Variation Of Shear Wave Velocity Research Articles

Evaluation of variability of shear wave velocity in India

The shear wave velocity is a significant component in seismic hazard studies. This involves standard practice in seismic hazard analysis through ground motion prediction equations (GMPEs), ground response analyses (GRAs), or the computation of amplification factors. In practice, the shear wave velocity may be obtained through invasive methods, noninvasive methods, transformation models, or correlation-based methods. Currently, the uncertainty component of shear wave velocity has either been overlooked or oversimplified. The aim was to quantify the range of variations in shear wave velocity across India. The database included data of reported shear wave velocities in 20 states collected from various project reports and publications. A brief review of the methods used to obtain the data is given, along with a systematic highlight of the range of uncertainties encountered and their statistics. Base-case profiles for response analyses are recommended, and their statistics are examined. These observations aid computation of amplification factors. The transformation uncertainties of the shear wave velocity and the standard penetration test (SPT) were examined. The outcomes of the research will guide reliable seismic hazard studies across countries. Furthermore, this study paves the way for future development toward addressing uncertainties in shear wave velocity and facilitates the use of reliability-based design procedures.

Effect of proteinuria at relapse on shear wave velocity assessed using ultrasound elastography in children with idiopathic nephrotic syndrome.

Shear wave velocity (SWV) is an ultrasound elastography technique that provides much information for kidney disease assessment. However, the factors that alter SWV are not fully understood; it is unclear whether the variation in SWV seen in proteinuria associated with disease progression is due to tissue or proteinuria. This study investigated the effect of proteinuria on SWV. This prospective observational study compared SWV at remission with SWV at relapse in children treated for idiopathic nephrotic syndrome (INS) between April 2020 and December 2023. All relapses without oral steroids during the observation period were measured. SWV at remission was defined as the date closest to relapse during which repeated measurements were taken approximately every 3months after steroid discontinuation. Eight patients were treated for INS with a median observation period of 21.9months (11.8-27.1). Of the 15 relapses, five that met the definition were considered for the study. The median interval between the measurement at relapse and remission was 40days (11-55). SWV was significantly lower at relapse than remission (2.40 ± 0.20m/s vs. 2.14 ± 0.15m/s, P < 0.01). SWV decreased in the presence of severe proteinuria at relapse compared to the remission measurements. Although more cases need to be studied, the decrease in SWV may reflect the mechanism by which protein leaks into the urine, not just a direct change caused by the presence of proteinuria.

The Effects of Layer-Induced Elastic Anisotropy on Microseismic Monitoring Records in Underground Coal Mines

Real-time microseismic observations are extensively used in underground hard rock mines. However, microseismic monitoring in coal mines is not common. In this study, the features of microseismic (MS) event waveforms were discussed under conditions of thin-layered sedimentary media. The possibility of source mechanism evaluation was considered for the MS event recorded in coal mine. Moment tensor (MT) inversion of the waveforms can be challenging due to influence of thin-layered medium structure. Splitting of shear SH- and SV-waves is one of the characteristic features that affect MS records in coal mines. The effect of splitting was demonstrated using real microseismic data collected by an underground seismic array installed in the Polosukhinskaya coal mine, Russia. Velocities of shear SH- and SV-waves were estimated from travel times. The results reveal significant variations in shear wave velocities composed of a radial anisotropy of 75%, thus confirming the importance of a thin-layered model for moment tensor inversion. The thin-layered model was built for geological conditions at the Polosukhinskaya coal mine and synthetic seismograms were generated using QSEIS (Pyrocko OS). A full moment tensor of a single typical event was found employing the layered 1D velocity model.

Spatial variability of shear wave velocity: implications for the liquefaction response of a case study from the 2010 Maule Mw 8.8 Earthquake, Chile

Assessing the potential and extent of earthquake-induced liquefaction is paramount for seismic hazard assessment, for the large ground deformations it causes can result in severe damage to infrastructure and pose a threat to human lives, as evidenced by many contemporary and historical case studies in various tectonic settings. In that regard, numerical modeling of case studies, using state-of-the-art soil constitutive models and numerical frameworks, has proven to be a tailored methodology for liquefaction assessment. Indeed, these simulations allow for the dynamic response of liquefiable soils in terms of effective stresses, large strains, and ground displacements to be captured in a consistent manner with experimental and in-situ observations. Additionally, the impact of soil properties spatial variability in liquefaction response can be assessed, because the system response to waves propagating are naturally incorporated within the model. Considering that, we highlight that the effect of shear-wave velocity Vs spatial variability has not been thoroughly assessed. In a case study in Metropolitan Concepción, Chile, our research addresses the influence of Vs spatial variability on the dynamic response to liquefaction. At the study site, the 2010 Maule Mw 8.8 megathrust Earthquake triggered liquefaction-induced damage in the form of ground cracking, soil ejecta, and building settlements. Using simulated 2D Vs profiles generated from real 1D profiles retrieved with ambient noise methods, along with a PressureDependentMultiYield03 sand constitutive model, we studied the effect of Vs spatial variability on pore pressure generation, vertical settlements, and shear and volumetric strains by performing effective stress site response analyses. Our findings indicate that increased Vs variability reduces the median settlements and strains for soil units that exhibit liquefaction-like responses. On the other hand, no significant changes in the dynamic response are observed in soil units that exhibit non-liquefaction behavior, implying that the triggering of liquefaction is not influenced by spatial variability in Vs. We infer that when liquefaction-like behavior is triggered, an increase of the damping at the shallowest part of the soil domain might be the explanation for the decrease in the amplitude of the strains and settlements as the degree of Vs variability increases.

Emerging technologies and advanced analyses for non-invasive near-surface site characterization

The in-situ small-strain shear modulus of soil and rock materials is a parameter of paramount importance in geotechnical modeling. It can be derived from non-invasive geophysical surveys, which provide the possibility of testing the subsurface in its natural and undisturbed condition by inferring the velocity of propagation of shear waves. In addition, for soil dynamics and earthquake engineering applications, the small-strain damping ratio plays a relevant role, yet its estimation is still challenging, lacking consolidated approaches for its in-situ evaluation. Recent advancements in instrumentation, such as distributed acoustic sensing (DAS), combined with advanced analysis methodologies for the interpretation of seismic wave propagation (e.g., machine learning and full waveform inversion), open new frontiers in site characterization. This paper presents and compares some advanced applications of measuring 1D and 2D variations in shear wave velocity and attenuation in-situ with reference to a specific case history.

Three-dimensional mapping of ultrasound-derived skeletal muscle shear wave velocity.

Introduction: The mechanical properties of skeletal muscle are indicative of its capacity to perform physical work, state of disease, or risk of injury. Ultrasound shear wave elastography conducts a quantitative analysis of a tissue's shear stiffness, but current implementations only provide two-dimensional measurements with limited spatial extent. We propose and assess a framework to overcome this inherent limitation by acquiring numerous and contiguous measurements while tracking the probe position to create a volumetric scan of the muscle. This volume reconstruction is then mapped into a parameterized representation in reference to geometric and anatomical properties of the muscle. Such an approach allows to quantify regional differences in muscle stiffness to be identified across the entire muscle volume assessed, which could be linked to functional implications. Methods: We performed shear wave elastography measurements on the vastus lateralis (VL) and the biceps femoris long head (BFlh) muscle of 16 healthy volunteers. We assessed test-retest reliability, explored the potential of the proposed framework in aggregating measurements of multiple subjects, and studied the acute effects of muscular contraction on the regional shear wave velocity post-measured at rest. Results: The proposed approach yielded moderate to good reliability (ICC between 0.578 and 0.801). Aggregation of multiple subject measurements revealed considerable but consistent regional variations in shear wave velocity. As a result of muscle contraction, the shear wave velocity was elevated in various regions of the muscle; showing pre-to-post regional differences for the radial assessement of VL and longitudinally for BFlh. Post-contraction shear wave velocity was associated with maximum eccentric hamstring strength produced during six Nordic hamstring exercise repetitions. Discussion and Conclusion: The presented approach provides reliable, spatially resolved representations of skeletal muscle shear wave velocity and is capable of detecting changes in three-dimensional shear wave velocity patterns, such as those induced by muscle contraction. The observed systematic inter-subject variations in shear wave velocity throughout skeletal muscle additionally underline the necessity of accurate spatial referencing of measurements. Short high-effort exercise bouts increase muscle shear wave velocity. Further studies should investigate the potential of shear wave elastography in predicting the muscle's capacity to perform work.

Lateral variations of shear wave velocity (VS) profile and VS30 over gentle terrain

Lateral variations of shear wave velocity (VS) profile and time-averaged shear wave velocity in the upper 30 m (VS30) are important indicators of site condition and significant contributors to uncertainty in earthquake ground motion and seismic hazards. Few previous studies involved in quantifying these lateral variations, especially at short distances, due to limited data at closely spaced sites. This study quantitatively investigated the lateral variation of VS profile and VS30 over plain and piedmont terrains, focusing on short distances ranging from hundreds of meters to several kilometers. 2510 plain and 749 piedmont borehole profiles from the mainland China borehole profile database were employed to examine these variations using the variogram as the geo-statistics tool. The main findings include: 1) Over plain terrain, the variation in site conditions between sites does not significantly increase with separation distance within a certain range (typically from 1 km to 3–5 km). Over piedmont terrain, site condition variation steadily increases with distance. 2) The variation of site condition over piedmont terrain is higher than that over plain terrain. Nonetheless, for both terrains, the dispersion of site condition between sites at distance of <1 km is engineering negligible, and the correlation of site conditions between sites within 5 km remains strong; 3) For both terrains, the lateral variation of VS in deeper layers is higher than that in shallower layers. The study outcomes emphasize the importance of incorporating the information of nearby measurements in site condition estimations. The study outcomes can also be utilized in the optimization of borehole sampling design for microzonation projects and the evaluation of site condition dispersion at inferred sites.

Shear wave crustal velocity structure in the Garhwal-Kumaon Himalaya based on noise cross-correlation of Rayleigh wave

The tectonics of the Garwal-Kumaon Himalaya is characterized by thrusts, tectonic windows, and klippen. We investigate crustal shear wave velocity variations using ambient noise cross-correlation tomography. The studied region encompasses the Kali River valley in the east to Satluj valley in the west, with the adjoining Indo-Gangetic Plain in the south covering the northern part of the Delhi-Haridwar ridge. The fundamental mode group velocities of Rayleigh waves are extracted from cross-correlation data of 33 broadband seismological stations from a regional seismic network. A total of 374 dispersion curves with a period range of 4–29 s show a group velocity variation between ∼2.3 and ∼ 3.4 km/s. The shear wave velocity structure of the uppermost lithosphere down to ∼50 km obtained by non-linear inversion of the Rayleigh wave dispersion data provides new insight into the geometry of the crust. A large variation in Vs in the range of ∼2.8 to ∼4.7 km/s corresponds to a variety of changes in the tectonic deformation, structure, and crustal thickness of the Himalayan wedge. Thick low-velocity sedimentary formations are identified beneath the Indo-Gangetic Plain and the frontal Himalaya. Anomalous low Vs zones are also observed in the mid-crust beneath the higher Himalaya and southern Tibet, indicating partial melting or the presence of aqueous fluid zones. The high-velocity anomalies may be correlated with duplex structures beneath the Lesser Himalaya and with lithospheric flexure.

The upper crustal shear wave structure of the Zagros collision zone from local earthquake Rayleigh wave tomography

The Zagros fold-and-thrust belt is the principal response to the Arabia–Eurasia continental collision, which generates significant seismic activity in Iran. Surface wave tomography can be a proper tool for crustal structure imaging in this region. We used observed dispersion curves of Rayleigh wave derived from high-quality local-earthquake waveforms to obtain 2D tomographic maps in the period range 1–28 s beneath the Zagros collision zone. Subsequently, local pseudo-dispersion curves for individual grid points are inverted for shear-wave velocity structure, in order to obtain a 3D velocity model of the crust. Our results indicate shallow low-velocity structures near the suture zone. Moreover, a low-velocity flexure shape in the south of the Zagros Mountain Front Fault could be due to the thick sedimentary basin beneath the Dezful embayment. Relatively an uplifting of the deeper structures beneath Sanandaj-Sirjan and the high Zagros might could be led to high-velocity structures at mid-crustal depths. The variation of shear-wave velocity along the Zagros indicates the inhomogeneous structures along this folding and thrust belt.

Time-lapse global inversion for surface-waves: A differential approach using a linear approximation of the Rayleigh wave phase velocity

Surface-wave methods have a wide range of possible applications in the monitoring of near-surface media. In classical monitoring methods, data measured at different times are inverted separately and independently, and the difference in the inversion results reflects the temporal changes in the medium. However, the results obtained in this way can be affected by the reproducibility of experimental measurements, measurement and inversion uncertainties, etc. In this study, we introduce Differential Inversion in surface-wave methods, which uses the difference between measured data as inversion input data, instead of the measured data themselves. More precisely, a linear approximation of Rayleigh wave phase velocity associated with its sensitivity kernel is used to relate the data difference with the model variations. This differential inversion approach is firstly tested with numerical data generated in a series of two-layer models in order to estimate the limitations of the linear approximation with a global inversion approach. It is shown that when the shear-wave velocity variation is more than 5%, the linear assumption is no longer relevant. With respect to the 5% of variation, we apply the differential inversion approach on laboratory data, obtained from three reduced-scale epoxy-resin two-layer models. The results show the feasibility of the differential inversion to estimate the shear-wave velocity differences between epoxy-resin models. The short calculation time is one of its advantages. However, a good estimation on the baseline is required in order to calculate the sensitivity kernel. Finally, the robustness of the proposed approach is verified by numerical data, using the Spectral Element Method to simulate wave propagation in the presence of an under-surface cavity.

Probabilistic shear wave velocity model based on downhole test database for Apennine flysch formations

Defining a reference shear wave velocity profile (Vs) is crucial for seismic-hazard assessment and seismic response analysis. However, selecting a reference velocity profile is not straightforward, mainly because of the high variability of shear wave velocity in complex formations. This study considered 26 shear wave profiles from downhole tests carried out in the Laga Basin (Gran Sasso, Central Apennines), where the soil can be categorised as complex owing to the chaotic succession of turbidite and Quaternary continental deposits. Downhole tests and stratigraphy were used to (i) develop a probabilistic model for predicting the shear wave profiles calibrated on the collected test data and (ii) propose an outlier detection method for estimating the first fundamental frequency of the deposit from horizontal-to-vertical spectral ratio (HVSR) analyses. Specifically, the stochastic shear wave model provides the uncertainty of the first fundamental frequency estimated from downhole tests based on a simplified mechanics-based formulation. The defined bounds, were forward uncertainty propagation following a Monte Carlo approach, have been used to define the criteria for outlier detection in HVSR frequency assessments, given an approximate estimate of the equivalent depth.

Characterization of a leaky earth dam using integrated geophysical surveys

Early earth dams were mainly constructed using a soil core wall, which is susceptible to a variety of potential risks such as inhomogeneity, poor compactness, and leakage due to multiple fillings and biological damage. To ensure safe operation of the dam, cost-effective geophysical non-destructive testing techniques should be considered during routine maintenance. This paper presents a case study of an earth dam designed for water storage and flood control that experienced a leak problem. To provide a basis for future leakage control, we carried out 2D integrated geophysical surveys utilizing Electric Resistivity Tomography (ERT), Ground Penetrating Radar (GPR), and Surface Wave (SW) methods along the dam axis. In conjunction with the fast attenuation characteristic of GPR signals, the low resistivity anomalies recovered by the ERT provide evidence to reveal the leakage area within the dam structure. Moreover, the shear wave velocity variation retrieved by the SW method helps to determine the distribution of bedrock and soil layers. By integrating the results mentioned above, the interpretation outcome yields a comprehensive understanding of the internal condition of the earth dam. The successful investigation of the dam structure and the mapping of the leakage area demonstrate the effectiveness of integrated geophysical surveys in characterizing earth dams. It also suggests that it is essential to consider the difference between various geophysical results and the underlying reason when integrating them to characterize leaky earth dams.

The seismic structure of Villarrica Volcano revealed by ambient noise tomography

We compute the first ambient noise tomography of Villarrica Volcano, obtaining a 3D Vs model. Using ambient noise recorded from May to July 2014 from 18 seismic stations, we measured the Rayleigh wave group velocity by stacking daily cross-correlations between station pairs. The resulting group velocity curves were inverted into 2D tomographic maps with periods ranging between 1.0 and 4.9 s. These tomographic maps were locally inverted through the neighborhood algorithm to obtain an accurate Vs model in depth. Our findings reveal velocity variations within the Villarrica Volcano region, reaching a depth of around 7.5 km. The resulting 3D shear-wave velocity model has provided novel insights into the internal structure of the area. Our study has identified slow velocity anomalies in the area of the Liquiñe-Ofqui Fault Zone, and horizontal variations in shear-wave velocity that run parallel to the volcanic lineament from the Mocha-Villarrica Fault Zone. We also note the presence of a negative velocity anomaly located southeast of the Villarrica summit, in a position consistent with the magmatic plumbing system proposed by earlier geophysical studies conducted in the region.

Density-dependent relationship between changes in ultrasonic wave velocities, effective stress, and petrophysical-elastic properties of sandstone

The present study aims to reveal the importance of density as a moderator variable in interpretation of possible relationships between variations in compressional and shear wave velocities (ΔVp and ΔVs), effective stress, and rock’s petrophysical and elastic properties. Towards this end, fourteen subsurface sandstone samples were collected and analyzed through the measurement of ultrasonic wave velocities at standard and reservoir conditions within a Triaxial-testing cell. The results were interpreted for two groups, i.e., low density (LD) and high density (HD) samples, where greater ΔVp and ΔVs were observed for the HD group samples that have similar average porosity and permeability compared to samples from LD group. Effective stress exhibits better fit with ΔVp and ΔVs for the LD compared to HD group samples. Density was revealed to be well fitted with ΔVp of LD and ΔVs of HD samples. Porosity exhibits good fit with ΔVs of LD and permeability exhibits good fit with ΔVp of both LD and HD groups. Also, variations in estimated elastic limit (ΔEd) is well matched with ΔVs, while changes in estimated Poisson’s ratio (Δν) manifests a good fitting with ΔVp. Finally, variation in deviatoric stresses from triaxial tests is in a good agreement with ΔVp. The results of this study provide convenient insights for conversion of wave velocities and elastic properties between standard and reservoir conditions.

Influence of shear wave velocity heterogeneity on SH-wave reverberation imaging of the mantle transition zone

SUMMARY Long-period (T &amp;gt; 10 s) shear wave reflections between the surface and reflecting boundaries below seismic stations are useful for studying phase transitions in the mantle transition zone (MTZ) but shear-velocity heterogeneity and finite-frequency effects complicate the interpretation of waveform stacks. We follow up on a recent study by Shearer &amp; Buehler (hereafter SB19) of the top-side shear wave reflection Ssds as a probe for mapping the depths of the 410-km and 660-km discontinuities beneath the USArray. Like SB19, we observe that the recorded Ss410s-S and Ss660s-S traveltime differences are longer at stations in the western United States than in the central-eastern United States. The 410-km and 660-km discontinuities are about 40–50 km deeper beneath the western United States than the central-eastern United States if Ss410s-S and Ss660s-S traveltime differences are transformed to depth using a common-reflection point (CRP) mapping approach based on a 1-D seismic model (PREM in our case). However, the east-to-west deepening of the MTZ disappears in the CRP image if we account for 3-D shear wave velocity variations in the mantle according to global tomography. In addition, from spectral-element method synthetics, we find that ray theory overpredicts the traveltime delays of the reverberations. Undulations of the 410-km and 660-km discontinuities are underestimated when their wavelengths are smaller than the Fresnel zones of the wave reverberations in the MTZ. Therefore, modelling of layering in the upper mantle must be based on 3-D reference structures and accurate calculations of reverberation traveltimes.

Relation of shear wave velocity variations with depth for different lithologies: A contribution towards mitigating the region's seismic risk

The state of Uttarakhand in India lies in a highly seismically active zone, therefore earthquake re- sistant design criteria have been followed for major construction practice in this region. One of the major inputs for earthquake-resistant design is average shear wave velocity (VS ) at 30 m depth. In the present work, at twenty-six different sites, the Horizontal to vertical spectral ratio (HVSR) technique and multichannel analysis of surface wave (MASW) is used for joint analysis to obtain a one-dimensional velocity model that supports both the HV spectrum and dispersion curve obtained from HVSR and MASW methods, respectively. Shear wave profiles obtained at various sites are compared with the bore log data obtained from drilling and it clearly shows that the (VS ) for the same formation have a strong dependence on the depth at which it occurred. Data from twenty- four shear wave profiles have been used to prepare a linear regression relation of (VS ) for different lithological formations with respect to their depth of occurrence. Root mean square error obtained from the developed relationship for various lithologies clearly shows that the results are fairly well within the range of acceptance. The developed relations have been further validated by calculating Vs profile from bore log data obtained at two new location sites that are not included in the data set used for the preparation of regression relations. A comparison of two velocity sections clearly shows that the velocity profile computed from regression relation matches closely with that obtained from the seismic survey and thereby establishing the efficacy of developed regression relationship for its practical implementation. This developed relation allows soil type classification for zoning purposes.

Spatial variation in mechanical properties along the sciatic and tibial nerves: An ultrasound shear wave elastography study

Ultrasound shear wave elastography has become a promising method in peripheral neuropathy evaluation. Shear wave velocity, a surrogate measure of stiffness, tends to increase in peripheral neuropathies regardless of etiology. However, little is known about the spatial variation in shear wave velocity of healthy peripheral nerves and how tensile loading is distributed along their course. Sixty healthy young adults were scanned using ultrasound shear wave elastography. Five regions of the sciatic (SciaticPROXIMAL, SciaticDISTAL) and tibial nerve (TibialPROXIMAL, TibialINTERMEDIATE, and TibialDISTAL) were assessed in two hip positions that alter nerve tension: 1) neutral in supine position; and 2) flexed at 90°. Knee and ankle remained in full-extension and neutral position. We observed spatial variations in shear wave velocity along the sciatic and tibial nerve (P < 0.0001). Shear wave velocities were significantly different between all nerve locations with the exception of SciaticDISTAL vs. TibialINTERMEDIATE (P = 0.999) and TibialPROXIMAL vs. TibialINTERMEDIATE (P = 0.708), and tended to increase in the proximal-distal direction at both upper and lower leg segments. Shear wave velocity increased with hip flexion (+54.3%; P < 0.0001), but the increase was not different among nerve locations (P = 0.233). This suggests that the increase in tensile loading with hip flexion is uniformally distributed along the nerve tract. These results highlight the importance of considering both limb position and transducer location for biomechanical and clinical assessments of peripheral nerve stiffness. These findings provide evidence about how tension is distributed along the course of sciatic and tibial nerves.

Radial anisotropy and S-wave velocity depict the internal to external zone transition within the Variscan orogen (NW Iberia)

Abstract. The cross-correlation of ambient noise records registered by seismic networks has proven to be a valuable tool to obtain new insights into the crustal structure at different scales. Based on 2 to 14 s period Rayleigh and Love dispersion data extracted from the seismic ambient noise recorded by 20 three-component broadband stations belonging to two different temporary experiments, we present the first (i) upper-crustal (1–12 km) high-resolution shear wave velocity and (ii) radial anisotropy variation models of the continental crust in NW Iberia. The area of study represents one of the best-exposed cross sections along the Variscan orogen of western Europe, showing the transition between the external eastern zones towards the internal areas in the west. Both the 2-D maps and an E–W transect reveal a close correspondence with the main geological domains of the Variscan orogen. The foreland fold-and-thrust belt of the orogen, the Cantabrian Zone, is revealed by a zone of relatively low shear wave velocities (2.3–3.0 km s−1), while the internal zones generally display higher homogeneous velocities (&gt; 3.1 km s−1). The boundary between the two zones is clearly delineated in the models, depicting the arcuate shape of the orogenic belt. The velocity patterns also reveal variations of the elastic properties of the upper crust that can be linked to major Variscan structures, such as the basal detachment of the Cantabrian Zone, the stack of nappes involving pre-Variscan basement, or sedimentary features such as the presence of thick syn-orogenic siliciclastic wedges. Overall, the radial anisotropy magnitude varies between −5 % and 15 % and increases with depth. The depth pattern suggests that the alignment of cracks is the main source of anisotropy at &lt; 8 km depths, although the intrinsic anisotropy seems to be significant in the West Asturian–Leonese Zone, the low-grade slate belt adjacent to the Cantabrian Zone. At depths &gt; 8 km, widespread high and positive radial anisotropies are observed, which we attribute to the presence of subhorizontal alignments of grains and minerals in relation to the pre- or syn-orogenic deformation associated with the Variscan orogenesis.

The possible use of equivalent homogeneous subsoil models for 1D seismic response analyses in seismic microzonation studies

The possibility is here explored to use an ‘equivalent’ homogeneous configuration to simulate 1D seismic response of heterogeneous engineering-geological bodies when relatively weak seismic impedance contrasts internal to the bodies (it was assumed a shear wave velocity variation between the alternating layers equal to 150 m/s) only exist above the seismic bedrock. This equivalent configuration is obtained by considering an equivalent Vs value the harmonic average of the actual Vs values and a linear combination of G/G0 and D curves relative to the lithotechnical components present in the actual configuration. To evaluate feasibility of this approach, a wide set of numerical simulations was carried out by randomly generating subsoil layering including sequences of alternating thin layers of geotechnical units (e.g. sands and clays) each characterized by a characteristic nonlinear curve. Outcomes of these simulations are compared with those provided by considering a single homogeneous layer characterized by equivalent nonlinear curves obtained as a weighted average of the original curves. By comparing the heterogeneous and the homogeneous columns seismic response in terms of amplification factors and fundamental period, the results confirm the possibility to model a 1D column characterized by a generic lithostratigraphic succession with an equivalent one without introducing significative errors that, at least for the studied cases, do not exceed the 6%. This conclusion is substantially confirmed by extending the comparison to a real case, i.e., the 113 m-thick heterogeneous soil profile at Mirandola site (Northern Italy), presented in the last part.

ОБ ОЦЕНКАХ ТОЧНОСТИ МЕТОДА МИКРОСЕЙСМИЧЕСКОГО ЗОНДИРОВАНИЯ

In this paper, the author considers the necessity of the microseismic sounding method (MSM) and possible approaches to estimation of its accuracy. The MSM method is a passive amplitude method for seismic tomography in which spatial variations in Rayleigh wave amplitudes are associated with variations in shear wave velocities. Two statistical methods for estimating the accuracy of the values given on the high-speed sections of the MSM method are discussed.