Shear Wave Velocity Profiles Research Articles

MASW? A critical perspective on problems and opportunities in surface-wave analysis from active and passive data (with few legal considerations)

In the last two decades, surface wave analysis has become a widespread practice for several geotechnical applications and the MASW (Multichannel Analysis of Surface Waves) acronym has become extremely popular. The standard MASW approach is based on the personal (i.e. subjective) interpretation of the phase-velocity spectrum of the vertical component of Rayleigh waves. This way, we do not invert the actual datum but a subjective interpretation that can be wrong and lead to erroneous shear-wave velocity (VS) profiles. Even in case data interpretation is correct, non-uniqueness of the solution obtained from inversion of just one observable remains a problem. A series of active and passive synthetic and field datasets are used to illustrate main issues in surface wave analysis and highlight the need for multi-component analysis possibly according to an approach that goes beyond the subjective interpretation of modal dispersion curves. The multi-component approach considered to overcome the ambiguities of single-component analysis is performed according to the FVS (Full Velocity Spectrum) approach, therefore overcoming subjective velocity-spectra interpretations. It is also shown that surface-wave analysis can be effectively carried out considering not phase velocities but group velocities, with the obvious benefit that while for the phase-velocity analysis multi-offset data are necessary, the definition of the group velocities can be accomplished from single-offset data (i.e. from the data of one single sensor). An example of erroneous VS determination made for an important public building is illustrated in order to highlight the importance of multi-component analysis and the need for stricter guidelines.

Site amplification prediction model of shallow bedrock sites based on machine learning models

Prediction of the site amplification is of primary importance for a site-specific seismic hazard assessment. A large suite of both empirical and simulation-based site amplification models has been proposed. Because they are conditioned on a few simplified site proxies including time-averaged shear wave velocity up to a depth of 30 m (VS30) and site period (TG), they only provide approximate estimates of the site amplification. In this study, site amplification prediction models are developed using two machine learning algorithms, which are random forest (RF) and deep neural network (DNN). A comprehensive database of site response analysis outputs obtained from simulations performed on shallow bedrock profiles is used. Instead of simplified site proxies and ground motion intensity measures, matrix data which include the response spectrum of the input ground motion and shear wave velocity profile. Both machine learning based models provide exceptional prediction accuracies of both the linear and nonlinear amplifications compared with the regression-based model, producing accurate predictions of both binned mean and standard deviation of the site amplification. Among two machine learning techniques, DNN-based model is revealed to produce better predictions.

Revealing the shallow soil structure of the Yigong Lake in the Tibetan Plateau using a portable distributed acoustic sensing interrogator

The harsh and extreme environmental and near surface conditions of the Tibetan Plateau have limited the conventional electrical-based seismic instruments from obtaining high-quality seismic data through long-term and continuous observations, setting challenges for environmental seismology study and natural hazard monitoring in this area. Distributed acoustic sensing (DAS) is an emerging technique based on optical fiber communication and sensing. It provides a possible solution for subsurface imaging in extreme conditions at high spatiotemporal resolution by converting fiber-optic cables into dense seismic strainmeters. We deploy two survey lines with armored optical fiber cables in the Yigong Lake area, Southeastern Tibetan Plateau, to record ambient noise for a week. The DAS interrogator is specifically designed in a portable size with very low power consumption (25 W/h). Hence, we can use a 12V-DC battery for power supply to adjust the power limitation during the field recording. Ambient noise interferometry and multichannel analysis of surface waves are used to get 2D shear wave velocity profiles along the fiber paths. The results highlight the great potential of DAS for dynamic monitoring of the geological evolution of lakes and rivers in areas of extreme environments as in the Tibetan Plateau.

Influence of heterogeneity and elevated temperatures on the seismic translational stability of engineered landfills

This paper proposes a new generalised closed-form solution to estimate the safety factors for translational failure of landfills considering the effects of heterogeneity and temperature variation with the depth of the landfill. The influence of heterogeneity was considered in the form of a shear wave velocity profile, while the influence of temperature was accounted for by considering the temperature-dependent shear strength properties. The proposed method also considered the influence of material damping and the mode change behaviour of the landfill. The safety factor for translational failure of the landfill was estimated using the limit equilibrium-based two-part wedge method. The proposed method was validated by comparing the safety factor obtained from this method with comparable analytical solutions. An extensive parametric study was conducted to demonstrate the influences of the landfill geometry, shear strength properties of municipal solid waste and liner components, parameters of strong ground motion, heterogeneity, and landfill temperature on the calculation of safety factors. The interfacial shear strength properties of liner components played a vital role in the translational stability of landfills compared to the shear strength properties of municipal solid waste. The frequency of base excitation showed a strong influence on landfill stability, while the material damping displayed a nominal effect. For the cases considered in this study, when the frequency of base excitation was close to the fundamental frequency of the landfill, the base acceleration was amplified by 6.7 times which reduced the safety factor by approximately 1.6 times. Heterogeneity and temperature have a significant influence on landfill stability. The increased degree of heterogeneity caused an approximately 27% reduction in the safety factor. For the given set of input parameters, the elevated temperature in the landfill alone decreased the safety factor by a maximum of 18%, while it reduced by approximately 60% under the combined influence of seismic, heterogeneity, and elevated temperature.

The Influence of Input Motion Scaling Strategies on Nonlinear Ground Response Analyses of Soft Soil Deposits

A key issue for the estimation of ground shaking is the proper selection of input motions at the seismic bedrock. At the same time, the effect of the input motion scaling strategy on structural response is typically studied disregarding the presence of the soil deposit. In this work, different soft soil deposits are selected by varying the shear wave velocity profiles and the depth to the seismic bedrock, modelling the soil behaviour through a nonlinear constitutive model implemented into a fully coupled FE code. Seven input motions are retrieved for several selection strategies in conjunction with two seismic intensity levels. Hence, more than 300 one-dimensional ground response analyses are performed. The results of the analysed cases, which are presented in terms of spectral response at ground surface and amplification factors, indicate that: (i) the use of an advanced elasto-plastic soil constitutive model accounts for nonlinear ground response effects, including higher site amplification in the mid-period range and deamplification of the peak ground accelerations; (ii) the different scaling strategies lead to comparable mean values of the amplification factors, and (iii) the variability of the amplification factors is significantly reduced when the scaling strategy seeks the compatibility with the target spectrum over a specified period range. The research will aid the prediction of local seismic site response over large areas, particularly in the absence of the fundamental period of a structure and facilitate its use in general recommendation for quantifying and reducing uncertainty.

Estimation of V S30 using shallow depth time-averaged shear wave velocity of Rawalpindi–Islamabad, Pakistan

The time-averaged shear wave velocity of top 30 m (V S30) is the most commonly used parameter to classify a site and evaluate its amplification characteristics for the seismic design. The in-situ seismic tests must be performed up to a depth of 30 m for obtaining the shear wave velocity (V S) profiles to estimate V S30. It is intimated that, in most of the cases, the measured V S profile does not extend up to 30 m due to numerous reasons including limitation of testing techniques and unfavorable field conditions. Since, the measurements of V S30 are unavailable for the majority of Pakistan and the world, the local geology and topographic slope or its combination are used to estimate V S30. However, there is no field-based validation of the estimated V S30 is performed in Islamabad–Rawalpindi region, the proxy-based estimation may lead to unrealistic results. To accommodate this, region specific extrapolation methods are developed. This study develops an empirical data-driven function of V S30 from shallow V S profiles by correlating V S30 with the time-averaged V S to depths less than 30 m. In this regard, 85 V S profiles are used from Rawalpindi-Islamabad region. A comparative analysis of the proposed procedure is carried out with the published methods. It is revealed that V S30 predicted by the proposed function results in close matches with the data measured in the western United States. In addition, the results indicate that the local geology and topographic slope proxies may not be acceptable for usage in the region due to their greater uncertainty. Finally, a procedure for extrapolating the V S profile from available shallow depth measurements up to 30 m is proposed.

Field and laboratory measurements of shear wave velocity in unsaturated soils

Shear wave velocity measurements in soil are important for assigning site soil profiles to seismic site classes and for calculating other dynamic soil properties. The seismic cone penetration test (SCPT) is commonly used to determine shear wave velocity profiles in the field. In the laboratory, bender elements provide an easy means to measure shear wave velocity of soil specimens from discrete depths. In this study, shear wave velocity measurements obtained from unsaturated soil profiles in the field and on field samples in the laboratory under similar stress conditions were analyzed and compared. Additionally, the influence of seasonal changes on the behavior of shear wave velocity was investigated. Comparisons of shear wave velocities from the field and laboratory were favorable for similar moisture and stress conditions. However, results showed that as the degree of saturation increases and suction decreases the shear wave velocity can decrease significantly.

Application of Microtremor Survey Technology in a Coal Mine Goaf

Goafs are one of the main factors that endanger the safety of mining areas, leading to roadway collapse, land subsidence and other problems. Determining how to detect goafs accurately and efficiently is an important issue faced in engineering geophysical exploration research. Microtremor survey technology, used in geophysical exploration, has been developed in recent years. It has the advantages of low cost, flexible construction, low topographic impact and high efficiency. In this paper, microtremor survey technology was applied to the detection of a coal mine goaf. In the known goaf area of the Taiyuan Nanling Coal Mine, a linear observation array was arranged to conduct rolling acquisition, observe the microtremor signal records of the natural field and use the extended spatial autocorrelation method to extract the dispersion curve. Through the inversion of the extracted dispersion curve, the apparent shear wave velocity profile of the underground medium clearly showed the location of the goaf. It was roughly consistent with the goaf data obtained from the mine. The detection results reflect the advantages of micromotion exploration. The goaf of Coal Seam #2 and its affected area appear to be due to an obvious low-speed phenomenon. The boundary of the apparent shear wave velocity profile of micromotion detection was basically consistent with the boundary of the goaf, indicating that micromotion exploration technology has good technical advantages and application prospects in the detection of coal mine goafs. The rolling acquisition method of linear arrays can be used to ensure accurate detection, improve exploration efficiency and save on exploration costs.

A database of ground motion recordings, site profiles, and amplification factors from the Groningen gas field in the Netherlands

A comprehensive database that has been used to develop ground motion models for induced earthquakes in the Groningen gas field is provided in a freely accessible online repository. The database includes more than 8500 processed ground motion recordings from 87 earthquakes of local magnitude ML between 1.8 and 3.6, obtained from a large network of surface accelerographs and borehole geophones placed at 50 m depth intervals to a depth of 200 m. The 5%-damped pseudo-acceleration spectra and Fourier amplitude spectra of the records are also provided. Measured shear-wave velocity (VS) profiles, obtained primarily from seismic Cone Penetration Tests (CPTs), are provided for 80 of the ∼100 recording stations. A model representing the regional dynamic soil properties is presented for the entire gas field plus a 5 km onshore buffer zone, specifying lithology, VS, and damping for all layers above the reference baserock horizon located at about 800 m depth. Transfer functions and frequency-dependent amplification factors from the reference rock horizon to the surface for the locations of the recording stations are also included. The database provides a valuable resource for further refinement of induced seismic hazard and risk modeling in Groningen as well as for generic research in site response of thick, soft soil deposits and the characteristics of ground motions from small-magnitude, shallow-focus induced earthquakes.

Advanced monitoring of tailings dam performance using seismic noise and stress models

Tailings dams retain the waste by-products of mining operations and are among the world’s largest engineered structures. Recent tailings dam failures highlight important gaps in current monitoring methods. Here we demonstrate how ambient noise interferometry can be applied to monitor dam performance at an active tailings dam using a geophone array. Seismic velocity changes of less than 1% correlate strongly with water level changes at the adjacent tailings pond. We implement a power-law relationship between effective stress and shear wave velocity, using the pond level recordings with shear wave velocity profiles obtained from cone penetration tests to model changes in shear wave velocities. The resulting one-dimensional model shows good agreement with the seismic velocity changes. As shear wave velocity provides a direct measure of soil stiffness and can be used to infer numerous other geotechnical design parameters, this method provides important advances in understanding changes in dam performance over time.

In-situ shear-strain profile assessment in geotechnical array with ground motion data

In-situ shear-strain profile can be used to evaluate soil dynamic behavior with depth, but also safety of buried lifeline and structures due to seismic wave propagation in soil during an earthquake. Using 199 sets of ground-motion data recorded by six geotechnical arrays with 3–7 sensors in 43 earthquakes, the seismic interferometry by deconvolution method is employed to extract shear-wave velocity profile (Vs, sw) of seismic wave in each array; ratio of peak particle velocity (PPV) and Vs, sw between two successive sensors, PPV/Vs, sw, is computed in each array and assumed to be proxy of the in-situ shear strain. The characteristics of in-situ shear strain profiles in the arrays are studied under seismic loadings in various earthquakes. The relationships of evaluating both the in-situ shear strain profile and the largest in-situ shear strains of the upper soil near the surface are derived from the regression analysis for D and E class sites, respectively. Based on the linear and volumetric threshold shear strains and PGAs and PGVs, the variations of soil behaviors (i.e., linear elastic, nonlinear and plastic behaviors) with the increase of the shear strain or PGA or PGV are studied in the shear strain profiles. The results are as follows. (1) With the increase of PGA and/or PGV in various earthquakes, the in-situ shear strain profiles move towards the larger shear strains and the soil behavior will change to a higher stage or a higher level in the same stage. (2) In the shear strain profile, with the increase of depth, the in-situ shear strains decrease gradually and the soil behavior will change to a lower stage or a lower level in the same stage. (3) Among the shear strain profiles, upper soil has the largest shear strain (γmax) and the shallower the soil depth, the larger the shear strain. The relationships between the γmax and PGV and between the γmax and PGA and PGV combination can be used to evaluate the dynamic behavior of upper soil with less than 30 m at a strong motion array or station for D and E class sites. (4) The in-situ shear strains are highly correlated with the PPV and average shear wave velocity measured by the drilling-hole methods (Vs,dh) combination and with the PPV and Vs,dh and center depth between two successive sensors (Hcd) combination, and increase with the increase of the PPV and decrease with the increase of the Vs,dh and/or Hcd, respectively. The relationships are recommended to evaluate the in-situ shear strain profile with very high accuracy in the geotechnical array with more than two sensors for D and E class sites.

Cluster analysis of velocity models around the Hudson Bay region, Eastern Canada

SUMMARYUnderstanding deep crustal structure can provide us with insights into tectonic processes and how they affect the geological record. The deep crustal structure can be studied using a suite of seismological techniques such as receiver function analysis, body and surface wave tomography. Using models of crustal structure derived from these methods, it is possible to delineate tectonic boundaries and regions that may have been affected by similar processes. However, often velocity models are grouped in a somewhat subjective manner, potentially meaning that some geological insight may be missed. Cluster analysis, based on unsupervised machine learning, can be used to more objectively group similar velocity profiles and, thus, put additional constraints on the deep crustal structure. In this study, we apply hierarchical agglomerative clustering to the shear wave velocity profiles obtained by previous studies focused on the region from the joint inversion of receiver functions and surface wave dispersion data at 59 sites surrounding the Hudson Bay. This location provides an ideal natural laboratory to study the Precambrian tectonic processes, including the 1.8Ga Trans-Hudson Orogen. We use Ward linkage to define the distance between clusters, as it gives the most physically realistic results, and after testing the number of clusters from 2 to 10, we find there are 5 main stable clusters of velocity models. We then compare our results with different inversion parameters, clustering schemes (K-means and GMM), as well as results obtained for profiles from receiver functions in different azimuths and find that, overall, the clustering results are consistent. The clusters that form correlate well with the surface geology, crustal thickness, regional tectonics and previous geophysical studies concentrated on specific regions. The profiles in the Archean domains (Rae, Hearne and Superior) are clearly distinguished from the profiles in regions influenced by Proterozoic orogenic events (Southern Baffin Island and Ungava Peninsula). Further, the crust of Melville Peninsula is found to be in the same cluster as the crust of the western coast of Ungava Peninsula, suggesting a similar crustal structure. Our study shows the promising use of unsupervised machine learning in interpreting deep crustal structures to gain new geological insights.

An approach for developing site signature consistent deep shear wave velocity profiles for the Mississippi embayment using generalized power-law functions

A new approach is presented to develop site signature consistent deep shear wave velocity profiles (VS profile) for the Mississippi embayment using generalized power-law functions. The approach utilizes 24 deep shear wave velocity profiles measured across the embayment to develop power-law shear wave velocity functions for the geologic units observed in the embayment. The velocity functions along with the layer interface boundaries are utilized to generate an initial VS profile, whose Vs is then adjusted to be consistent with the fundamental site frequency (i.e., the site signature). Using the developed approach, model VS profiles at 24 sites throughout the embayment are generated and compared to the corresponding measured Vs profiles. All modeled VS profiles matched the site fundamental frequency within one standard deviation. Percent differences calculated between the modeled VS profiles and measured VS profiles demonstrated good agreement with less than a 10% difference at most depth ranges. However, differences of up to 30% are observed for near surface layers. The VSavg of the modeled and measured VS profiles have a strong association with a Pearson correlation coefficient of 0.95, while the VS30 of the modeled and measured Vs profiles have a Pearson coefficient of 0.49, indicating a weaker association. As an independent verification, model VS profiles are generated at 11 independent sites and compared with the measured VS profiles at these sites. Percent differences calculated between the modeled and measured VS profiles at the independent sites are below 15% at most depth ranges, with a Pearson correlation coefficient calculated for VS150 of 0.75, indicating a strong association. With reasonable layer interface models, this new approach can be used to develop an updated 3D shear wave velocity model for the Mississippi embayment.

Seismic Characterization of Swiss Strong-Motion Borehole-Station Sites by Inversion of Full Microtremor Horizontal-to-Vertical Spectral Ratios [H/V(z,f)

ABSTRACT The assessment of the local site amplification during an earthquake requires, among other input information, a reliable estimate of the shear-wave velocity profile, including the contact with engineering and seismic bedrocks. We determine the shear-wave velocity (VS) profiles at two Swiss strong-motion borehole-station sites at Visp (Valais) and Buochs (Nidwalden) by inversion of microtremor horizontal-to-vertical spectral ratio [H/V(z,f)] curves measured at the surface and at different depths. These borehole stations were built to monitor not only the seismic activity in Switzerland and the surrounding areas but also the nonlinear site response, especially liquefaction processes during strong local and regional earthquakes. The boreholes are equipped with accelerometers at various depths, with the deepest borehole located at 102 m below the surface. In the first part, we review the forward modeling algorithm of the full-microtremor H/V(z,f), with a focus on the computational cost and accuracy. In the second part, we perform a temporal analysis of the H/V(z,f) curves obtained from the accelerometers. The results show seasonal variabilities in H/V between summer and winter. The third part presents the inversions of the H/V curves for a single day in summer and winter at both sites. From the full H/V(z,f) inversion, we obtain shear-wave velocities in the upper 30 m (VS30) of 216 and 209 m/s at Visp in winter and summer, respectively. At Buochs, the corresponding VS30 are 269 and 345 m/s. The depths of the seismic bedrock are at 219 and 210 m at Visp, and at Buochs they are at 293 and 213 m. The estimated velocity profiles compare well with independent estimates from array measurements of ambient seismic vibrations, gravimetry, and geological logging information. Finally, we use the obtained seismic velocity profile information to model the theoretical 1D shear wave transfer function. The latter result compares well with amplification function results obtained using earthquake recordings.

Study of Local Site Conditions for Seismic Hazard Assessment at the BHU Campus, Varanasi, North India, using Ambient Noise Measurements

Abstract The ambient noise measurement approach has been applied for detailed seismic hazard mapping in the Banaras Hindu University (BHU), campus, Varanasi. Thirty three single stations, two standard penetration tests (SPT), and two array micro tremor measurements were carried out to determine the local site characteristics. In this study, H/V spectral ratio confirms that the sites have an average predominant frequency of around 0.53Hz and an average amplitude of 7.94. These resonant frequencies are related to the thickness of the sediments in the region. Based on the available lithological data and estimated sub-surface structures, shallow and deeper cross-sections through two profiles within the BHU campus are presented. The estimated shallow sub-surface structures are correlated with 150 m boreholes data for a better understanding of sub-soil stratification. These studies show the presence of clay up to a depth of ~40 m, mixed with sand and pebbles at some locations, followed by layers of sand, and clay with pebbles ~200 m. Findings of the present study are also well correlated with inverted shear wave velocity profiles. The Vs observed from the array measurements and N-value from standard penetration test (SPT) are positively correlated to each other and give a better understating of Vs. Thus, the inverted 1-D shear velocity models clearly show low shear wave velocity having sediment thickness of 30 to 40 meters. Finally, fundamental frequency, amplification, vulnerability index, and shear wave velocity maps are presented. Additionally, the borehole data are used to calibrate the results of the present study of the shallow sub-surface structure. Subsequently, computed maps are a very useful tool for engineers to quantify the intensity of earthquake ground motion, which will be helpful to determine the capacity of buildings to resist earthquakes.

Phase-velocity and intercept-time scanning (PIS) technique for extracting experimental dispersion characteristics of surface waves

In active surface wave methods, the ability to extract the true dispersion trends from surface wave field data is critical for inferring accurate shear-wave velocity profiles. However, dispersion images such as those generated by the conventional wavefield transformation method are usually accompanied by numerical artifacts including side-lobes and aliasing, both of which decrease the dispersion image resolution and may lead to misinterpretation of dispersion modes. For example, the side lobes can be misinterpreted as higher or lower modes, which would introduce additional errors into the inverted stiffness profiles. A new experimental dispersion analysis method is presented to improve the resolution and sharpness of dispersion images and minimize the side lobes. The proposed method employs scanning of the phase-velocities and intercept-times of a series of harmonic signals obtained by Fourier transformation of raw multichannel data in the space-time (x-t) domain. Results obtained from synthetic and real field data demonstrate that the improved method can yield high-quality dispersion images compared to those of the conventional dispersion-imaging method. In addition, the improved method do not rely on the assumption that the impact point is the generation point of the Rayleigh waves, which is needed for the conventional method.

Linear site responses from U.S. borehole arrays: Primary site-response parameters and proxies

Site response, site resonance in particular, is a great concern in earthquake engineering. However, scientists and engineers have yet to reach a consensus on the most important parameters or proxies to characterize or quantify site response. In this investigation, we identified the most important parameters at 12 borehole arrays in the United States by analyzing weak-motion empirical and theoretical site responses in terms of soil-to-rock spectral ratios and ratios of response spectra at each site in the frequency range of engineering interest, 0.1–10 Hz (i.e., a period range of 0.1–10 s). From these ratios, we extracted two sets of parameter pairs: the fundamental and peak mode frequencies, f0 and fp, respectively, and their corresponding amplifications, A0 and Ap, respectively. We also characterized the Vs profiles using the shear-wave velocity ratio (Rj) and impedance ratio IRj and determined the maximum velocity and impedance ratios (Rmax and IRmax) for these borehole arrays. Both the linear empirical and theoretical results confirm that site response is site-specific and controlled predominantly by each site's in situ shear-wave velocity (Vs) profile in terms of shear-wave velocity ratios or impedance ratios, the maximum velocity or impedance ratio in particular. Our studies suggest that a minimum bedrock-sediment shear-wave velocity ratio of about 2.5 (i.e., Rmax≥2.5) or impedance ratios of about 3.0 (i.e., IRmax ≥ 3.0) is required to generate site resonance. Our results show that the fundamental mode is the most important mode for quantifying site response for engineering purposes at sites where amplifications are controlled by site resonance because it is the dominant mode on the ratios of response spectra. We also compared five proxies derived from each site's velocity profile—the time-averaged shear-wave velocity for the top 30 m (VS30), depth to bedrock or basin depth (Zb), time-averaged shear-wave velocity of sediment (Vs‾), ratio of bedrock shear-wave velocity (Vsb) to Vs‾ (Rmax), and ratio of bedrock impedance to the average sediment impedance (IRmax)—with the empirical fundamental resonant mode parameters. The comparisons revealed that Zb and Rmax (or IRmax) are the best pair of proxies to parameterize the fundamental resonant mode parameters. The comparisons also show that VS30 is not the best proxy and that no single proxy is sufficient to parameterize the fundamental resonant mode parameters.

Shear Wave Velocity Estimation Based on Deep-Q Network

Geoacoustic inversion is important for seabed geotechnical applications. It can be formulated as a problem that seeks an optimal solution in a high-dimensional parameter space. The conventional inversion approach exploits optimization methods with a pre-defined search strategy whose hyperparameters need to be fine-tuned for a specific scenario. A framework based on the deep-Q network is proposed in this paper and the environment and agent configurations of the framework are specially defined for geoacoustic inversion. Unlike a conventional optimization method with a pre-defined search strategy, the proposed framework determines a flexible strategy by trial and error. The proposed framework is evaluated by two case studies for estimating the shear wave velocity profile. Its performance is compared with three global optimization methods commonly used in underwater geoacoustic inversion. The results demonstrate that the proposed framework performs the inversion more efficiently and accurately.

Shear wave velocity and site period measurements for the western portion of the Mexico City Basin following the Mw7.1 2017 Puebla–Morelos, Mexico, earthquake

Dynamic site characterization was performed at 25 sites located on the western portion of the Mexico City Basin that were severely damaged during the Mw7.1 2017 Puebla–Morelos, Mexico, earthquake. Testing was conducted using active and passive seismic surface wave methods and the microtremor horizontal-to-vertical spectral ratio method to determine site periods and develop one-dimensional (1D) shear wave velocity ( Vs) profiles for the first 60 m of the subsoil. The measured site periods were compared to site period maps developed in 2004 and 2020 along with values computed using the Design Seismic Actions System (SASID) software following the 2020 version of the Complementary Technical Norms for Seismic Design (NTC-DS). The most noticeable biases in the predictions from the 2004 site period map were observed between the boundary of Zone II and Zone IIIa, at which site periods are overestimated. These estimates were improved upon in the 2020 site period map and showed a close similarity with SASID computed site period values. The Vs, depth, and thickness of the lacustrine clay layer were also determined to be quite variable within the basin. The softest sites are located between the lakebeds with a Vs between 45 and 57 m/s. Sites located toward the outer rim of the North lakebed have a higher Vs between 80 and 100 m/s. The thickness of the clay layer varies significantly in the western side of the Basin with values ranging between approximately 3 and 34 m. Overall, the results of this study indicate good agreement with the model embedded in the SASID software. The results (1) emphasize the need to regularly monitor changes that occur over time in the lacustrine clay layer, (2) complement the development of models that improve our understanding of wave propagation within the Basin, and (3) update and improve Mexico City’s Norms.

Influence of Uncertainty in Vs Profiles in the Southeastern Korea On Site Response Analyses

The effect of uncertainty on the shear wave velocity (VS) profiles from the Korean soil conditions on the seismic site response analysis (SRA) was investigated by comparing the amplification factors (AFs) computed from 170 VS profiles to those from three (representative) statistically derived VS profiles (Approach 1: a constant standard deviation of VS, σln(Vs), Approach 2: a depth-dependent, σln(Vs), and Approach 3: 5% and 95% percentiles for generating lower and upper bounds). All of the VS profiles are divided into three site classes (B, C, and D) by the National Earthquake Hazards Reduction Program (NHERP) site classification system. Seven pairs of ground motion records (two horizontal components) that represent Korean bedrock conditions are entered into all of the three types of VS profiles to run the SRA with a single type of modulus reduction and damping curves. The root-mean squared difference (RMSD) of the mean and standard deviation, (μln(AF), and σln(AF)) of the AFs in natural log-scale between the three types of VS profiles is estimated, which results in Approach 3 with the smallest RMSD measure for all the site classes. Conventionally, characterizing the uncertainty of the VS profiles is typically assessed through the representative profiles of Approach 1; however, this practice suggests that the inclusion of the percentile bounds (Approach 3) can be supplemental to better describe the VS uncertainty while taking the Approach 1 as primary representative profiles.