Shear Wave Velocity Distribution Research Articles

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.

Three-dimensional shear wave velocity prediction by integrating post-stack seismic attributes and well logs: application on Asmari formation in Iran

Understanding the distribution of shear wave velocity (VS) in hydrocarbon reservoirs is a crucial concern in reservoir geophysics. This geophysical parameter is utilized for reservoir characterization, calculating elastic properties, assessing fractures, and evaluating reservoir quality. Unfortunately, not all wells have available VS data due to the expensive nature of its measurements. Hence, it is crucial to calculate this parameter using other relevant features. Therefore, over the past few decades, numerous techniques have been introduced to calculate the VS data using petrophysical logs in wells with limited information. Unfortunately, the majority of these methods have a drawback they only offer insight into the location of the wells and do not provide any details regarding the distribution of VS in the space between the wells. In this article, we employed three-dimensional post-stack seismic attributes and well-logging data integration to predict the distribution of VS in the Asmari formation in an Iranian oil field. To accomplish this objective, the model-based seismic inversion algorithm was utilized to convert the seismic section into the acoustic impedance (AI) section. Then, AI and seismic data were utilized in the cross-validation method to determine the relevant attributes for predicting the spatial distribution of VS throughout the entire reservoir area, using an artificial neural network. The proposed method was shown to provide 94% correlation and 109 m/s error between the actual and estimated VS. Also, the calculated VS section has a high correlation with the actual logs at the location of the wells.

Automatic Extraction of Surface Wave Dispersion Curves Using Unsupervised Learning and High‐Resolution Tau‐p Transform

AbstractDispersion curves for surface wave recordings are required input for many surface wave inversion methods to image subsurface shear wave velocity distribution, while the conventional extraction of dispersion curves requires significant amount of human interaction. This step impedes efficiency enhancement of surface wave analysis and its automation. In this paper, we present an unsupervised learning scheme to achieve efficient automatic picking of dispersion curves of the fundamental mode for surface wave gathers. This scheme is composed of four major steps: computing a frequency velocity(f‐v) spectrum for the surface wave gather using a high‐resolution Tau‐p transform improved by the iteratively shrinkage Thresholding algorithm (ISTA) algorithm, generating clusters points along the dispersion energy in the f‐v spectrum via a weighted Kmeans algorithm, filtering these cluster points by principal component analysis (PCA) and Local Outlier Factor (LOF) algorithms to remove the erroneous clusters, and fitting the remaining clusters to form the dispersion curve. Tests with synthetic and field noisy surface wave recordings demonstrated the effectiveness of this approach and its potential in automatic processing of noisy surface wave data sets.

Subsoil density field reconstruction through 3-D FWI: a systematic comparison between vertical- and horizontal-force seismic sources

SUMMARY Bulk-density (ρ) of soil is an important indicator of soil compaction and type. A knowledge of the spatial variability of in situ soil density is important in geotechnical engineering, hydrology and agriculture. Surface geophysical methods have so far shown limited success in providing an accurate and high-resolution image of 3-D soil-density distribution. In this pursuit, 3-D seismic full-waveform inversion (FWI) is promising, provided the robustness and accuracy of density inversion via this approach can be established in the near-surface scale. However, simultaneous reconstruction of ρ and seismic wave velocities through multiparameter FWI remains a challenging task. Near-surface seismic data are commonly dominated by dispersive surface waves whose velocities are controlled by the value and distribution of shear-wave velocity (VS). One major difficulty in estimating reliably ρ from near-surface seismic data is due to the relatively low sensitivity of the seismic wavefield to ρ compared to seismic velocities. Additionally, the accuracy of the estimated ρ decreases due to error in VS—an issue known as parameter coupling. Parameter coupling makes it difficult to estimate accurately ρ within the framework of conventional gradient-based FWI. More sophisticated optimization approaches (e.g. truncated Newton) can reduce the effect of parameter coupling, but these approaches are commonly not affordable in near-surface applications due to heavy computational burden. In this research, we have investigated how choosing correctly the force direction of the seismic source can contribute to a higher accuracy of ρ estimates through 3-D FWI. Using scattered wavefields, the Hessian, and inversion tests, an in-depth and systematic investigation of data sets corresponding to different force directions has been carried out. A comparison of the scattered wavefields due to a point-localized ρ perturbation for different force directions shows the robustness of the horizontal-force data set to noise compared to the vertical-force data set. Furthermore, for a point-scatterer model, an analysis of the gradients of the misfit function using the Hessian shows that utilizing a horizontal-force source enables one to reconstruct the high-resolution gradient with relatively small parameter coupling. Finally, inversion tests for two different subsoil models demonstrate that 3-D FWI on a horizontal-force-source seismic data set is capable of providing a more accurate 3-D ρ distribution in soil compared to a vertical-force-source data set. Our results show that the use of a horizontal-force source might allow avoiding computationally demanding, costly optimization approaches in 3-D FWI.

Magma and hydrothermal sources below the northern part of Paramushir Island (Kuril Arc) inferred from ambient noise tomography

Paramushir is the northernmost island of the Kuril Arc comprising several Holocene volcanoes, of which two are presently active: Ebeko and Chikurachki. We present the first crustal-scale three-dimensional seismic shear-wave velocity model of the northern part of Paramushir derived with the use of data of 20 temporary stations operated in 2021–2022 and one permanent station. The continuous seismic data were used to obtain the Rayleigh wave dispersion curves for periods ranging from 0.5 s to 12 s through computing ambient seismic noise cross-correlation functions. Then the 3D shear wave velocity distribution was constructed by seismic tomography. The synthetic tests demonstrate that the model has sufficient resolution down to 7–10 km depth. We observe a series of low-velocity anomalies at the depths of 4–6 km below the volcano-origin Vernadsky Ridge. They may likely represent magma reservoirs that are responsible for Holocene eruptions along the ridge. These anomalies are overlain by the high-velocity anomalies associated with the rigid cover consisting of an upper-crustal granite-granodiorite body and consolidated magma intrusions. Along the eastern flank of Ebeko we reveal a thin low-velocity anomaly at a depth of ∼1 km that can be interpreted as a water-saturated layer. The contact of this layer with hot magma intrusions leads to the formation of a large amount of steam that is ejected during the frequent phreatic eruptions of Ebeko and from numerous fumaroles in the summit area. On the other hand, this layer, which is following down to the Pacific Coast, might be promising for the geothermal energy exploitation.

LANDSLIDE POTENTIAL ANALYSIS ON NEW ROAD OF UNDIP-JANGLI CAMPUS, SEMARANG USING MICROTREMOR METHOD

Tembalang is one of the sub-districts in the Tembalang District where landslides or landslides often occur (BPS Kota Semarang, 2021). The New Undip-Jangli Semarang Campus Road is a research area which is one of the areas with a medium level of vulnerability to ground motion (PVMBG, 2022). The study was conducted using the HVSR microtremor method with a total distribution of 40 measurement points. This study aims to identify areas prone to ground movement or landslides based on subsurface characteristics from the distribution of seismic susceptibility index (Kg) values, ground shear strain (GSS) values, identify profiles of subsurface characteristics laterally based on the distribution of shear wave velocity values (Vs). , around Jalan Baru Campus Undip-Jangli Semarang. Based on the research results, the distribution value of the seismic vulnerability index (Kg) around Jalan Baru Campus Undip-Jangli is between 0.27-17.27. The distribution value of Ground Shear Strain (GSS) in the study area is between 2.46x10-4-1,39x10-3. The value of the shear wave velocity distribution (Vs) around Jalan Baru Campus Undip-Jangli Semarang is between 179-2,981 m/s. Earth movement vulnerability analysis based on the parameters above indicates several zones of vulnerability to ground movement, namely at points 3, 4, 5, 6, and 7. Points 9, 10, 13, 14 and 16 and between points 16, 24 and 32. Points 22, 30 and 38 are to the east of the New Undip-Jangli Campus Road. The zone between points 3, 4, 5, 6 and 7 is to the south of the New Undip-Jangli Campus Road. At points 9, 10, 13, 14, and 16 the possible ground motions are rotational avalanches moving south.

Improving Surface Wave Retrieval From Traffic Noise by Deconvolution of the Decomposed Wavefield

AbstractTraffic noise is an important type of passive seismic data because it usually includes strong dispersive surface wave components and can be easily accessed. It can be used to extract virtual surface waves via seismic interferometry algorithms for the purpose of imaging subsurface shear wave velocity distribution. In this paper, we propose a scheme to improve the retrieval of surface waves from traffic noise recorded using linear arrays along traffic roads. By deconvolving the decomposed traffic noise wavefield, robust surface wave traces can be computed from a short noise record. First the far‐field component of the traffic noise recording is extracted and separated into unidirectionally propagating components. Then deconvolution interferometry is applied to these separated far‐field wavefield to extract surface wave Green's function. With this scheme, crosstalk noise and near‐field artifacts are excluded from the computation, and surface wave traces with high signal‐to‐noise ratio (SNR) are achieved using short traffic noise traces. In a synthetic test virtual surface waves estimated with the proposed method show significantly higher SNR than those computed with the conventional interferometry workflows, and matches well with simulated active source traces. A field data example with traffic noise recorded in a distributed acoustic sensing experiment also shows that surface waves estimated using the proposed methodology demonstrate higher SNR than those computed with the conventional interferometry schemes and that the virtual surface waves generated using 4 s of traffic noise demonstrate signal quality comparable to the surface waves recorded in this experiment with an active source.

Determination of the Earth’s structure based on intermediate-period surface wave recordings of tidal gravimeters: a case study

Tidal gravimeters can detect intermediate-period surface waves with high accuracy. Three gravimetric stations with estimated transfer functions and co-located with seismic stations were selected: two in Belgium (Membach and Rochefort, in Western Europe) and one in Germany (Black Forest, in Central Europe). The compatibility of gravimetric and seismic recordings of earthquakes in the period range of 10-180 s has been presented. The series of monochromatic signals separated from surface waves for selected events have been calculated using the multiple-filtering procedure, and averaged fundamental-mode Rayleigh wave group-velocity curves have been estimated on a regional scale. Next, averaged dispersion curves for three regions (Italy, Greece, and Western Turkey) were inverted by weighted linear inversion methods. Additionally, a quantitative analysis of resolution tests of inverted models was presented to show the capabilities of the gravimetric data to retrieve a shear-wave velocity distribution with a depth. In particular, a method for determining the depth range of the inverted model has been proposed. Results obtained based on gravimetric data were verified by results from seismic data obtained by applying the same procedure. This study shows the novel application of tidal gravimetric data in the determination of the regional Earth’s structure based on intermediate-period surface waves recordings, as well as a comprehensive approach to the quantitative estimation of a final model resolution.Graphical

Seismic Hazard Zonation in Gedebage Future Development in Bandung City Using HVSR Inversion

Greater Bandung, the largest economic growth corridor in Indonesia after Jabodetabek, is sitting on the Bandung basin. This deep sedimentary basin is situated just 12 km south of the active Lembang fault. Gedebage is an area intended to be the new economic and business center located in the easternmost of Bandung. The research aims to identify the vulnerability of Gedebage area against seismic ground motions. The area's morphology is dominated by a flat area with 0 - 8% of slope and predominantly composed of an old lake deposit. Topography, including basement morphology, sediment thickness, and physical properties, plays a great role in escalating/de-escalating seismic ground motions. A specific morphology may trap and prolong seismic shaking. Furthermore, stiffness and bedrock depth are instrumental in passing the spectral ground motions to the surface. The HVSR inversion method is applied to map subsurface conditions that successfully applied in Palu and its surrounding area. The research shows that Gedebage areas are vulnerable to the seismic hazard, referring to the shear wave velocity (Vs30) distribution and seismic hazard micro zonation maps. The discussion of the research findings is useful for future infrastructure development in the research area. The area is categorized as soft soil and medium soil classes, and it has a high vulnerability for destruction if there is an earthquake. The area should be cleared from vital infrastructures such as government buildings, schools, or hospitals.

KECEPATAN GELOMBANG GESER (VS) DAN KETEBALAN SEDIMEN (H) DI KABUPATEN KLATEN DARI DATA MIKROTREMOR

Research on Shear Wave Velocity (vs) and Sediment Thickness (h) in Klaten Regency, Central Java from field microtremor data aims to determine the thickness of sediment or soft sediment as one factor of high and low earthquake vulnerability. Microtremor data was collected using a three-component TDS 303 seismometer covering 111 measuring points in the Klaten Regency area. In this study, the sediment thickness was calculated using the equation shear wave velocity (vs) divided by four times the dominant frequency (fg). The dominant frequency (fg) value is obtained from the HVSR curve. In contrast, the shear wave velocity value (vs) used in this study is the inversion value of the HVSR curve, which is modeled based on borlog data at the Wedi Church, equated for all measurement points. The map of the shear wave velocity distribution at each microtremor measuring point has a vs value; this value is used to calculate the thickness of the sediment at each end. So that the distribution of the thickness value of the sediment at each point is obtained, then a thickness map is made covering the study area. The results showed that, in general, the research area has a Vs value in the effects of the HVSR curve inversion ranging from 130 m/s to 570.83 m/s, Vs average 275.68 m/s. The areas with vs are relatively high in the southern part of Gantiwarno, Wedi, and Bayat subdistricts directly adjacent to Gunung Kidul. Areas with relatively low-value of vs are in Prambanan Subdistrict, Northern Wedi Subdistrict, Trucuk Subdistrict, Jogonalan Subdistrict, South Klaten District, Central Klaten District, and Kalikotes Subdistrict. For sediment thickness (h) ranges from 5.105 m to 113.648 m. Areas with relatively high sediment thickness are located in parts of Wedi Subdistrict with a thickness of up to 110 m. Areas with medium thickness (around 60 to 90 m) are in the Gantiwarno sub-district, Prambanan district, and the southern part of the Jogonalan sub-district. In Bayat District, it has a low thickness (about 5 to 30 m) located in the proves and mountains of the Paseban area and the tomb of Sunan Pandanaran. The result shows that areas with high sediment thickness, namely in Wedi District, have a high level of earthquake vulnerability.

Reconstruction, with tunable sparsity levels, of shear wave velocity profiles from surface wave data

SUMMARYThe analysis of surface wave dispersion curves is a way to infer the vertical distribution of shear wave velocity. The range of applicability is extremely wide: going, for example, from seismological studies to geotechnical characterizations and exploration geophysics. However, the inversion of the dispersion curves is severely ill-posed and only limited efforts have been put in the development of effective regularization strategies. In particular, relatively simple smoothing regularization terms are commonly used, even when this is in contrast with the expected features of the investigated targets. To tackle this problem, stochastic approaches can be utilized, but they are too computationally expensive to be practical, at least, in case of large surveys. Instead, within a deterministic framework, we evaluate the applicability of a regularizer capable of providing reconstructions characterized by tunable levels of sparsity. This adjustable stabilizer is based on the minimum support regularization, applied before on other kinds of geophysical measurements, but never on surface wave data. We demonstrate the effectiveness of this stabilizer on (i) two benchmark—publicly available—data sets at crustal and near-surface scales and (ii) an experimental data set collected on a well-characterized site. In addition, we discuss a possible strategy for the estimation of the depth of investigation. This strategy relies on the integrated sensitivity kernel used for the inversion and calculated for each individual propagation mode. Moreover, we discuss the reliability, and possible caveats, of the direct interpretation of this particular estimation of the depth of investigation, especially in the presence of sharp boundary reconstructions.

Landslide characterization using active and passive seismic imaging techniques: a case study from Kerala, India

The unusually intense precipitations of the 2018 monsoon triggered numerous landslides in the Western Ghats region, southwest of India. Although the landslides caused no casualties, significant damage to property and infrastructure was observed. We present, as a case study, the results of active and passive seismic prospecting at two of those landslides with the goal of characterizing them, in a first application of shallow seismic exploration to landslides in the region. Our deployments included both sites perturbed by the landslides and unperturbed slopes adjacent to them with the purpose of identifying possible structural differences between slopes that underwent landsliding from slopes that were not affected. We analyze seismic sections obtained using the multi-channel analysis of surface waves technique and compare the results with seismic noise analyzed using seismic interferometry. We show that different analyses give similar results. The lateral variations observed in the shear-wave velocity distribution below the different profiles are well correlated with dominant frequency determined from seismic noise horizontal-to-vertical spectral ratios. Our measurements were taken after the landslides occurred. In hindsight, our results suggest that soil thickness played a major role in the triggering of landslides.

Nevşehir Castle Region in Turkey Interpreted by the Use of Seismic Surface Wave and Electrical Resistance Measurements Together

Abstract The underground city beneath the Nevşehir Castle located in the middle of Cappadocia region in Turkey with approximately cone shape is investigated by jointly utilizing the modern geophysical techniques of seismic surface waves and electrical resistivity. The systematic void structure under the Nevşehir Castle of Cappadocia, which is known to have widespread underground cities, is studied by the use of 33 separate two-dimensional profiles ~4-km long where electrical resistivities and seismic surface waves are concurrently measured. Seismic surface wave measurements are inverted to establish the shear-wave velocity distribution while resistivity measurements are inverted to resolve the resistivity distribution. Several high-resistivity anomalies with a depth range 8-20 m point to a systematic void structure beneath the Nevşehir Castle. We were able to effectively isolate the void structure from the embedding structure since the currently employed resistivity instrument has provided us high resolution quality measurements. Associated with the high resistivity anomalies there exist low-velocity depth zones acquired from the surface wave inversions also pointing to a systematic void structure where three-dimensional visualization techniques are used to show the extension of the void structure under the studied area.

Ground subsidence and its implication on building seismic performance

Ground subsidence due to underground water extraction leads to changes in dynamic properties such as shear wave velocity distribution and shear modulus degradation and damping curves, and more importantly, soil layer thickness and shape. These effects modify site predominant periods, frequency content, and spectral accelerations in both the free field and soil-structure systems during an earthquake. These variations can be substantial in highly compressible very soft clays, such as those found in the Mexico City basin. This paper examines the complex role that ground conditions, including settlement variability and structural tilting due to regional ground subsidence, had in the seismic performance of buildings during the Mw 7.1 2017 Puebla-Mexico City earthquake. A numerical study was undertaken to simulate the seismic response of conventional buildings supported by a compensate box-like foundation in soft clay, representative of those that exhibited major damage or collapse. Sets of three-dimensional finite difference models were developed with FLAC3D. Initially, the overall static behavior of the soil-structure system was assessed, considering both ground settlement and structure tilting due to ground consolidation, resembling the conditions prevailing right before the earthquake. Then, the seismic performance evaluation during the dynamic event was conducted at several consolidation times (i.e. 0, 25, 32 and 45 years). From the results gathered herein, it was clearly established the effect of the variation of the fundamental period of the soil deposit, and the initial ground deformation and structural tilting, on the seismic demand acting on the structure during the earthquake, which in turn, lead to most of the observed failures.

Surface wave tomography of the arctic from seismic Rayleigh and Love wave group velocity dispersion data

The results of studying the deep structure of the Earth’s crust and upper mantle of the Arctic from surface wave data are presented. For this purpose, based on the frequency-time analysis procedure, a representative dataset of group velocity dispersion curves of seismic Rayleigh and Love waves (1555 and 1265 paths, respectively) in the period range from 10 to 250 s is obtained. With the use of a two-dimensional tomography technique for a spherical surface, group velocity distributions are calculated at separate periods. Overall, 18 maps for each type of surface waves are constructed and the horizontal resolution of the mapping is estimated. For four tectonically different regions of the Arctic, the dispersion curves calculated from the tomography results are inverted for the velocity sections of the SV- and SH-waves. Based on the obtained distributions, the main large-scale features are analyzed in the deep structure of the Earth’s crust and upper mantle of the Arctic, and the revealed velocity irregularities are correlated to various geological structures. The results of the study are of considerable interest for further constructing the three-dimensional model of the shear wave velocity distributions and for studying the anisotropic properties of the upper mantle of the Arctic, as well as for building the geodynamical models of the region.

Surface Wave Tomography of the Arctic from Rayleigh and Love Wave Group Velocity Dispersion Data

The results of studying the deep structure of the Earth’s crust and upper mantle of the Arctic from surface wave data are presented. For this purpose, based on the frequency-time analysis procedure, a representative dataset of group velocity dispersion curves of seismic Rayleigh and Love waves (1555 and 1265 paths, respectively) in the period range from 10 to 250 s is obtained. With the use of a two-dimensional tomography technique for a spherical surface, group velocity distributions are calculated at separate periods. Overall, 18 maps for each type of surface waves are constructed and the horizontal resolution of the mapping is estimated. For four tectonically different regions of the Arctic, the dispersion curves calculated from the tomography results are inverted for the velocity sections of the SV- and SH-waves. Based on the obtained distributions, the main large-scale features are analyzed in the deep structure of the Earth’s crust and upper mantle of the Arctic, and the revealed velocity irregularities are correlated to various geological structures. The results of the study are of considerable interest for further constructing the three-dimensional model of the shear wave velocity distributions and for studying the anisotropic properties of the upper mantle of the Arctic, as well as for building the geodynamical models of the region.

3D joint inversion of gravity data and Rayleigh wave group velocities to resolve shear-wave velocity and density structure in the Makran subduction zone, south-east Iran

In this study, we developed a method to invert jointly Rayleigh wave group velocities and gravity anomalies for velocity and density structure of the lithosphere. We applied the method to the Makran accretionary prism, SE Iran. The reason for using different data sets is that each of these data sets is sensitive to different parameters. Surface wave group velocities are sensitive mainly to shear wave velocity distribution in depth but do not well resolve density variations. Therefore, joint inversion with gravity data increases the resolution of density distribution. Our approach differs from others mainly in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for the properties of only four layers: thickness, P- and S-wave velocities and densities and their vertical gradients in sediments, upper-crust, lower-crust and upper mantle. The method is applied first to synthetic models in order to demonstrate its usefulness. We then applied the method to real data to investigate the lithosphere structure beneath the Makran. The resulting model shows that Moho depth increases from Oman Sea (18–33 km) and Makran fore-arc (33–37 km) to the volcanic-arc (44–46 km). The crustal density is high in the Oman Sea as should be expected for the oceanic crust. We also find a high-velocity anomaly in the upper mantle under the Oman Sea corresponding to the subducting slab. The crust under the fore-arc, volcanic-arc and back-arc settings of Makran subduction zone is characterized by low-velocity zones.

Multimodal Layered Transdimensional Inversion of Seismic Dispersion Curves With Depth Constraints

AbstractMuLTI (Multimodal Layered Transdimensional Inversion) is a Markov chain Monte Carlo implementation of Bayesian inversion for the probability distribution of shear wave velocity (Vs) as a function of depth. Based on Multichannel Analysis of Surface Wave methods, it requires as data (i) a Rayleigh‐wave dispersion curve and (ii) additional layer depth constraints, the latter we show significantly improve resolution compared to conventional unconstrained inversions. Such depth constraints may be drawn from any source (e.g., boreholes, complementary geophysical data) provided they also represent a seismic interface. We apply MuLTI to a Norwegian glacier, Midtdalsbreen, in which ground‐penetrating radar was used to constrain internal layers of snow, ice, and subglacial sediments, with transitions at 2 and 25.5 m, and whose Vs is assumed to be in the range 500–1,700, 1,700–1,950, and 200–2,800 m/s, respectively. Synthetic modeling demonstrates that MuLTI recovers the true model of Vs variation with depth. Significantly, compared to inversions without depth constraints, in this synthetic case MuLTI not only reduces by about a factor of 10 the error between the true and the best fitting model, but also reduces by a factor of 2 the vertically averaged spread of the distribution of Vs based on the 95% credible intervals. We further show that using frequencies above about 100 Hz lead to unconverged solutions due to mode ambiguities associated with fine spatial structures. For our acquired data on Midtdalsbreen, we use 14‐100 Hz data for which MuLTI produces a large‐scale converged inversion.

Effect of toe dorsiflexion on the regional distribution of plantar fascia shear wave velocity

BackgroundThe plantar fascia is exposed to repetitive tensile stress induced by cyclic loads associated with daily activities, such as walking and running. Due to overuse or abnormal foot alignment, insertional and distal (i.e., mid-substance) regions within the plantar fascia may exhibit microtears, which leads to plantar fasciopathy. Ultrasound shear wave elastography is an imaging technique to measure shear wave velocity propagating through biological tissues, considered herein as an index of tensile stress. This study aimed to quantify the effect of toe dorsiflexion on the regional distribution of plantar fascia shear wave velocity. MethodsShear wave velocity of the plantar fascia was measured in the insertional and distal regions using ultrasound shear wave elastography in sixteen healthy participants (7 males and 9 females). The measurements were performed while the toes were maintained in neutral or dorsiflexed positions. FindingsWhen considering the insertional region, there was no significant difference in shear wave velocity between neutral toe position [mean (SEM): 5.4 (0.6) m/s] and dorsiflexed toe position [5.5 (0.5) m/s] (P = 0.88; effect size = 0.05). When considering the distal region, there was a significant difference in shear wave velocity between the neutral position [7.8 (0.4) m/s] and dorsiflexed position [9.9 (0.3) m/s] (P = 0.002; effect size = 0.88). The difference in shear wave velocity between the insertional and distal regions showed a large effect size for either neutral (P = 0.010; effect size = 0.75) or dorsiflexed toe position (P = 0.003; effect size = 0.86). InterpretationIn contrast to clinical beliefs, these findings suggest that toe dorsiflexion induces non-homogeneous changes in tensile stress within the plantar fascia.

Seismic microzonation of Srinagar city, Jammu and Kashmir

The seismic vulnerability of Kashmir basin is evident from the historical and instrumental earthquake records in general and the capital city of Srinagar in particular. An attempt is made here to deliver the seismic microzonation map of this scenic city. To do so, probabilistic and geotechnical hazard representing the primary seismic hazard, secondary seismic hazard and site conditions are used as input themes. The themes considered for the holistic integrated hazard quantification are the peak ground acceleration at engineering bedrock, shear wave velocity distribution representing site conditions, liquefaction potential index, site amplification ratio and predominant frequency distribution. All these themes have been assigned appropriate weights and their features. The highest weight was given to peak ground acceleration, followed by shear wave velocity distribution, liquefaction potential index, site amplification ratio and predominant frequency themes. These weighted themes are then integrated in the GIS environment to generate seismic microzonation map of the Srinagar city. This map is expressed in terms of the Seismic Hazard Index (SHI) and is accordingly grouped into two classes: very high and severe. The results demonstrate that Srinagar city is susceptible to very high seismic hazard, especially when predicted peak ground acceleration is of the order of 0.605–0.63 g at the bedrock.