Seismic Site Characterization Research Articles

Seismic site characterization of shallow bedrock sites in peninsular India using multiple geophysical methods

Seismic site characterization of shallow bedrock sites in peninsular India using multiple geophysical methods

Seismic Site Characterization and Region-Specific Seismic Site Parameter Relationships of Essex County, Ontario, Canada

Abstract We perform a multimethod in situ seismic field campaign to assess variability in local site conditions throughout Essex County, Ontario, Canada. Fundamental peak frequencies (f0HV) determined at 86 sites from microtremor horizontal-to-vertical spectral ratios (MHVSRs) increase southward from Windsor (∼1.7 Hz) to Amherstburg (>4 Hz) and eastward within Amherstburg (up to 17 Hz). We determine similar VS for individual subsurface layers at the 11 array sites from constrained shear wave velocity (VS) depth (z) profiles obtained by joint inversion of the site’s fundamental-mode Rayleigh-wave dispersion curve and f0HV. This indicates that the spatial variation in f0HV is driven primarily by the resonator depth, which shallows southward. We compile our 11 noninvasive VS depth profiles with an additional 86 VS profiles from previous invasive field testing, to develop a VS–depth relationship for Essex County’s postglacial sediments. Use of most other VS–z relationships available in eastern Canada underpredict the fundamental site frequency in Essex County because the average VS typical of those regions is lower. We find that the velocities for Montreal and Charlevoix (Canada) are closest to the velocities of our study area and consistent with Holocene alluvium deposits in the U.S. Geological Survey national crustal model. Compilation of in situ measurements at 172 sites throughout Essex County enables determination of three region-specific predictive relationships of important seismic site characterization metrics that are applied to generate regional seismic microzonation maps.

Seismic Site Characterization of Coimbatore city, Tamil Nadu, India using the Multi-channel Analysis of Surface Waves (MASW) test and Correlations between shear-wave velocity and SPT - N

Seismic Site Characterization involves the qualitative assessment of top-soil properties that have the capability of amplifying the generated earthquake ground motions. The geotechnical properties of topsoil refer to the top 30-m subsurface profile which plays a vital role in seismic microzonation and Ground Response Analysis (GRA) studies. Among various geotechnical parameters, shear-wave velocity (Vs) of the top 30 m subsurface is mainly linked to seismic site characterization and amplification studies. The average shear-wave velocity of the top 30 m subsurface, Vs(30), has been used for seismic site classification in accordance with the National Earthquake Hazard Reduction Program (NEHRP) and various building codes. In this study, an attempt has been made to retrieve the geospatial variation of average shear-wave velocity for Coimbatore city using the active Multi-channel Analysis of Surface Waves (MASW) test which is one of the non-destructive geophysical tests. To retrieve the spatial distribution of shear-wave velocity (Vs), the test was carried out at 35 locations in the vicinity of important structures, schools, colleges, and hospitals within the city. The seismic records have been acquired in the field and analyzed using the winMASW software. From the one-dimensional MASW test, the study area has an average Vs(30) in the range of 640 m/s to 909 m/s and has been classified as site-class “BC” (soft rock) according to NEHRP standards. These test results have been validated using the collected SPT bore log data from various locations, including 40 sites in the vicinity of the conducted MASW tests. The site-specific correlation between the shear-wave velocity (Vs) and the corrected SPT N- Value, N1(60), and between Vs and shear modulus (G) have been developed for Coimbatore city with a regression coefficient of 0.79 and 0.83 respectively. From the fundamental site period map, the study area has a site period in the range of 0.1 to 0.2 s, which indicates that 1- to 2- storey buildings that are densely distributed throughout the city may lead to damage in case of probable future earthquakes. This study bridges the connectivity from the evaluated bedrock acceleration using the Seismic Hazard Analysis (SHA) and provides insights for evaluating surface acceleration using GRA studies.

Seismic site characterization baseline data for microzonation and site response analysis of Otuasega Town, Bayelsa State, Niger Delta region of Nigeria

This study presents the findings of a comprehensive geotechnical and seismic site investigation conducted at Otuasega Town located in Bayelsa State within the Niger Delta region of Nigeria. Subsurface exploration involved advancing 10 boreholes to 30 m depth using hollow stem auger drilling. Continuous disturbed and undisturbed soil sampling was performed at 1.5 m intervals for detailed geotechnical testing. Laboratory tests on the recovered soil samples established the index properties, classification, densities and consistency limits of the stratified deposits. The subsurface profile comprised alternating layers of clay, silt and sand typical of deltaic sediments, with the clay fractions exhibiting medium to high plasticity. Shear wave velocity (Vs) profiling using Multichannel Analysis of Surface Waves (WASW) techniques categorised the site predominantly as Site Class C and D based on international standards. The Standard Penetration Test (SPT) N-values ranged from 5 to 10, indicating soft normally consolidated clay conditions typical of the Niger Delta region. Predictive empirical models developed from the field and lab data showed strong correlations for estimating key geotechnical parameters such as SPT blow count, Vs and liquefaction resistance. Ground response analyses using the Vs and SPT data indicated significant site amplification potential, with peak ground accelerations up to 1.5 times the bedrock motion. Liquefaction analysis based on the empirical SPT-based methods revealed a high potential for liquefaction in the sandy layers, especially under strong earthquake shaking. The study characterized the complex sedimentology and provided baseline information for seismic microzonation and site-specific ground response analyses to advance understanding of geohazards in this delta environment.

A Comparison of Three Geophysical Methods for Determining the Depth to Bedrock

The location of the study site for the presented research here is at Illinois in USA. Geophysical investigations were conducted to characterize the subsurface. At the site, borehole control, downhole seismic (DHS), seismic refraction tomography (SRT) and multichannel analysis of surface waves (MASW) data were acquired for the purpose of seismic site characterization. Shear wave and compressional wave velocities were used to estimate depth to bedrock. The data analyses showed the depth to bedrock determined by DHS, MASW and SRT is in good agreement with borehole data.

Techniques to identify microtremor wave contributions and impact to seismic site characterization

Techniques to identify microtremor wave contributions and impact to seismic site characterization

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.

Probabilistic seismic hazard analysis and site characterization of a power plant site in Chittagong, Bangladesh

Probabilistic seismic hazard analysis and site characterization of a power plant site in Chittagong, Bangladesh

Seismic microzonation mapping of Greater Vancouver based on various site classification metrics

The goal of the multi-year seismic microzonation mapping project for Greater Vancouver, British Columbia, Canada, is to produce seismic hazard maps inclusive of local site effects, in particular seismic hazard specific to one-dimensional site response and three-dimensional Georgia sedimentary basin amplification, as well as liquefaction and landslide hazard potential. We explore the variability in key seismic site characterization measures most often used for seismic microzonation mapping to evaluate the impact on mapping and communication of seismic microzonation of Greater Vancouver. This study focuses on the comparison of seismic microzonation maps of Greater Vancouver based on up to three seismic site term parameters and their associated classification schemes: 1) the time-averaged shear-wave velocity (Vs) of the upper 30 m (Vs30) and associated Canadian National Building Code (NBC) site class; 2) Vs30-based site classification proposed for the updated Eurocode 8; 3) site period (T0) determined from microtremor site amplification spectra; and 4) a hybrid site classification based on T0 and the average Vs and thickness of soil. 810 Vs30 and 2,200 T0 values are determined to evaluate sub-regional differences in these important seismic site parameters in Greater Vancouver. We find that the seismic microzonation of Greater Vancouver depends on the chosen seismic site parameter (Vs30, T0, or a combination of parameters) and that classification schemes with greater class divisions are beneficial to communicating the great variability in seismic site conditions in Greater Vancouver. We recommend that either one hybrid classification map or two classification maps of Vs30 and T0 together are required for effective communication of the seismic microzonation of Greater Vancouver.

Simplified engineering geomorphic unit-based seismic site characterization of the detailed area plan of Dhaka city, Bangladesh

Severe failure of improperly designed and poorly constructed structures may occur due to the amplified and prolonged ground motion during an earthquake, and hence prediction of the ground motion characteristics at the soil surface is crucial. In this study, based on the prepared simplified engineering geomorphic map, we performed a one-dimensional (1D) nonlinear site response analysis for seismic site characterization of the recently proposed Detailed Area Plan (DAP) area of Dhaka City, the Capital of Bangladesh. The engineering geomorphic unit-based map was prepared from image analysis and verified with the collected borehole data and surface geology map. The study area was classified into three major geomorphic units and seven sub-units subject to the subsurface soil profiles. Nine earthquake time histories, seven from the PEER NGA WEST2 data set and two synthetics, and seven identified subsurface soil profiles were used for nonlinear site response analysis, along with the BNBC 2020 uniform hazard spectrum as the target spectrum. For the selected earthquake ground motions, the near-surface soil response of the DAP area showed de-amplification of acceleration in the short period and amplification of acceleration in the long period. The amplified long-period acceleration could cause severe damage in inappropriately designed and poorly constructed long-period structures. The outcome of this study could be used to prepare a seismic risk-sensitive land use plan for the future development of the DAP of Dhaka City.

Seismic site characterization using MASW and correlation study between shear wave velocity and SPT[sbnd

A seismic site characterization study is carried out in the south of the foothills of tectonically active Himalayan region. During an earthquake, the dynamic characteristics of the shallow subsurface strata can influence seismic shaking in the region, causing damage to civil infrastructures. The seismic site characterization study with shear wave velocity (Vs) is performed using the Multi-channel Analysis of Surface Wave (MASW) technique, which is particularly challenging to implement in the densely inhabited areas. However, the standard penetration test (SPTN) commonly measured by civil engineers can be utilized to compute Vs of such areas by establishing a correlation between Vs and the SPT-N values.In the study area, MASW data were acquired at twelve different locations, whereas SPT-N tests were available from 26 boreholes. Twelve borehole locations closer to the MASW survey lines were paired to establish relationships between SPT-N and Vs. The developed linear and non-linear correlation functions were found to be statistically similar. Furthermore, the linear correlation function is used for estimating Vs from SPT-N in remaining 14 boreholes. The new correlation functions were further compared with previously reported empirical relationships from other areas. The current relationship can be used for geoengineering purposes at different sites with similar geological conditions.Shear wave velocity derived from the MASW surveys and estimated from the new empirical relationship was utilized for seismic site characterization based on average shear wave velocity up to 30 m depth (Vs30). The Vs30 map of the study site is prepared and classified in accordance with the National Earthquake Hazard Reduction Program (NEHRP-2020). The calculated Vs30 value of the study site ranges from 285 to 375 m/s. The major portion of the study area lies under Site Class CD, with a minor portion under Site Class D. In the event of an earthquake, some zones can potentially have stronger ground amplification, particularly in Site Class D and may have damaging effects. A series of average shear wave velocity maps for different depths are prepared to help geotechnical engineers in identifying and mitigating geotechnical risks. Furthermore, there are potential geohazard risks as certain depths show low shear velocity.

Seismic signatures and site characterization of an intermittent stream in dry and flood conditions: an implication for soil losses and landslide triggering

Identifying ambient noise-based (ANb) signatures together with the erosion-prone site conditions retrieved from georadar attribute analysis of streams can help in the estimation of their erosive potential (EP) that promotes reverie landslides and soil losses in the fluvial valleys. This is particularly imperative on flooding or rainy days, leading to stronger erosion-prone conditions (colluvium and boulders) of the valley beds. Developing such research direction can benefit the local communities, as is the case with the Cerrado region of Brazil, where these phenomena have high destructive potential with social, economic, and climatic implications. For the present study, a seasonal stream in the Federal District of Brazil was investigated by ANb monitoring supported by ground penetration radar (GPR) for site characterization. The ANb monitoring was conducted (at a safe distance) with a seismometer over several durations of dry and rainy conditions. The power spectral density (PSDs) as a function of several weather conditions (rainfall, wind speed, and pressure), time–frequency spectrograms, and ambient noise displacement root mean square (dRMS) were computed. This analysis also considered the single station horizontal-to-vertical spectral ratio (HVSR), where rain, wind, pressure, river flow and anthropogenic signatures were evident (at selective frequency ranges). Multi-peaks that emerged on the HVSR curve were further analyzed to identify amplitude and frequency changes, with the three peaks shifting on average to a lower position during the rainy period. The GPR amplitude and waveform variation features were attributed to the stratigraphy of the floodplain and regions susceptible to erosion, such as erosion-prone lithological spots, which provide the basis for non-destructive monitoring tools that enable the detection of “seismic signatures” and weak spots of the fluvial channels for improving environmental management.

Seismic site characterization using multichannel analysis of surface waves in the Singhbhum region of Jharkhand, India: a case study

Seismic site characterization using multichannel analysis of surface waves in the Singhbhum region of Jharkhand, India: a case study

Near-Surface 2D Imaging via FWI of DAS Data: An Examination on the Impacts of FWI Starting Model

Full waveform inversion (FWI) and distributed acoustic sensing (DAS) are powerful tools with potential to improve how seismic site characterization is performed. FWI is able to provide true 2D or 3D images of the subsurface by inverting stress wave recordings collected over a wide variety of scales. DAS can be used to efficiently collect high-resolution stress wave recordings from long and complex fiber optic arrays and is well-suited for large-scale site characterization projects. Due to the relative novelty of combining FWI and DAS, there is presently little published literature regarding the application of FWI to DAS data for near-surface (depths < 30 m) site characterization. We perform 2D FWI on DAS data collected at a well-characterized site using four different, site-specific 1D and 2D starting models. We discuss the unique benefits and challenges associated with inverting DAS data compared to traditional geophone data. We examine the impacts of using the various starting models on the final 2D subsurface images. We demonstrate that while the inversions performed using all four starting models are able to fit the major features of the DAS waveforms with similar misfit values, the final subsurface images can be quite different from one another at depths greater than about 10 m. As such, the best representation(s) of the subsurface are evaluated based on: (1) their agreement with borehole lithology logs that were not used in the development of the starting models, and (2) consistency at shallow depths between the final inverted images derived from multiple starting models. Our results demonstrate that FWI applied to DAS data has significant potential as a tool for near-surface site characterization while also emphasizing the significant impact that starting model selection can have on FWI results.

Seismic site characterization considering directional near-field seismogenic active faults in Aswan area, Egypt

Seismic site characterization considering directional near-field seismogenic active faults in Aswan area, Egypt

DeltaVs: A Method for Detecting Significant Layer Boundaries in Surface Wave Inversion Results

Surface wave testing is a powerful tool for noninvasive seismic site characterization. It includes a wide variety of active-source and passive-wavefield methods [e.g., spectral analysis of surface waves (SASW), multichannel analysis of surface waves (MASW), microtremor array measurements (MAM)] that can be applied to many different field conditions. The dispersion data collected with all these methods can be inverted to produce (one-dimensional) 1D shear wave velocity (VS) profiles of the subsurface. A critical part of this inversion procedure is the need to develop several trial model parameterizations with different numbers of layers as a means for investigating epistemic uncertainty and nonuniqueness when attempting to fit the experimental dispersion data. This is especially important when a priori information either is not available or does not extend to great enough depths to constrain the inversions. These trial parameterizations are used to vary the number of potential layers present in the subsurface models and to set the acceptable search ranges for depths to boundaries and stiffness in each layer. The inversion results are highly affected by the parameterizations. When performing high-quality inversions, it is important to use different parameterization options to fully explore potential conditions at the site and characterize the epistemic uncertainty of the inversion process. While this practice allows for a robust analysis of the experimental dispersion data and its uncertainty, it also generates many potential subsurface models for the site, which can represent a challenge for engineers when deciding which VS profiles best represent the true subsurface layering for use in subsequent analysis and design (e.g., seismic site response). This paper presents an approach, called the DeltaVs method, for systematically evaluating the depths of layer boundaries across all acceptable models determined from several different inversion layering parameterizations as a means to identify distinct clusters of boundaries. Using these clusters, the number and location of meaningful layer boundaries can be determined for many sites, thereby allowing for the elimination of some parameterization options containing more, or fewer, boundaries and a reduction in epistemic uncertainty. The distributions of layer boundaries within these clusters can then be used to determine statistics for the depths of layer boundaries that are consistent with the epistemic uncertainty of the models produced by the inversion procedure. The DeltaVs method is demonstrated on 12 synthetic ground models as well as one field data set.

Probabilistic approach for seismic microzonation integrating 3D geological and geotechnical uncertainty

A novel probabilistic methodology for regional seismic site characterization is proposed and applied to a region with highly heterogeneous surficial geology and varying soil sediment thickness and stiffness. The method combines various sources of geological and geotechnical uncertainties to develop a three-dimensional (3D) shear-wave velocity ( Vs) model and evaluate the associated uncertainties. A 3D geological model of the unconsolidated deposits was developed using geostatistical interpolation and simulation methods. Sequential indicator simulations produced a quantitative geologic model that explicitly quantified geological uncertainties based on the likelihood of specific soil types occurring. In situ measurements and multivariate statistical analysis allowed the development of empirical correlations between Vs, geotechnical parameters, depth, and soil types. The resulting 3D Vs values were estimated on the basis of Vs-depth correlations and the probability of occurrence of each soil type. In this approach, the propagated uncertainty was also quantified by considering the combined variance. Seismic microzonation mapping was then conducted by transforming the 3D Vs model into two-dimensional (2D) maps that represent the spatial distributions of the time-averaged shear-wave velocity of the top 30 m ( Vs,30) and the fundamental site period ( T0), along with their respective uncertainties using Monte Carlo simulations. The results indicate that microzonation maps and their uncertainties are influenced by the thickness, occurrence probability, and geotechnical properties of soils. The proposed method can be used to assess the probabilistic seismic risk at local and regional scales in areas with geologically and geotechnically complex soil properties.

Assessment of Kappa Values in the Chilean Subduction Zone for Interface and In-Slab Events

Abstract One way to study the physical process that occurs behind earthquakes and how they affect different sites depending on the source type and the geological structure of the site is the shape of the Fourier spectrum. A parameter related to the shape of the spectrum is the spectral decay factor—kappa (κ), which characterizes how the amplitude of the Fourier acceleration spectrum decays at high frequencies. The parameter κ can be important in the characterization and estimation of the surface seismic demand, being useful in, for example, the adjustment of ground-motion prediction equations. We calculate the values of κ and its site component κ​0 from 36 seismic stations of the National Seismological Network of Chile to determine the dependence that this parameter has to the site conditions as well as to the properties of the path in which greater values of κ are observed for subduction earthquakes that occur in the interface between the Nazca and the South American plates, compared with the values obtained from events occurring inside the subducting plate, known as in-slab earthquakes. We find that κ​0, calculated using the hypocentral distance correlates more closely with the site fundamental frequency f​0, rather than the commonly used V​S30 (time-averaged shear-wave velocity in the top 30 m). Our kappa value results are field estimates of near-surface attenuation, which can be used to calculate the minimum site-specific damping or crustal attenuation in seismological models that have a strong impact on seismic site characterization, particularly, in subduction settings.

Flexible multimethod approach for seismic site characterization

We describe the flexible multimethod seismic site characterization technique for obtaining shear-wave velocity (VS) profiles and derivative information, such as the time-averaged VS of the upper 30 m (VS30). Simply stated, the multimethod approach relies on the application of multiple independent noninvasive site characterization acquisition and analysis techniques utilized in a flexible field-based approach through a decision process based on primary factors such as the investigator’s available equipment, field logistics (e.g., available array deployment space, urban versus rural), geologic setting, and funding among other primary factors. In a multimethod approach, sites ideally should be characterized using both active and passive noninvasive (i.e., no drilling and only minimal disturbance to the ground surface) methods. Almost without exception, we recommend the use of at least one active-source technique for analyzing surface waves, which in the current state of practice would include one or more of the following: spectral analysis of surface waves (SASW; commonly Rayleigh waves) and multichannel analysis of surface waves (MASW; Rayleigh and/or Love waves). In addition, passive-source microtremor array methods (MAMs) using linear (one-dimensional) and two-dimensional arrays may be appropriate or even required for characterizing a given site. Similarly, the microtremor horizontal-to-vertical spectral ratio (mHVSR) method thacan be valuable for identifying shallow rock sites. Finally, P- and SH-wave seismic refraction methods warrant consideration, especially at rock and shallow-rock sites.

Retraction: Seismic site characterization in northeast India: a holistic GIS based approach

Retraction: Seismic site characterization in northeast India: a holistic GIS based approach