Seismic Site Effects Research Articles

Seismic site effects in Lisbon: the role of complex geological and morphological conditions

Lisbon’s historical seismicity, socioeconomic importance and population density contribute to a moderate to high seismic risk. The geological setting of the city includes cases of inclined layers, interbedding sedimentary rock layers in soil deposits, sand and clay layers in the same geological unit, leading to cases of shear wave velocity inversion and a large scatter of geotechnical properties within each geological unit. The morphological setting of the city is characterised by the existence of several hills and relatively shallow, stream-carved valleys filled with alluvial deposits. The seismic site effects in Lisbon were assessed through numerical simulation using the linear equivalent method and adopting the two types of seismic action defined in the Portuguese National Annex of Eurocode 8: (i) one-dimensional subsoil models covering the city, at sites where borehole data and geophysical data were available; (ii) two-dimensional subsoil models along three cross-sections representative of the geological settings and morphology. The distribution of amplification factors in the city revealed a pattern related to ground characteristics that impact seismic soil response, such as the presence of high-thickness cover deposits, significant shear-wave variations, alluvial valleys, a crest or significant slope variations and inclined layers. The 2D/1D spectral ratio highlighted the areas were 2D seismic effects are more important. The soil factor determined in the numerical analyses was consistently greater than the soil factor values indicated in Eurocode 8.

Are the Italian microzonation level 2 abacuses applicable in the Friuli Venezia Giulia (Italy) plain? Comparison between the national abacuses and the numerically simulated amplification factors and between the derived elastic response spectra

Seismic shaking of an area is strongly affected by the local geology. The so-called local site effects must be considered for the estimation of seismic effects on structures and urban planning. Thus, the seismic microzonation is the process aimed at identifying and mapping the subsoil local response in a given area, usually at urban/municipality scale and in terms of ground shaking parameters and susceptibility to ground instabilities. In Italy, for areas that can be schematised as a 1D subsoil model (e.g. alluvial plain), a simplified approach is proposed to quantify the seismic amplification (amplification factor, AF). This approach consists of tables of correspondences, called seismic abacuses, available for the whole national area as well as for some regional territories, and derived for simplified subsoil models. In this work, the results of the comparison between the AF values retrieved from national abacuses applied in the Friuli Venezia Giulia (Italy) plain municipalities and those from 1D numerical simulations are presented. In general, the abacuses underestimate the local seismic site effects a part for sites with a shallow bedrock. No correlations/trends were identified between the AF derived from abacuses and those from numerical simulations. Moreover, considering the elastic acceleration response spectra, it emerges that in the 49.5% of the FVG analysed sites the abacuses approach, even though it underestimates the real seismic response, is a more suitable approximation compared to the soil class simplified approach proposed by the Italian regulation. Finally, what emerges is that the limit of 30 m, as indicated in the Italian regulation, to consider a deep or shallow bedrock seams underestimated, and the AFs are not correlated with the seismic bedrock depth when it is higher than 100 m.

Performance evaluation of parameters as estimators of seismic site effects in northern South America

In this study, we investigate the performance of several parameters as estimators of seismic site effects for sites in northern South America. Investigated parameters include the conventional time-averaged shear wave velocity in the upper 30 m (VS30), the predominant period of the site, and horizontal-to-vertical spectral ratios. To assess the performance of these parameters, we estimate the amplification of seismic waves within shallow soil layers (i.e., site effects) at recording stations across northern South America. The site amplification is estimated by analyzing the residuals calculated from ground-motion records with respect to model predictions for hard-rock site conditions. Several functional forms were introduced to express the linear site amplification in terms of the explored parameters. We evaluate the efficacy of the parameters, as well as their advantages and limitations, by considering their impact in the site-to-site standard deviation of the residuals between observations and predictions using the introduced functional forms.

Seismic Microzonation Map of Chiang Mai Basin, Thailand

The seismic site effect which is controlled by local geological conditions is a key parameter of seismic hazards analysis. This paper presents an observational study of microtremor data to investigate the dynamic characteristics of soil in the Chiang Mai basin (Chiang Mai and Lumphun province), Northern Thailand. The Chiang Mai basin was formed on terrace sediments and alluvium sediments. The horizontal vertical spectral ratio (HVSR) analyses of ambient noise data at 101 sites in the basin were analyzed for an average smoothed HVSR curve to estimate the amplification factor and fundamental resonance frequency of each observation point. We also evaluated the shear wave velocity using the HVSR inversion technique based on the diffuse field assumption. Tests undertaken include a comparison of the HVSR of a significant earthquake that was detected at stations in the basin and a nearby bedrock site. The results indicate that the resonance frequency ranges between 0.15 - 0.4 Hz at sites having large thicknesses of soft sediments. The lowest resonance frequencies occur in the center of the basin, whereas higher resonance frequencies were observed in the areas of the shallowest bedrock in the west and east of the basin. It can be inferred that the western margin of the Chiang Mai basin is a steep slope, while the eastern margin of the basin is a low-angle west-dipping basement. The amplification factor ranges between 3 - 5 times, in the middle of the basin. Most of the Chiang Mai basin area is classified as site D soil (stiff soil) relative to alluvium sediments, and a region of class C soil (very dense soil) conforms to the terrace sediments located on the eastern edge of the basin. The soil classification is based on shear wave velocity (Vs30) determined by HVSR inversion. HIGHLIGHTS HVSR analysis and inversion of HVSR from 101 survey points in Chiang Mai basin The Chiang Mai basin area is at high risk from seismic amplification that amplifies a ground motion between 3 - 5 times in the middle of the basin at a dominant frequency range from 0.15 - 0.4 Hz The lowest Vs30 values location is in the middle of the basin classified as site class E (soft soil), while most of the basin area is classified as site D soil (stiff soil) corresponding to alluvium sediments. The class C soil (very dense soil) area conforms to the terrace sediments located on the eastern edge of the basin GRAPHICAL ABSTRACT

Contribution of HVSR, MASW, and geotechnical investigations in seismic microzonation for safe urban extension: A case study in Ghabt Admin (Agadir), western Morocco

Ghabt Admin, located in western Morocco, is prone to seismic activity and site effects due to its position within an alluvial basin situated between two faulted mountain belts. The region has experienced several devastating earthquakes, including the 1960 Agadir earthquake that resulted in the loss of 12,000 lives and the destruction of over 75% of the city. Moreover, the area has recently undergone rapid urbanization, necessitating the implementation of seismic studies to make informed decisions regarding land use. This study aims to conduct the first seismic microzonation and site characterization studies by using HVSR, 1D-MASW, and geotechnical investigations. The datasets consist of 80 microtremors, 15 1D-MASW profiles, detailed geological observations, boreholes, and soil/rock identification tests. Due to challenges related to anthropogenic sources and soil heterogeneities, only 31 H/V measurements met the SESAME criteria. The HVSR results indicate the following: (i) the predominant peak frequency (f0) ranges from 1.4 to 15.7 Hz, with lower values (1.4–3 Hz) observed in the Oued Issen alluvial fans and the sand dune area, while moderate to high values (4–15.7 Hz) are found in the lacustrine limestone and Tagragra’s dome. (ii) The amplitude peak frequency (A0) ranges from 2 to 7.7, with higher values observed in the southwestern zone and certain parts of the glacis-fans zone, particularly in the Oued Issen zone. (iii) The seismic vulnerability index (Kg) ranges from 0.28 to 39.5. The Oued Issen area, which is composed of recent unconsolidated deposits and affected by the Oued Issen fault, exhibits high Kg values. Analysis of HVSR curve typology reveals complex geology. The VS30 map shows VS values ranging from 179 m/s to 830 m/s, classifying the study area into soil classes A, B, and C according to Eurocode 8. Furthermore, the 1D-MASW results are compatible with the HVSR results and the distribution of the geological units.

Geotechnical Zonation and Vs30 Classification of Soil for Seismic Site Effects Evaluation in the Western Extension of New Aswan City, Egypt

Geotechnical Zonation and Vs30 Classification of Soil for Seismic Site Effects Evaluation in the Western Extension of New Aswan City, Egypt

Seismic site effect models for the Turkiye-Izmir-Bayrakli Basin

Seismic site effect models for the Turkiye-Izmir-Bayrakli Basin

Horizontal-to-Vertical Spectral Ratios and Refraction Microtremor Analyses for Seismic Site Effects and Soil Classification in the City of David, Western Panama

The City of David constitutes one of the most important commercial centers of the Republic of Panama. However, it is located on a coastal plane, close to an area with high seismic activity and has been affected by significant earthquakes (18 July 1934, Mw = 7.4 and 12 March 1962, Mw = 6.7). The goal of this study is to estimate the seismic effects and to classify the soil in the City of David. The experimental work entailed the measurement of environmental noise for H/V spectral ratio (HVSR) analyzed at 22 stations. A series of microtremor refraction studies (ReMi) at six stations distributed from north to south of the city was also performed. The stations were distributed around urban areas of the city, which are characterized by the presence of water supply, sewerage, buildings, roads, etc. The spectral analysis of environmental noise allowed the generation of three different types of maps: First, predominant frequency maps (f0) with zones composed mainly of rigid and semi-rigid soils in the southern end of the city and rigid soils in the central and northern regions. Secondly, maximum H/V amplitude maps (A0) which evidence a low range of HVSR amplitude in the city, ranging from 1.1 to 3.8. Finally, liquefaction vulnerability (Kg) maps, with values less than 2 Hz−1, representative of a low liquefaction risk. Soil classification using ReMi and calculation of the corresponding Vs30 reveal type D soils, which correlate well with results obtained using HVSR analyses. A comparison between HVSR and ReMi shows that HVSR curves that exhibit clear peaks tend to be related to ReMi stations that presented relatively large shear-wave velocity contrasts at some depth. The results from this research are intended to aid the decision-making process related to the future development of the city, as well as government level maintenance and mitigation plans.

Geophysical surveys in the Kashmir valley (J&K Himalayas) part II: Anomalous seismic site-effects and exploration of alternative site classification schemes

Extensive geophysical surveys (MHVSR and MASW) conducted for the microzonation of the Kashmir Valley, Himalayas, revealed unexpected dynamic characteristics at certain sites pointing out the weak relationship between fundamental frequency (f0) and time-averaged shear wave velocity over 30 m depth (VS,30). Unusual low-frequency amplification at stiff soil sites and high-frequency amplification at weathered rock sites was obtained. On the contrary, high-frequency amplification was attained at a soft soil site over shallow bedrock. Instances of topographic amplification on slopes, hills, and valleys were encountered. Consequently, the commonly used VS,30-based single-proxy site classification failed to explain these atypical site effects, thus underscoring the caution to be exercised for site classification in geologically complex regions. These findings motivated the documentation of the anomalies and the search for the most suitable site characterisation scheme for the geological deposits of the Kashmir Valley. The coupled MHVSR-VS,30 proxy approach accomplished the best results for the study region.

Seismic site-effects assessment in a fluvial sedimentary environment: case of Oued Martil floodplain, Northern Morocco

Seismic site-effects assessment in a fluvial sedimentary environment: case of Oued Martil floodplain, Northern Morocco

Shaking table test for near-valley subway station - Part I: Seismic site effects

As a typical local irregular topography, valley would have a significant influence on the seismic distribution around the local site as well as on seismic responses of near-valley underground structures, which has been evidenced by ground motion observation and field seismic records. However, current knowledge on the seismic site effects of the valley is limited to analytical solutions or numerical methods, and lack of experimental data from physical models. In this paper, a physical model test method based on 1g shaking table for arc-shaped valley topography is proposed, and a series of shaking table tests are carried out to investigate the seismic site effects induced by the valley topography. Details of experimental setup include not only the dynamic similitude design, the laminar model container and the model soil, but also the design and fabrication of arc-shaped valley topography with three kinds of depth-to-width ratios, i.e. 1/3, 1/4, and 1/8, as well as procedure for simulation of seismic excitations considering harmonic waves and actual seismic records. The free-field test case without valley site is also involved and taken as a benchmark comparison to better understand the seismic site effects of valley terrain. Acceleration sensors are placed and embedded in the model soil with different buried depths and distances to the local valley site in order to record the overall free-field seismic responses as well as the local topography effects. Results show that the dynamic response on the surface of model soil exhibits strong spatial variability due to the impact of the local valley topography. Results also indicate the ground acceleration increases within the local site, and the amplification effect is most pronounced at the valley top. The influence of the valley on the acceleration response of the ground is more significant when the predominant frequency of the seismic motions is higher and closer to the natural frequency of the site under different seismic motions. Furthermore, when the depth-to-width ratio of the valley increases, the acceleration amplitude at the valley top gradually increases, and the predominant frequency of Fourier spectrum at the valley top is also amplified. Notably, the Arias intensity amplification factor of the surrounding strata increases significantly due to the local valley topography, and the Arias intensity is also increased with the depth-to-width ratio of the valley.

The influence of boundary conditions on nonlinear time domain site response analysis using LS-DYNA

The dynamic response of nuclear power plants and other critical infrastructure under seismic conditions is a major issue, and the earthquake-resistant capacity design of such facilities and their interior equipment should be based on prediction of dynamic response. Therefore, field geological investigations and corresponding site response analyses facilitate the evaluation of seismic site effects for the entire construction process, including site selection and detailed design. During site response analysis, soil parameters are usually obtained from laboratory tests using specimens retrieved from boreholes or from empirical equations. Conventional site response analysis software assumes horizontal layering in a 1D soil model, though the actual conditions of underground stratification are far more complicated. Some research efforts have undertaken 3D site response and soil-structure interaction analysis using LS-DYNA; however, the influence of different boundary conditions in these studies has been rarely discussed. The research reported on in this paper utilized LS-DYNA to perform nonlinear time-domain site response analysis and evaluated the difference between 1D and 3D model results. The results of both 1D and 3D site response analyses conducted using LS-DYNA were comparable to those of traditional approaches using a constrained boundary. The 3D models that adopt a non-reflective boundary condition can be affected by energy attenuation near the model boundary, causing a significant difference in the results. We suggest that the constructed model should have an area of at least 600 m × 600 m (plan view) to ensure accuracy in a 200 m × 200 m area in the center of the 3D simulation model using non-reflecting boundary conditions. Furthermore, four soil profile from real sites have been verified that the suggestion is also applicable in multilayered scenario.

On the time-stability of resonance frequencies in deep basins

SUMMARY Determining the resonance frequencies of sediment-filled basins is important in seismic site effects assessment and to infer information about the geometrical and mechanical properties of the basins. Being intrinsic properties of elastic bodies, resonance frequencies are not expected to change over time, at least in the short term and under small excitations, in this type of basins. By analysing multi-annual time-series at some seismic stations located on markedly alpine and subalpine 2-D basins, we first state under what type of exciting function (ambient noise) these resonances can be identified and with what uncertainty. The analysis will reveal a clear annual and daily oscillation of the resonance frequencies, increasing in the summertime and at daytime (i.e. directly correlated with temperature). We attempt to provide different explanations to this not yet so systematically documented experimental evidence. A clear and unique answer is yet to come.

Contribution to the seismic microzonation of Lisbon based on the integration of geological, geophysical, and geotechnical data

Lisbon has been affected by several moderate to strong earthquakes that caused considerable damage and produced significant economic and social impacts (e.g., the November 1st, 1755, earthquake). Studies on the potential damage due to a seismic event have been done considering Lisbon's moderate to high seismic hazard, but the seismic site effects have not yet been investigated at the city scale. The local ground conditions can locally modify the characteristics of surface seismic motion.Based on a multidisciplinary dataset composed of surface geology, a geotechnical database, and a geophysical database that compiles all the available geophysical tests carried out in Lisbon, an integrated approach to seismic ground characterisation was performed.A regression analysis of the shear-wave velocity values compiled in the geophysical database was done for each geological unit. The peak frequency distribution in the city, assessed from the ambient vibration measurements, showed a pattern in peak frequency variation. It decreases from the oldest to the most recent geological formations that outcrop in the SW-NE direction. Three zones based on peak frequency value distribution were proposed.An estimation of the impedance contrast depths was computed through two processes, considering the previous results: (i) the shallower contrast depth was assessed through a sharp variation of NSPT values available in the geotechnical database, and (ii) the deeper contrast depth was estimated based on geology data and deep borehole information. The results showed that for the upper ∼100 m of depth, a consistent relationship between the contrasts identified through different data was obtained, even if uncertainty increased with depth.

Semiempirical model for the estimation of site amplification in Northern South America

This article proposes a semiempirical model to estimate seismic site effects based on a predominant-period classification scheme for application in earthquake ground-motion models (GMMs). The proposed model introduces the use of the peak amplitude of the average horizontal-to-vertical ratio of the spectral response of the site obtained from earthquake records. The site effects model is implemented within a GMM estimated using ground-motion records from northern South America. The research compares the performance of the proposed site function with other formulations based on VS30 or site predominant period. This approach yields a smaller within-event variability than alternative methodologies, providing confidence for the reliability of the proposed site classification scheme as a viable alternative for the estimation of site effects, especially for sites for which VS30 measurements are not available.

Scaling ratios for spectral accelerations caused by seismic site effect under site condition classifications in China

The spectral accelerations (Sa), which are widely used as ground motion inputs in structural seismic designing, are significantly affected by local site conditions classified by near-surface geology. A novel approach of quantifying the scaling ratios for Sa on site class I, II, III, and IV under the site condition classifications in Chinese seismic codes, was proposed. In this integrated approach, the scaling ratios for Sa on each site class were subordinated to three constituents, i.e., scaling ratios for peak ground acceleration (PGA), scaling ratios for PGA-normalized Sa, and non-linear decay exponents. The scaling ratios for peak ground acceleration were derived from recent studies and numerical simulations of 1,138 borehole models in China, the scaling ratios for PGA-normalized Sa were derived from 3,584 strong motion records in NGA West two database, and the non-linear decay exponents were derived from about 140 thousand borehole observation data recorded by KiK-net. Consequently, this approach was solidly based on statistics of observation data in company with numerical simulations, which resulted in more reasonable and more reliable scaling ratios for Sa caused by the seismic site effect under site condition classifications in China.

Seismic site effect in a coastal area: 1D and 2D modelling of a tuff cliff in Sorrento Peninsula, Italy

Seismic site effect in a coastal area: 1D and 2D modelling of a tuff cliff in Sorrento Peninsula, Italy

Strategy for eliminating high-frequency instability caused by multi-transmitting boundary in numerical simulation of seismic site effect

A multi-transmitting boundary is a local artificial boundary widely used for numerically simulating seismic site effects. However, similar to other artificial boundaries, the multi-transmitting boundary has instability issue in numerical simulation. Based on the concept of multi-directional transmitting formula, a strategy for eliminating the high-frequency instability of the transmitting boundary is studied and a measure is proposed using a neighbour node of a boundary node to realize smoothing filtering. The proposed measure is verified through numerical analysis. The smoothing coefficient chosen for this measure provides a reference for deriving the coefficient of multidirectional transmitting formula in the time domain.

Complementary Dense Datasets Acquired in a Low-to-Moderate Seismicity Area for Characterizing Site Effects: Application in the French Rhône Valley

Abstract Superficial geological layers can strongly modify the surface ground motion induced by an earthquake. These so-called site effects are highly variable from one site to another and still difficult to quantify for complex geological configurations. That is why site-specific studies can greatly contribute to improve the hazard prediction at a specific site. However, site-specific studies have historically been considered difficult to carry out in low-to-moderate seismicity regions. We present here seismological datasets acquired in the framework of the French–German dense array for seismic site effect estimation project in the heavily industrialized area surrounding the French Tricastin Nuclear Site (TNS). TNS is located above an ancient canyon dug by the Rhône River during the Messinian period. The strong lithological contrast between the sedimentary fill of the canyon and the substratum, as well as its expected confined geometry make this canyon a good candidate for generating site effects that are variable on short spatial scales. To investigate the impact of this geological structure on the seismic motion, we conducted complementary seismic campaigns in the area. The first main campaign consisted of deploying 400 nodes over a 10 × 10 km area for one month and aimed at recording the seismic ambient noise. A second seismic campaign involved the deployment of 49 broadband stations over the same area for more than eight months. This complementary campaign aimed at recording the seismicity (including local, regional, and teleseismic events). These different designs allowed us to target a variety of seismic data at different spatial and temporal scales. Beyond the interest for local operational seismic hazard applications, these datasets may be valuable for studying seismic wave propagation within complex kilometer-scale sedimentary structures. In this article, we present the deployment designs as well as initial analyses to provide information on the characteristics and the overall quality of the data acquired to future users.

Two-dimensional basin-scale seismic site effects in the Kitimat Valley, British Columbia, Canada: A practical example of using a fast hybrid FE/BE method

The two-dimensional (2-D) basin-scale seismic site response of the Kitimat valley, located in the North Coast region of British Columbia, Canada, was investigated. The valley was subjected to a synthetic ground motion corresponding to a realistic local crustal earthquake of moderate magnitude consistent with the prevailing seismic hazard in the Kitimat region. SiteQUAKE, a fast hybrid finite element (FE)/boundary element (BE) numerical code was used for analyses, in which linear and equivalent-linear soil behavior models were implemented. The near-field (the sedimentary basin infill) and far-field (surrounding bedrock) were modeled by the FE and BE methods, respectively. Responses at different locations along a cross-section were compared with the input motion to illustrate the importance of basin effects. By dissociating the peak ground responses affected by the sub-surface topography (basin emptied of its infill) and by the presence of the sediment infill, it was shown that one-dimensional (1-D) analyses can either underestimate or overestimate 2-D seismic responses, highlighting their inability and insufficiency to correctly address basin effects.