The ambient vibration Horizontal to Vertical Spectral Ratios (HVSR) is a widely used technique to identify the seismic resonance phenomena induced by the presence of seismic impedance contrasts at depth. Moreover, the HVSR curve can be used to constrain the shear wave velocity (Vs) profile in numerical inversion procedures: for this purpose, different HVSR forward modeling were developed in the last decades, which differ from each other both for the basic theoretical assumptions related to the ambient vibration wavefield simulation and for the phases of the involved seismic waves. Recently, some works showed that strong similarities between these HVSR models exist. In particular, these approach were considered: one based on vertically propagating body waves; one based on the ellipticity of the fundamental mode of Rayleigh waves; one based on the contribution of uniformly distributed random sources at the surface and the full wavefield and one based on a diffuse random wavefield assumption. As concerns the last two models, full wavefield and surface wave only were taken into account. In view of these conclusions, the aim of this work is to perform a comparison of the different theoretical HVSR modeling with experimental HVSR curves. To accomplish this purpose, HVSR measurements were carried out at test sites belonging to the down-hole database of the Tuscany Region administration (Central Italy; https://www.regione.toscana.it/-/banca-dati-vel). In particular, more than 50 sites with Vs profiles characterized by the presence of the seismic bedrock (Vs≥800m/s) and strong impedance contrasts were selected. Velocimetric acquisitions were carried out using the three-directional 24-bit digital tromograph Tromino™ (https://moho.world/) and the ambient vibrations were acquired for 20 min with a sampling frequency of 128 Hz. In particular, the spectra of the single components were computed by averaging 20-s-long non-overlapping windows; a detrend and a 5% cosine taper were applied to each window, and the spectra were smoothed by using a triangular moving window with a frequency-dependent half-width (10% of central frequency). The horizontal components were combined with the geometric average. Theoretical HVSR curves were simulated considering the models mentioned above and taking into account the Vs and Vp profiles of the selected down-holes; density values were deduced from Vp by empirical relationships and, for not purely elastic models, damping values for Vp and Vs are assumed equal to 0.01 for all the layers. Finally, these curves were compared with the respective experimental ones in order to evaluate the differences in terms of frequency and peak amplitude as well as of overall trend.

How to cite: Tanzini, A., Paolucci, E., and Albarello, D.: Comparison using different models between theoretical and experimental HVSR curves., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1778, https://doi.org/10.5194/egusphere-egu23-1778, 2023.

Kashmir Valley is one of the two administrative divisions of the Jammu and Kashmir region, lying within the Himalayan belt at the confluence of the Indian and Eurasian tectonic plates. It is an elongated basin filled with deep sedimentary deposits in the central portion, bounded by the Himalayan Mountain ranges at the periphery. The Pleistocene Karewa deposits overlain by the Recent Alluvium form the major types of sediments in the valley, showing a thickness of over 1300m (Burbank and Johnson, 1982). To evaluate the seismic response of these geological deposits, we recorded microtremors and performed multichannel analysis of surface waves (MASW) testing at ~190 sites using TROMINO^® equipment. Horizontal-to-vertical-spectral ratio (HVSR) curves and dispersion curves were respectively developed from the raw data, which were further used to estimate the site fundamental frequency (f₀) and H/V amplitude (A) as well as average shear wave velocity over 30m depth (V_S,30). A comparison of f₀ and A with V_S at each site was conducted to assess the suitability of codal provisions (NEHRP, EC8, etc.) and the specified amplification factors.

We found certain issues with the use of V_S,30 as the sole parameter for estimating the expected amplification at a site for seismic design. A direct correlation between amplification and stiffness (V_s30) at the sites was not attained, as opposed to the most common assumption in the prevalent code-based site classifications. Low amplification can be expected even at soft soil sites (NEHRP E) in the absence of an impedance contrast, and vice versa in stiff deposits. Observations in the deep sedimentary deposits in the valley show the importance of the contribution of deeper stratigraphy beyond the assumed 30m depth in the codes. Moreover, the notion of f₀ being directly proportional to V_s30 irrespective of the geological conditions is not true. Besides, rock sites (NEHRP A and B) may not always present the conventional flat HVSR response, instead, significant high frequency amplifications can occur due to weathered or fractured material. Test results also revealed unusually high amplitudes within the fractured portion of a fault zone, signifying that the assumption of all rock sites being safe in terms of negligible amplifications may not be entirely correct. Topographic amplifications near basin edges, hill slopes, and within small valleys resulted in broad peaks over wide frequency range in the HVSR curves, which cannot be explained through the limited information contained in the V_s30 parameter.

These incongruities observed at various sites in the Kashmir region point out the inadequacy of the site classifications based on the single-parameter (V_s30)approach, which do not always explain the actual amplifications observed during earthquakes in different geological conditions. The results obtained in the Kashmir Valley, backed by the similar arguments provided in literature (e.g., Lombardo and Rigano, 2006; Castellaro et al., 2008; Rovelli et al., 2009; Panzera et al., 2014; etc.), clearly indicate the need for adopting alternate site classification schemes (Di Alessandro et al., 2012; Pitilakis et al., 2019; Paolucci et al., 2021).

How to cite: Zahoor, F., Ansari, A., Rao, K. S., and Satyam, N.: Atypical Site Effects and Insufficiency of the Conventional Seismic Site Classification Methods: Experimental observations in the Geological Deposits of the Kashmir Valley (NW Himalayas), Jammu and Kashmir, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15497, https://doi.org/10.5194/egusphere-egu23-15497, 2023.

The alpine valleys are peculiar geological environments, as the thickness of their sedimentary infill increases from few meters at the valley borders to several hundred meters at its centre. This setting determines distinctive earthquake response effects. A number of works have examined such effects, which can be summarized as: i) 2D/3D resonance phenomena; ii) edge-generated surface waves; iii) trapped seismic waves (i.e. waves remaining trapped in the valleys’ basin due to the impedance contrast with the surrounding bedrock). Although several studies have tackled one or more of such effects, only recently attempts have been made to systematically identify – in numerical modelling studies – which subsurface parameters control these phenomena and to quantify their effect on local site response.

In the framework of the “Alpine valleys” project, we have compiled a database to empirically observe and systematically characterize earthquake site effects in alpine valleys at the national scale of Switzerland; this work builds on the data and results of the national amplification model developed in the framework of the project Earthquake Risk Model for Switzerland. The “Alpine valleys” database comprises earthquake observations, geophysical measurements and morphological, topographic parameters. In particular, the database includes:

• A dataset of empirical amplification functions estimated (by means of spectral modelling technique) at about 275 (urban) free-field seismic stations of the Swiss networks. These instrumented sites cover a variety of geological and morphological settings (alpine valley beds and flanks, as well as sites in the plains of the Swiss Plateau for comparison with the former).
• A dataset of waveforms recorded by the seismic stations mentioned above, collecting local and regional earthquake records from the period 2000-2022.
• A dataset of measured S-wave velocity profiles, derived from geophysical measurements performed at a portion (~120 sites) of the set of considered seismic stations.
• A dataset of 1750 single-station noise recordings, processed in terms of horizontal-to-vertical spectral ratio. 160 of such measurements coincide with a seismic station with measured site amplification function.
• A map of the unconsolidated sediments-bedrock interface, covering all Switzerland. The map allows to quantify the geometry of the valleys’ sedimentary infill (e.g. depth, width, shape ratio), one of the key elements determining the peculiar site effects observed in valley beds.
• Maps of multi-scale topographical parameters. Maps of planform, profile and standard curvature were derived for the entire Swiss territory, at 7 spatial scales between 75 and 7800 m.
• A classified map of the Swiss alpine valley beds, obtained with a GIS algorithm combining topographical and morphological data.

The ambition of our work is cross-referencing the empirical earthquake observations (waveforms, local amplification functions) with geophysical measurements and geological, topographic and morphological parameters, in order to single out the site effects proper of deeply incised valleys, identify the frequencies of ground motions where these appear, and map the morphological and geological settings where they become significant. The implications for the soil classification of the Swiss seismic building code will also be assessed.

How to cite: Bergamo, P., Chieppa, D., Panzera, F., Perron, V., and Fäh, D.: A database for the empirical observation and characterization of earthquake site effects in alpine valleys, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6904, https://doi.org/10.5194/egusphere-egu23-6904, 2023.

The time averaged shear wave velocity of the top 30 meters (Vs30) is the most widely used parameter for the geotechnical characterization of site conditions. However, the spatial availability of Vs30 observations are rather limited except specific areas where conducted micro-zonation studies include closely spaced measurements suitable for assessment of earthquake site effects. In order to infer Vs30, global models use slope or morphological terrain classes as proxies. In a regional scale, these proxies are commonly combined with geologic and geotechnical data to improve the accuracy of Vs30 predictions. So far, a region specific Vs30 model that would aid seismic hazard assessments is not yet constructed for Türkiye and its near vicinity. In this study, a new Vs30 prediction strategy is developed using data from Türkiye and California, and its performance is compared with others.

At first, Vs30 measurements are classified into 4 sedimentary rock classes according to their ages (Quaternary-Pliocene, Miocene, Paleogene, Pre-Paleogene) and 3 non-sedimentary rock classes (Intrusive, Extrusive, Metamorphic). Observations from Quaternary-Pliocene rocks are most abundant and characterized by large data scatter, thus further divided into 2 major terrain classes. Since the reduction in Vs30 due to fluid saturation is pronounced, especially in unconsolidated young units, Quaternary-Pliocene rocks are also differentiated as saturated if the water table depth is less than 30 meters and unsaturated otherwise. In California, saturation is determined by using available groundwater measurements. Throughout Türkiye, flat areas with elevation differences less than 30 meters from water bodies (sea, lake, and major rivers) are mapped out as saturated zones. After the elimination of outliers, slope and elevation based Vs30 prediction equations are developed separately for sub-classes of Quaternary-Pliocene, Miocene, and Paleocene aged sedimentary rocks using multi-variable linear regression while Vs30 is fixed to class average in others. Resultant model misfits and comparisons with results of micro-zonation study conducted across İstanbul, clearly indicate that our proposed Vs30 prediction strategy is performing better, especially in younger sedimentary units and thus provide a new, more accurate Vs30 model of Türkiye. 

How to cite: Okay, H. B. and Özacar, A. A.: A New Vs30 Prediction Strategy Taking Topography, Geology, Terrain and Water Saturation into Account: Application to Türkiye and California, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7445, https://doi.org/10.5194/egusphere-egu23-7445, 2023.

The importance of vertical ground motions for design continues to gain recognition, as more evidence shows that vertical ground motions can significantly exceed their horizontal counterparts on very soft soils, at short source-site distances, and at short spectral periods. Assuming that the vertical component is largely comprised of compressional P-waves, new approaches and models are required to constrain the expected linear and nonlinear site response of the vertical component.

In this study, we combine empirical analysis with laboratory experiments, to study and define the nonlinear behavior of the vertical component. We use 27 Kik-net stations to analyze nonlinearity of dry sandy deposits, comparing the full vertical component with the P-wave window, to help define the partial contribution of P-waves to the entire vertical component. We develop modulus degradation and damping (MRD) curves for the case of uncoupled and coupled shear-compression response. In addition, we compare the empirical MRD curves with experimental MRD curves, describing the response of sandy soil to cyclic compressional loading under Ko conditions. We show that vertical ground motions are less nonlinear than their horizontal counterpart, for the same incoming ground motion. We also show that pure P-waves are less nonlinear than the full vertical motion, suggesting that the vertical component is comprised of a combination of P and SV waves, thus implying that vertical site-response analysis should include both shear and compression-related properties and procedures.

How to cite: Kamai, R. and Frid, M.: Nonlinear site-response of vertical ground motions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4093, https://doi.org/10.5194/egusphere-egu23-4093, 2023.

Liquefaction hazard analysis is critical to the safety and cost-effectiveness of structures. Ramsar, the westernmost city of northern Iran's Mazandaran province, borders the Caspian Sea to the north. Since the soil strata of the Ramsar region are predominantly composed of low-grade sand and the groundwater is at low depths, it is highly vulnerable to liquefaction. Understanding how sedimentary basins react to seismic energy generated by earthquakes is a major concern for seismic hazard assessment and risk analysis. The primary purpose of this study is to determine the distribution of the natural frequency value, the amplification factor value and the soil vulnerability index. These were carried out as indicators for potential liquefaction sites in the city of Ramsar based on measurements of seismic ambient noise or microtremors. In this regard, ambient seismic noise was collected at 100 stations in the Ramsar region by Nanometrics Trillium40s intermediate sensors. Data are processed using the horizontal-to-vertical spectral ratio (HVSR) method provided by Nakamura (1989). Based on the results, the vulnerability index Kg is determined, which can be used as a parameter for calculating the region's liquefaction potential. Huang and Tseng (2002) suggested that the HVSR of microtremor data can be a good alternative indicator of the liquefaction potential of an area. It is revealed by previous researchers that improving the accuracy of geology and geomorphology-based liquefaction susceptibility map is accomplished by supplementing it with subsurface data (e.g., SPT, CPT, shear wave velocity data). The results of this study show that with the use of seismic ambient noise, the accuracy of the assessment of liquefaction susceptibility and zonation improves substantially.

How to cite: Shabani, E.: Site response and liquefaction hazard analysis in Ramsar city, North of Iran, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4084, https://doi.org/10.5194/egusphere-egu23-4084, 2023.

The Outer Western Carpathians represent an accretionary wedge of the Alpine-Himalayan orogenic zone in central Europe, where the Mesozoic and Cenozoic sedimentary (flysch) rocks were deformed and thrust over the European foreland during the Paleogene and Neogene. The thrusting processes terminated in the Upper Miocene, and the contemporary instrumental earthquake distribution records suggest that OWC are already stabilized and belong to the European Plate. This hilly to mountainous region has been intensively affected by various types of shallow slides, debris flows as well as deep-seated slope failures of different magnitudes. Due to the lack of direct evidence of intense seismicity, the permafrost thawing on a turn of the Pleistocene and intense rainfalls in the Holocene have been considered as their principal triggers. However, our current research revealed landforms attributed to the coseismic Holocene polyphase strike-slip faults´ surface ruptures. The associated coseismic sedimentary structures include injected sand and flame structures of fluviolacustrine sediments, large angular boulders in riverbed fluvial sediments, etc. Alongside with those structures and landforms, we documented also evidence for coseismic slope failures in the close surrounding of the faults near Lidečko Village and in the summit area of the Javorníky Mts. in the E part of the Czech Republic and north-western Slovakia, respectively. Several distinct faulting phases with offsets up to a few meters were dated from ca. 10.000 ¹⁴C ka up to 1240 ¹⁴C ka BP. Morphological analysis of LiDAR digital elevation models with a 1 m resolution revealed clustered populations of prehistoric presumably coseismic landslides that accompany the fault traces at both sites for a distance up to a couple of kilometres. Field inspections allowed description of their source zones, which were usually several meters up to first tens of meters deep and up to 500 m long. The landslide masses travelled for a distance up to 1-2 km, while being often transformed to debris flows and rock avalanches. They are characteristic with their distinctly depleted source zones at generally gentle slopes ranging from 8 to 20°. The contribution presents the faulting styles, particular events and associated coseismic landslide characteristics in detail and broader regional context, providing the first comprehensive evidence for possible coseismic origin of the deep-seated slope failures in the Outer Western Carpathians.

The research was supported by the international bi-lateral project “Earthquake-triggered landslides in recently active and stabilized accretionary wedges” of the Czech Science Foundation (GAČR 22-24206J) and the Taiwanese Ministry of Science and Technology (MOST 111-2923-M-008-006-MY3).

How to cite: Baroň, I., Melichar, R., Hartvich, F., Bíl, M., Klimeš, J., Černý, J., Šutjak, M., Kociánová, L., Pánek, T., Janál, J., Braucher, R., Dong, J.-J., Hu, J.-C., Tseng, C.-H., Chen, Y., and Nguyễn, T.-T.: Direct field evidence for polyphase active faulting and associated coseismic landslides in the accretionary wedge of the Outer Western Carpathians, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15165, https://doi.org/10.5194/egusphere-egu23-15165, 2023.

    Since the 1999 Chi-Chi Earthquake, earthquake engineering has devoted much effort to the issues of fault rupture-engineering structure interaction in Taiwan. So far, the Central Geology Survey of Taiwan has announced 36 active faults. Due to natural terrain conditions, linear traffic structures are difficult to avoid crossing the fault zone. However, the current design code rarely considers the effects of long-term fault creeping displacement in the assessment of structural performance. This study aims to develop a numerical-based performance examination approach for the viaduct with pile group foundations subjected to long-term fault offset. The area where the High-Speed Rail of Taiwan crosses the Chegualin Fault was selected as the case study. Remote-sensing survey and structural monitoring both indicated that the viaduct has been offset by approximately 30 cm due to fault creeping since 2006. The ratio between right-lateral and uplift movements is approximately 7:1. This study adopted coupled FDM-DEM technique as the numerical tool, and first validated its performance by physical sandbox tests. We used non-cohesive soil as the overburden material and polyethylene hollow foam tube as the foundation piles. Several key factors, including the Riedel shear bands, fault extended distance, tri-shear zone, pile cap deformation, were compared between numerical and sandbox models. Next, full-scale numerical modeling was calibrated based on in-situ structural monitoring data. The comparison of bridge pier displacement shows approximately 80% agreement between simulation and monitoring data. We found that the deformation of piles was dominated by the location of the fault tip. Simulation shows that the maximum rigid-body rotation of the pile cap will occur close to the fault tip. The cap of each pile group exhibits differential rotations and displacements behaviors, resulting significant superstructure distress. Based on the simulation, the performance of the High-Speed Rail in the study area was expected to fail meeting the current design code of Taiwan after 50 years.

How to cite: Liu, Y.-L., Lin, C.-H., and Lin, M.-L.: Examination of the performance of viaduct with pile groups underoblique-slip fault creeping: a case study in Chegualin Fault, Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3023, https://doi.org/10.5194/egusphere-egu23-3023, 2023.