed across K-NET, KiK-net, Japan Meteorological Agency, and municipal seismic observation points. Based on the S-wave velocity structure obtained from microtremor array observations at the aforementioned 60,000 sites, the relationship between AVS30 and ground amplification coefficients was evaluated.
How to cite: senna, S.: Study on Ground Amplification Characteristics Based on the Microtremor Observation Database in Japan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2052, https://doi.org/10.5194/egusphere-egu26-2052, 2026.
Ground-shaking site effects cause localized anomalous macroseismic effects and are often responsible for the greatest damage observed during earthquakes and great loss of life. The best known are effects in surface sedimentary layers, sedimentary basins and valleys.
Less known and thus less investigated are specific site effects such as those related to lateral boundaries of sedimentary layers or sudden changes of the thickness of sedimentary layers. Pioneering numerical-modelling studies by Moczo and Bard (1993) and Kawase (1996) indicated interesting and important phenomena in the semi-infinite horizontal layers for the SH motion and P-SV motion, respectively, in relation to observed macroseismic effects.
We present a unified analysis of seismic motions due to incidence of plane SH, SV and P waves, by investigating simultaneously the translational motion, rotational motion and axial strain rate in the time, frequency and time-frequency domains. We pay a special attention to the effect of damping by considering frequency-independent as well as frequency-dependent attenuation in the sediment layer. We identify the main anomalous characteristics of seismic motion at the free surface of the sediment layer. Receivers located at short distances from the discontinuity undergo significantly larger translational motion than predicted by the local 1D response, and large rotational motion and axial strain rates. At longer distances (up to forty times the layer thickness), significant deviations from the pure 1D behaviour can be seen especially on the rotational motion and axial strain rate, and on the duration of translational motion as well.
How to cite: Moczo, P., Bard, P.-Y., Babaadam, N., Kristek, J., Kristekova, M., and Galis, M.: Effect of a Strong Lateral Discontinuity on Translational and Rotational Motion – Comprehensive Analysis in the Time, Frequency and Time-frequency Domains, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5355, https://doi.org/10.5194/egusphere-egu26-5355, 2026.
Standard Fourier transfer function (modulus of the complex Fourier transfer function) is widely accepted and used for both theoretical/numerical analyses and analyses of real recordings, especially for identifying resonance peaks and/or the amplified frequency bands. This is because it characterizes (linear, time-invariant) transfer properties of the medium between a source and receiver in the frequency domain. Being, however, just an amplitude Fourier spectrum of the time-domain impulse response, it cannot alone provide any information on temporal development of the response, that is, the temporal distribution of the different reverberations that cause amplifications, and therefore on their origin.
However, it is reasonable to assume that the missing information could be useful for interpreting complicated resulting motions in local surface structures because they are often due to multiple wave reflections and transmissions, conversions, interference, diffraction, scattering and resonance.
Therefore, we introduce a new tool for analysing seismic response including its temporal development – a time-frequency transfer function, TFTF, based on the continuous wavelet transform of the impulse response. Modulus of TFTF provides information on the temporal development of frequency-dependent amplification and its duration, linking amplified frequency bands to specific arrivals and reverberation trains rather than to spectral peaks alone.
We present numerical examples for several sedimentary structures close to pure 1D layers, proving that TFTF is much more informative than the standard Fourier transfer function: in particular, it allows to identify late arrivals and long-lasting reverberations, providing a deeper insight on their physical origin.
How to cite: Kristekova, M., Moczo, P., Kristek, J., Babaadam, N., Bard, P.-Y., and Galis, M.: Time-frequency Transfer Function – A New Tool for Modelling and Analysing Site Effects, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5589, https://doi.org/10.5194/egusphere-egu26-5589, 2026.
The continuous Earth’s seismic wave field is predominantly generated in the oceans through nonlinear interactions of ocean surface gravity waves. Using pressure sources close to the ocean surface derived from state-of-the-art ocean-wave model, we simulate secondary microseim surface waves. Previous modeling approaches accounted only for an ocean layer of variable thickness overlying a homogeneous half-space at each source location. Here, we incorporate spatially varying 1-D velocity models from CRUST1.0 at both source and receiver locations. Within this framework, we derive analytical expressions for source and receiver site coefficients that depend solely on local velocity model. Our results show that ocean-bottom sediments can strongly modulate the excitation and amplification of SM Rayleigh waves —by up to a factor 100— consistent with the observations. We further demonstrate that our modeling is valid for frequencies below 0.2 Hz; at higher frequencies, the contribution of Scholte modes must be taken into account to avoid misidentifications of surface wave modes. Finally, we show that this new modeling approach accurately reproduces SM amplitudes recorded both at the ocean bottom and on land.
How to cite: Stutzmann, E., Xu, Z., Farra, V., Devapriyan, D., and Crawford, W.: Theoretical Modeling of Secondary Microseisms Considering Source and Receiver Site Structures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5799, https://doi.org/10.5194/egusphere-egu26-5799, 2026.
Ambient seismic noise H/V spectral ratios are widely applied in the assessment of local site effects, particularly in sedimentary basins, yet their reliability can be compromised in the presence of strong lateral heterogeneity and non-uniform noise source distributions. Disentangling these contributions remains a key challenge for the interpretation of H/V measurements in complex geological settings. In this study, we use three-dimensional numerical modelling to investigate how basin structure and ambient noise source characteristics jointly control H/V amplitude and polarization.
The work is motivated by pronounced temporal variability observed in long-term H/V measurements at a station in the Mygdonian sedimentary basin (northern Greece), including strong changes in amplitude and polarization and occasional shifts in the dominant frequency. The station is located above a blind fault that separates a shallow northern basin from a deeper southern basin, making it a suitable test case for studying site effects in a laterally heterogeneous environment. We compute 3D viscoelastic Green’s functions for a simplified yet geologically representative basin model and exploit source–receiver reciprocity to simulate ambient seismic wavefields for a range of spatially variable surface noise source distributions.
Synthetic H/V ratios are analyzed at receivers located on both sides of the fault to evaluate the sensitivity of site-response indicators to structural contrasts and source directivity. The simulations show that lateral heterogeneities associated with basin geometry and faulting significantly affect H/V amplitudes and polarization patterns, with the strongest effects observed near the fault zone and within the deeper basin. Variations in the spatial distribution of noise sources are identified as a first-order control on H/V measurements and can apparent spatial or temporal variations that mimic structural effects. In particular, sources located in the shallow basin preferentially excite surface waves trapped in the upper layers that propagate toward the deeper basin, imprinting the shallow basin signature on deep basin H/V ratios, while the reciprocal effect is not observed.
Polarization analysis reveals systematic differences across the fault, with preferred orientations generally parallel to the fault trace in the deep basin and perpendicular in the shallow basin, reflecting the underlying structural control. However, strongly directional noise sources can partially obscure this signature, underlining the need for caution when interpreting polarization results based on short time windows of H/V ratios. Finally, comparison with elastic Diffuse Field Theory (DFT) shows reasonable agreement near the H/V peak frequency for isotropic source distributions, but significant deviations arise in the presence of attenuation and strong lateral contrasts. These findings demonstrate the importance of 3D numerical simulations in separating the effects of source distribution and basin structure on H/V measurements, and emphasize the benefit of extended, spatially dense ambient noise monitoring in complex geological settings.
How to cite: Gregor, D., Chaljub, E., Wathelet, M., Hollender, F., Perron, V., and de Martin, F.: Assessing the impact of noise source directivity and 3D basin structure on ambient seismic H/V ratios using numerical modelling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6515, https://doi.org/10.5194/egusphere-egu26-6515, 2026.
Understanding how seismic waves propagate through the subsurface is crucial for evaluating potential impacts on buildings and infrastructure. Velocity profiles provide essential information for seismic hazard mitigation, including estimation of average shear-wave velocity (i.e., VS30), assessment of nonlinear site effects, and numerical wave propagation simulations. These profiles can be obtained using various techniques, such as P-S logging, multiple-channel analysis of surface waves (MASW), and microtremor array analysis. However, differences in resolution and investigation depth among these methods can complicate their integration for geophysical and engineering applications.
In this study, we combine results derived from seismic reflection and microtremor array measurements (MAMs) to construct layered velocity models. Layered P-wave velocity (Vp) profiles are derived from seismic-reflection velocity analysis. The Dix equation is used to convert root-mean-square velocities into interval velocities for each layer. The depths of P-wave velocity interfaces are then used as constraints for S-wave velocity (Vs) inversion, performed using the software HV-Inv (García-Jerez et al., 2016). Monte Carlo sampling and the simplex downhill method are employed to generate ensembles of Vs, Vp, and density profiles, which are evaluated by Rayleigh-wave phase-velocity dispersion curves inversion.
The resulting layered Vp and Vs models enable investigation of the relationships between Vp, Vs, Vp/Vs, and Poisson’s ratio, as well as the factors controlling the variations in the shallow subsurface. These studies aim to provide a robust framework for integrating P-wave and S-wave velocity profiles to characterize shallow seismic site conditions.
How to cite: Juan, Y.-W. and Kuo, C.-H.: Analysis of Shallow Velocity Characteristics Using Vs Inversion Constrained by Seismic Reflection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16108, https://doi.org/10.5194/egusphere-egu26-16108, 2026.
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.
How to cite: Hakimov, F., Hürtgen, J., Havenith, H.-B., and Reicherter, K.: Assessment of Seismic Site Effects in Aachen (Germany) to Support Seismic Microzonation: Geophysical Observations and Numerical Modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21990, https://doi.org/10.5194/egusphere-egu26-21990, 2026.
The free-field strong earthquake stations of the Taiwan Strong Motion Instrument Program (TSMIP), distributed throughout Taiwan, have provided rich and high-quality seismic data over the years. Its data indirectly enhances Taiwan's earthquake-prevention and disaster-reduction capabilities. Due to the importance of seismic site effects in the research of strong-ground motion, the National Center for Research on Earthquake Engineering (NCREE) cooperated with the Central Weathering Administration (CWA) to carry out the Engineering Geological Database for TSMIP (EGDT) containing three site parameters, including Vs30, Z1.0, and κ0, and site classification. However, the CWA gradually renewed TSMIP stations since 2017, leading to the establishment and elimination of some stations. The new stations lack any earthquake site characteristics or classification information, which will make future research and the application of various strong earthquakes difficult. Besides, while Taiwan has produced numerous site characteristic studies based on microtremor or earthquake HVSR analysis, a comprehensive and systematic study of HVSR site characteristics for TSMIP strong-motion stations has been lacking. This study uses continuous seismic data from the GDMS-2020 database of CWA to extract seismic ambient noise and earthquake data for assessing site characteristics and the predominant frequencies of each TSMIP station using HVSR analysis. By establishing representative HVSRs for different site classifications using stations with well-known site parameters, we evaluated the site classifications of new stations without site information. Finally, an HVSR site database of the new TSMIP network is established and comprehensively discussed.
How to cite: Lin, C.-M., Kuo, C.-H., and Huang, J.-Y.: Site characteristics of TSMIP strong-motion stations in Taiwan using the Horizontal‐to‐Vertical Spectral Ratio method, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22169, https://doi.org/10.5194/egusphere-egu26-22169, 2026.
Posters virtual: Tue, 5 May, 14:00–18:00 | vPoster spot 1b
EGU26-21425 | ECS | Posters virtual | VPS24
Evaluating SASW/CSWS-Derived Proxies for Seismic Site AmplificationTue, 05 May, 14:24–14:27 (CEST) vPoster spot 1b
The alternative proxy parameters for seismic site amplification beyond the conventional time-averaged shear wave velocity of the upper 30 m (VS,30) are investigated in this study with a focus on quantities that can be derived or constrained from surface wave-based measurements such as Spectral Analysis of Surface Waves (SASW) and Continuous Surface Wave System (CSWS) testing. Surface wave methods provide dispersion curves that are inverted to obtain near-surface shear wave velocity profiles, which are then used to construct synthetic one-dimensional layered models for ground response analysis. For each profile, two different candidate site parameters are evaluated, including VS,30 and the impedance ratio between the surface layer and the underlying half-space. These parameters are chosen to reflect what can realistically be inferred from SASW/CSWS-derived velocity profiles, particularly the shallow stiffness and impedance contrasts that strongly influence amplification. Correlation analyses are carried out to quantify how well each parameter explains the variability in amplification across the synthetic suite. The results are used to assess whether the impedance ratio provides stronger or more consistent correlation with amplification than VS,30, thereby offering guidance on how surface wave–based site characterization can be better integrated into proxy-based amplification and site classification schemes in seismic design practice.
How to cite: Singh, V. and Baidya, D. K.: Evaluating SASW/CSWS-Derived Proxies for Seismic Site Amplification, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21425, https://doi.org/10.5194/egusphere-egu26-21425, 2026.
