Assessing the influence of local site conditions on seismic ground motion is crucial for seismic hazard analysis and earthquake engineering research and applications. This study analyzes site effects in the Kumamoto area, Japan, using 985 high-quality horizontal strong-motion records from 45 aftershocks (Mj = 2.7-4.9) recorded within 24 hours following the 2016 Kumamoto M_j 7.3 earthquake, as observed by 51 K-NET and KiK-net stations. For the generalized inversion technique (GIT), a reference station is required as a standard. In the GIT process, the number of events available for analysis is limited to those recorded by the reference station, and the stations whose site effects can be estimated are restricted to those that record common events with the reference station. To overcome this limitation, we apply the “transfer-station generalized inversion method (TSGI),” a modified GIT, to obtain the site responses for all stations and the average S-wave quality factor (Q_S) in the study area. It is found that Q_S is proportional to frequency in the 0.4-3 Hz range, while at frequencies above approximately 3 Hz, the dependence of Q_S on frequency becomes weak and Q_S can be regarded as constant. However, the results of GIT and TSGI are relative to the reference station, which may itself exhibit site effects. Therefore, we additionally apply a reference-independent technique, i.e., genetic algorithm (GA), to obtain the absolute site amplifications. Our result shows that at frequencies greater than about 1 Hz, the site response of the reference station is substantially lower than the theoretical amplification factor of 2, resulting in an overestimation of the site responses at other stations. When the results of GIT are corrected with the site response of the reference station obtained from GA, these two results agree very well for most of the stations. This indicates that the results of GIT are reliable if the reference station is an ideal surface rock station. The GA method yields accurate absolute site amplification factors for the stations investigated this study, demonstrating the effectiveness of GA in site effect analysis. In addition, we analyze the characteristics of S-wave high-frequency attenuation parameter (κ) in the Kumamoto area, and establish κ models for different site conditions and an empirical κ₀-V_S30 relationship.

How to cite: Zhang, W. and Zhou, T.: Estimation of site effects in the Kumamoto area, Japan, using aftershock acceleration records of the 2016 Kumamoto Mj 7.3 earthquake, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-222, https://doi.org/10.5194/egusphere-egu26-222, 2026.

The 2020 earthquake sequence in Croatia caused significant damage, particularly to cultural assets and older masonry buildings in areas of pronounced topography in Northern Croatia (EMS intensity VI). The observed damage distribution aligns closely with topographical features, with higher intensities recorded in hilly areas—such as Hrvatsko Zagorje, Ivanščica, Kalnik, and Međimurje—compared to adjacent alluvial basins.

To investigate these phenomena, this study presents results from microtremor measurements using the Horizontal-to-Vertical Spectral Ratio (HVSR) method across five localities characterised by distinct geological and morphological configurations. We integrated HVSR fundamental frequencies with local geological data to derive detailed seismic microzonation maps that quantify the terrain's resonance potential. These maps illustrate critical correlations between the slope/height of the dominant hill axis and the measured site frequencies.

Our analysis confirms that topographic site effects are primarily driven by the focusing of seismic waves at ridge crests, a process governed by diffraction, reflection, and wave type conversions. It is observed that amplification is highly frequency-dependent; resonance is strongest when the incoming wavelength aligns with the ridge’s frequency characteristics. Furthermore, the steepness of the topography plays a major role, with the uppermost portions of hills consistently showing stronger resonant motion than lower slopes.

Preliminary site amplification factors calculated for the 2020 earthquake scenarios (Zagreb Mw 5.4 and Petrinja Mw 6.4) reveal complex interactions between topographic irregularities and wave propagation. These findings underscore the necessity of explicitly incorporating topographic site effects into seismic microzonation studies. This approach is essential for producing reliable ground-shaking models and refining the local seismic-hazard assessment, particularly for preserving vulnerable historical structures in seismically active regions of Northern Croatia.

How to cite: Stanko, D., Novak, L., Jug, J., Hrnčić, N., Markušić, S., and Kovačić, M.: Assessing Topographic Site Effects and Seismic Microzonation in Northern Croatia: Case Study Insights from the 2020 Earthquake Sequence, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6376, https://doi.org/10.5194/egusphere-egu26-6376, 2026.

Reliable shear-wave velocity (V_S​) profiles and their quantified uncertainties are essential for the robust characterization of site conditions and the accurate interpretation of seismic waveforms. This study presents uncertainty-quantified V_S​​ profiles extending from the surface to depths of approximately 1 km for 100 seismic strong-motion stations across the southern Korean Peninsula. At each site, active and passive surface-wave dispersion data were acquired via microtremor array measurements and multichannel analysis of surface waves, spanning a broad frequency range from ~1 to 10 Hz and ~5 to over 50 Hz, respectively. These datasets were jointly inverted using a trans-dimensional and hierarchical Bayesian framework, which treats the number of layers and dataset-specific error levels as unknown parameters. This approach yields an ensemble of V_S​​ profiles for each station, which inherently captures depth-dependent uncertainties. From these ensembles, key seismic site parameters, including V_S​30​, bedrock depth, and resonance frequency, were estimated with rigorous uncertainty bounds to construct a comprehensive site flatfile. The estimated profiles and parameters were validated against independent in-situ borehole data, showing high consistency within the quantified uncertainty intervals. Furthermore, we derived region-specific regression equations among the site parameters, facilitating the generation of high-resolution maps for site parameters and their associated uncertainties. These outputs provide a foundation for correcting site effects in seismic waveforms, refining site terms in ground-motion prediction equations, and supporting regional seismic hazard assessments.

How to cite: Jeon, Y. and Kim, S.: Uncertainty-Quantified VS​ Profiles for 100 Strong-Motion Stations and Regional Site-Parameter Maps in the Southern Korean Peninsula, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16331, https://doi.org/10.5194/egusphere-egu26-16331, 2026.

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.

How to cite: van Ginkel, J., Janusz, P., Shynkarenko, A., and Bergamo, P.: Developing a seismic soil class map for Switzerland using geophysical parameters , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6651, https://doi.org/10.5194/egusphere-egu26-6651, 2026.

Gas extraction from the Groningen field has induced a substantial number of earthquakes that, despite their typically low magnitudes, produce notable ground motions at the surface due to their shallow depths of approximately 3 km. These ground motions pose risks to society and infrastructure. Therefore, an accurate ground motion simulation is essential for seismic hazard assessment. Previous studies have demonstrated that near-surface unconsolidated layers significantly influence ground motion amplification. However, less attention has been devoted to understanding the role of deeper structures. In the Groningen region, significant amplification and de-amplification effects are anticipated due to the complex subsurface, thickness variations across relatively short lateral distances, compositional heterogeneity within sedimentary sequences, and the presence of the Zechstein salt layer overlying the reservoir formation.

This study investigates how subsurface heterogeneity, both shallow and deep, affects seismic wave propagation and the corresponding ground motion observed at the surface. To analyze this, we employ 3D full waveform modeling using the spectral element method (SEM). First, to optimize mesh resolution, determined by the local S-wave velocity and the target design frequency, we conduct simulations across a range of frequencies and corresponding spatial resolutions to analyze their impact on wavefield accuracy and computational cost. Second, we simulate seismic wave propagation through a synthetic velocity model representative of the Groningen subsurface and compute Peak Ground Acceleration (PGA) for different earthquake scenarios using various Centroid Moment Tensor (CMT) source solutions. Since amplification effects are highly location-dependent, we evaluate multiple earthquake scenarios with varying source characteristics and locations. We then compare these results with PGA values computed for a homogeneous half-space model that preserves the bulk elastic properties of the realistic heterogeneous model, using identical earthquake sources. This comparison produces amplification factor maps that reveal distinct spatial patterns of amplification and de-amplification across the study region. To isolate the contributions of individual factors, we examine the influence of source frequency, the depth and thickness of velocity layers, the presence of velocity inversions within the stratigraphic sequence, and subsurface interface topography.

These tests allow us to identify how each parameter contributes to the resulting amplification and de-amplification patterns. This framework can provide physical explanations for the spatial distribution of observed ground motion variations, offering valuable insights that are instrumental for current and future seismic hazard assessments in areas of subsurface resource exploitation throughout the Netherlands.

How to cite: Khoveiledy, T., Fadel, I., Dahal, A., and van der Meijde, M.: Effects of subsurface heterogeneity on ground motion amplification in Groningen, the Netherlands, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13455, https://doi.org/10.5194/egusphere-egu26-13455, 2026.

The seismic response of the Taipei Basin is heavily influenced by its basin geometry and thick sedimentary deposits. These conditions focus seismic energy and govern surface shaking, resulting in prolonged durations and enhanced long-period content that pose significant risks to high-rise buildings and critical infrastructure. In structural design, the corner period (T₀) of the response spectrum is a concise measure of frequency content that integrates source, path, and site effects. With the basin's dense strong-motion network, this study directly derives T₀ from observations to refine seismic microzonation and design evaluations.

This study compiled a comprehensive dataset of moderate-to-large earthquakes (M_W ≥ 5 or local PGA ≥ 10 gal) recorded in Taiwan from 1992 to 2024. Records underwent a unified processing workflow of baseline correction, filtering, and 5%-damped response spectra generation, after which events were categorized into crustal, subduction-interface, and intraslab types. For each category, the T₀ was determined using the mean plus one standard deviation spectrum as the target. Results indicate pronounced spatial variations in T₀ for crustal and subduction-interface earthquakes. Values are longest in the northwestern to north-northeastern regions (exceeding 1.5 sec) and shortest along the southeastern region. In contrast, intraslab events exhibit minimal spatial variation. Correlation analysis confirms that T₀ distribution is strongly controlled by geological conditions, specifically bedrock depth and sediment thickness. By incorporating these geological parameters into spatial interpolation, this study enhances the resolution and physical interpretability of the microzonation, providing a more robust and detailed reference for seismic design in the Taipei Basin.

How to cite: Yang, C.-H., Gao, J.-C., Dong, J.-J., and Lee, C.-T.: Improving Seismic Microzonation of the Taipei Basin Using Response Spectra and Geology Informed Interpolation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4355, https://doi.org/10.5194/egusphere-egu26-4355, 2026.

Extensive microtremor surveys can provide valuable constraints on the deep structure of seismically active Quaternary intramontane basins. This study investigates two study areas, namely the Colfiorito and Annifo basins, which were affected by a seismic sequence during the years 1997–1998  [1]. Within the framework of the First ILGE Transnational Access (TNA) and National Open Access (NOA) Call (within the PNRR MEET WP3 project), a dense microtremors dataset was acquired in 2024 to improve the geological characterization of the two Quaternary basins. A total of 160 single-station microtremor measurements were collected over six days using 48 seismic nodes equipped with 4.5 Hz triaxial sensors, with recording time windows ranging from a few hours to two days. Two helicoidal nodal arrays were deployed in the northern Colfiorito plain and an additional one was installed in the southern area of the Annifo basin, to derive detailed shear-wave velocity profiles [2]. Moreover, two temporary stations equipped with 5-s Lennartz sensors and Reftek dataloggers were also operated for a few days. H/V spectral ratio analysis was carried out and revealed contrasting behaviors between the two basins. In Annifo, H/V peaks exceed 1.0 Hz, with the main depocenter located in the southern part of the basin. In Colfiorito, two main H/V frequency ranges have been observed: one is characterized by low-frequency peaks, between 0.6 and 1.0 Hz, located between the central and northeastern sectors of the basin, whilst a second, with frequencies between 1.0 and 6.0 Hz, characterizes the rest of the basin. In Colfiorito, the spatial distribution of resonant frequencies is consistent with a recent and independent gravimetric survey results [3], which identifyed two significant gravity minima in the central sector of the basin.

Acknowledgments

This publication results from work carried out under transnational     /national open access  (TNA/NOA) action under the support of WP3 ILGE - MEET project, PNRR - UE Next Generation Europe program, MUR grant number D53C22001400005.

References

[1] Messina, P.; Galadini, F.; Galli, P.; Sposato, A. Quaternary Basin Evolution and Present Tectonic Regime in the Area of the 1997–1998 Umbria–Marche Seismic Sequence (Central Italy). Geomorphology 2002, 42, 97–116, doi:10.1016/S0169-555X(01)00077-0.

[2] Di Giulio, G.; Cornou, C.; Ohrnberger, M.; Wathelet, M.; Rovelli, A. Deriving Wavefield Characteristics and Shear-Velocity Profiles from Two-Dimensional Small-Aperture Arrays Analysis of Ambient Vibrations in a Small-Size Alluvial Basin, Colfiorito, Italy. Bulletin of the Seismological Society of America 2006, 96, 1915–1933.

[3] Di Filippo, M.; Mancinelli, P.; Cavinato, G.P.; Pauselli, C.; Sabatini, A.; Mirabella, F.; De Franco, R.; Barchi, M.R. Bouguer Gravity Anomaly in the Colfiorito Quaternary Continental Basin, Northern Apennines, Central Italy. Journal of Maps 2025, 21, 2503244, doi:10.1080/17445647.2025.2503244.

How to cite: Ercoli, M., Di Giulio, G., Porreca, M., Safarzadeh, E., Alaia, G., and Brunori, C. A.: Revealing the buried structure of Apennines intermontane basins through dense nodal microtremor surveys: the case of Colfiorito and Annifo basins (central Italy)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3884, https://doi.org/10.5194/egusphere-egu26-3884, 2026.

Seismic-wave attenuation in near-surface deposits is a key factor in site-effect modelling and local seismic hazard assessment. We investigate S-wave attenuation at station CTL8 of the Italian National Seismic Network, located in the Po Plain (northern Italy), by applying the borehole-to-surface deconvolution technique and comparing the results with estimates obtained using the kappa-based approach. The station is equipped with a surface accelerometer and a borehole velocimeter installed at 162 m depth, providing a suitable configuration for near-surface attenuation studies.

The analysed dataset consists of 109 pairs of surface and borehole recordings selected for their high signal-to-noise ratio, associated with local earthquakes with magnitudes between 3.0 and 5.8 and epicentral distances ranging from 36 to 256 km. Assuming predominantly vertical S-wave propagation between the borehole and the surface, identical time windows around the S-wave arrival were selected on the transverse component. The orientation of the borehole sensor was determined using tele-seismic events and corrected prior to the analysis.

Following the deconvolution procedure, up-going and down-going S-wave pulses were successfully isolated in the time domain. The spectral ratio between these pulses was used to estimate attenuation, yielding a surface-borehole kappa difference of Δκ₁₆₂= (11.3 ± 1.1) ms. The time separation between the pulses also allowed the estimation of the time-averaged S-wave velocity between the borehole and the surface, resulting in Vs₁₆₂ = (364 ± 7) m/s.

The results are consistent with previous estimates obtained at the same site using standard kappa-based methods and with synthetic deconvolution signals derived from a previously developed velocity profile. These findings indicate that borehole-to-surface deconvolution is a reliable and complementary tool for estimating near-surface attenuation and average S-wave velocity, provided that sufficient borehole depth and data quality allow a clear separation of the up-going and down-going wavefields.

How to cite: Franceschina, G. and Tento, A.: Estimating S-Wave Attenuation in Sediments by Deconvolution Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13214, https://doi.org/10.5194/egusphere-egu26-13214, 2026.

The Alhama de Murcia Fault (AMF), located in southeastern Spain, is one of the most active and hazardous fault systems in the region due to its elevated tectonic activity and its capacity to generate damaging earthquakes. The most recent significant event, the 11 May 2011 Lorca earthquake (Mw 5.1), resulted in nine fatalities, numerous injuries, and substantial material losses. While some authors interpret this earthquake as a purely natural occurrence, others suggest that its rupture may have been influenced by crustal unloading processes associated with groundwater extraction, potentially affecting the timing of the event. This debate underscores the importance of distinguishing between natural and induced seismicity in regions with high societal vulnerability.

Previous studies on the AMF have focused on its structural characteristics, seismic activity, and hazard potential through various methodologies, including paleoseismology and satellite data. However, integrated multidisciplinary analyses remain limited.

As a part of the MADRIZ project, this study aims to advance the seismic characterization of the AMF by compiling and reanalyzing seismic data from the nearest stations of the Spanish National Seismic Network, accessible through the EPOS Data Portal, together with open-access data from additional seismic networks that operate in the region. By applying both one-dimensional and three-dimensional location methods in conjunction with digital waveform analysis, we obtain highly precise hypocentral locations. These solutions form the basis for calculating focal mechanisms to better constrain the geometry and kinematics of active faults in the study area.

This integrated approach provides new insights into the seismic behavior of the AMF, contributing to the ongoing discussion on the interplay between natural tectonic processes and potential anthropogenic influences, and ultimately supporting more refined seismic hazard assessments for southeastern Spain.

How to cite: Núñez, D., Roman, D., Martínez, C., Córdoba, D., Carrillo, R., and Fernández, J.: Active Faulting in Southeastern Spain: New Evidence from the Seismic Characterization of the Alhama de Murcia Fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18842, https://doi.org/10.5194/egusphere-egu26-18842, 2026.

How to cite: Ayoqi, N. and Marchetti, E.: Detailed 3D bedrock geometry in the Firenze area from HVSR seismic noise measurements, seismic noise inversion and dispersion curves of surface waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2237, https://doi.org/10.5194/egusphere-egu26-2237, 2026.