Reduction In Shear Wave Velocity Research Articles

Seismic vulnerability assessment and fragility analysis of tunnels in Oman: Development of risk matrices and functionality guidelines

Past seismic events worldwide inflicted severe damage on tunneling and underground infrastructure projects and resulted in considerable socioeconomic loss. In this study, the seismic vulnerability of tunnels in the Sultanate of Oman is assessed, accounting for tunnel typology and localized site conditions. Seismic Fragility Curves (SFCs) were developed for two seismic zones (Zone 1 and Zone 2) based on the proposed intensity-response relationship, incorporating two different lining properties. Results consistently indicated a 39.56 % decrease in raking ratio for every 15 m/s reduction in shear wave velocity (VSST) across all tunnel models, spanning soft to stiff strata conditions. Employing the fragility functions developed for the Sultanate of Oman, project-specific risk matrices and operational route maps for Al-Sharqiyah Expressway Tunnel-2 (AST2) were formulated, along with an assessment of post-seismic operational accessibility. The insights garnered from this investigation provide fundamental guidance for engineers, designers, and consultants involved in seismic analysis and design of underground structures, encompassing both new constructions and retrofitting endeavors. Furthermore, the data generated herein offers valuable input for governmental stakeholders, facilitating policy formulation and the establishment of national-level design directives concerning tunneling projects within the Sultanate of Oman.

Noise-based ballistic wave passive seismic monitoring – Part 2: surface waves

SUMMARYWe develop a new method to monitor and locate seismic velocity changes in the subsurface using seismic noise interferometry. Contrary to most ambient noise monitoring techniques, we use the ballistic Rayleigh waves computed from 30 d records on a dense nodal array located above the Groningen gas field (the Netherlands), instead of their coda waves. We infer the daily relative phase velocity dispersion changes as a function of frequency and propagation distance with a cross-wavelet transform processing. Assuming a 1-D velocity change within the medium, the induced ballistic Rayleigh wave phase shift exhibits a linear trend as a function of the propagation distance. Measuring this trend for the fundamental mode and the first overtone of the Rayleigh waves for frequencies between 0.5 and 1.1 Hz enables us to invert for shear wave daily velocity changes in the first 1.5 km of the subsurface. The observed deep velocity changes (±1.5 per cent) are difficult to interpret given the environmental factors information available. Most of the observed shallow changes seem associated with effective pressure variations. We observe a reduction of shear wave velocity (–0.2 per cent) at the time of a large rain event accompanied by a strong decrease in atmospheric pressure loading, followed by a migration at depth of the velocity decrease. Combined with P-wave velocity changes observations from a companion paper, we interpret the changes as caused by the diffusion of effective pressure variations at depth. As a new method, noise-based ballistic wave passive monitoring could be used on several dynamic (hydro-)geological targets and in particular, it could be used to estimate hydrological parameters such as the hydraulic conductivity and diffusivity.

Reduction in stiffness of proximal leg muscles during the first 6 months of glucocorticoid therapy for giant cell arteritis: A pilot study using shear wave elastography.

To investigate muscle stiffness changes in patients treated for giant cell arteritis (GCA) with high-dose oral glucocorticoids. Using ultrasound elastography, shear wave velocity (SWV) was measured in the quadriceps, hamstrings and biceps brachii muscles of 14 patients with GCA (4 male, mean age±SD, 68.2±4.3years) within the first 2weeks of initiating glucocorticoid treatment (baseline) and repeated after 3 and 6months treatment. Muscle strength and performance tests were performed at each visit. Baseline measures were compared with those from 14 healthy controls. Linear mixed models were used to test for change in patient measures over time. At baseline, muscle SWV in patients was not significantly different from controls. With glucocorticoid treatment, there was a reduction in SWV in the leg but not the arm muscles. SWV decreased by a mean of 14% (range 8.3%-17.3%; P=.001) after 3months and 18% (range 10.2%-25.3%; P<.001) after 6-months in the quadriceps and hamstrings during the resting position. The baseline, 3 and 6months mean SWV (±SD) for the vastus lateralis were 1.62±0.16m/s, 1.40±0.10m/s and 1.31±0.06m/s, respectively (P<.001). In the patient group as a whole, there was no significant change in muscle strength. However, there were moderate correlations (r=.54-.69) between exhibiting weaker muscle strength at follow-up visits and a greater reduction in SWV. Glucocorticoid therapy in patients with GCA was associated with a significant reduction in proximal leg muscle stiffness during the first 6months. Future research should study a larger sample of patients for a longer duration to investigate if diminished muscle stiffness precedes signs of glucocorticoid-induced myopathy.

Degradation of the mechanical properties imaged by seismic tomography during an EGS creation at The Geysers (California) and geomechanical modeling

Using coupled thermal-hydro-mechanical (THM) modeling, we evaluated new seismic tomography results associated with stimulation injection at an EGS demonstration project at the Northwest Geysers geothermal steam field, California. We studied high resolution seismic tomography images built from data recorded during three time periods: a period of two months prior to injection and during two consecutive one month periods after injection started in October 2011. Our analysis shows that seismic velocity decreases in areas of most intense induced microseismicity and this is also correlated with the spatial distribution of calculated steam pressure changes. A detailed analysis showed that shear wave velocity decreases with pressure in areas where pressure is sufficiently high to cause shear reactivation of pre-existing fractures. The analysis also indicates that cooling in a liquid zone around the injection well contributes to reduced shear wave velocity. A trend of reducing compressional wave velocity with fluid pressure was also found, but at pressures much above the pressure required for shear reactivation. We attribute the reduction in shear wave velocity to softening in the rock mass shear modulus associated with shear dislocations and associated changes in fracture surface properties. Also, as the rock mass become more fractured and more deformable this favors reservoir expansion caused by the pressure increase, and so the fracture porosity increases leading to a decrease in bulk density, a decrease in Young modulus and finally a decrease in Vp.

A New High-Pressure Triaxial Apparatus for Inducing and Tracking Hydro-Mechanical Degradation of Clayey Rocks

Abstract This paper presents the development of a new high-pressure triaxial apparatus specifically prepared for inducing and tracking the degradation of clayey rocks. Total suction—used to induce the hydraulic degradation—is applied with vapor transfer technique by controlling the relative humidity of air in contact with the material. The evolution of the degradation process along different hydro-mechanical stress paths is continuously tracked with bender elements, as well as with air or water permeability measurements on partially saturated or saturated states, respectively. Relevant test results on a low porosity clayey rock (Lilla claystone, Spain) are presented to evaluate the main capabilities of the new equipment. The results bring up the high sensitivity to water of the material, which is evidenced by the important reduction of shear wave velocity induced on first wetting and drying at low confining stress, as well as by the significant increase in the air permeability of the degraded material (around four orders of magnitude larger than the intact material). Test results also showed clear differences in the volume change and shear strength behavior of undisturbed, saturated, and degraded samples, highlighting the relevance of degradation on macroscopic behavioral features.

Study of the western edge of the African Large Low Shear Velocity Province

It is well established that there is a large low shear velocity province (LLSVP) in the lowermost mantle beneath Africa, extending from beneath the southeastern Atlantic Ocean to the southwestern Indian Ocean. The detailed 3‐D geometry of the LLSVP is crucial to the understanding of how this prominent lower mantle feature developed and evolved. Most studies have concentrated on mapping the southern and eastern edges of African LLSVP using the Kaapvaal array at South Africa. Here we use data from recently deployed arrays in the western Mediterranean to study its northwestern edge and evaluate the sharpness of its boundaries. Travel time and waveform modeling of S and SKS phases suggest the existence of sharp low velocity anomalies in the lowermost mantle beneath the eastern Atlantic Ocean, which agrees with global tomography models. However, the S‐SKS differential times vary up to 6 s across the array. To match the large travel time variations and waveforms, the existence of a slow velocity structure with a sharp top is required. The structure has a 3.5% reduction in shear wave velocity and a height of ∼600 km above the core‐mantle boundary, which is lower in topography than the southern and eastern part of the African LLSVP. Further 3‐D synthetic waveforms and modeling calculations demonstrate the existence of these sharp boundaries for the northwestern portion of the African LLSVP. The strong lateral variation in both travel times and waveforms suggests that this part of the African LLSVP may be as complex as the mid‐Pacific LLSVP. To explain the observed sharp top and large dome‐like structure, a thermochemical origin of the African LLSVP is supported.

An unsually large ULVZ at the base of the mantle near Hawaii

Previous studies have documented the presence of ultra-low-velocity-zones (ULVZs) at the base of the mantle, through observations of body wave complexities. Geometrically their heights are in the range of ∼5–30km, while little is known about their lateral extent beyond about 102km, due to limitations in sampling. Here we show remarkable features in the waveforms of S/Sdiff phases of western Pacific events observed at stations in North America that indicate the presence of a very large ultra-low-velocity-zone (ULVZ) at the base of the mantle, centered ∼11° to the southwest of Hawaii, within and near the northern border of the Pacific LLSVP. Waveform complexities include strongly delayed (>30s) postcursors. Measurements of travel times, beamforming analysis of out-of-plane energy, and full waveform comparisons with 3D numerical simulations, constrain the location, lateral extent, height and velocity reduction of the ULVZ with some level of trade-off. The simplified 3D model consists of a cylindrical ULVZ with a large aspect ratio of ∼20km in height and ∼910km in diameter at the CMB. The shear wave velocity reduction is ∼20%. This is to our knowledge the largest ULVZ mapped to date and the first time the lateral extent of a ULVZ has been constrained with some precision. Its location suggests that it may be the root of a long-lived, stable plume responsible for the Hawaiian volcanic chain, the hotspot track with the largest buoyancy flux.

Use of SASW Measurements to Evaluate the Effect of Lime Slurry Conditioning in Drilled Shafts

Spectral-Analysis-of-Surface-Waves (SASW) testing was conducted in untreated and treated drilled shafts at the NGES “TXHOUSTO” test site as part of a study on lime-slurry conditioning of soil in drilled shafts for pier foundation construction. Results of SASW testing in an untreated shaft revealed a reduction in shear wave velocity ([Formula: see text]; a measure of small-strain shear stiffness) within an annular zone slightly less than one shaft diameter in thickness. This reduction is attributed to stress relief and soil disturbance during excavation. Results of SASW testing in a treated shaft revealed that [Formula: see text] of the soil surrounding the shaft depended upon whether the measurement was performed above or below the water table. Above the water table, a thin (less than [Formula: see text]) annulus of stiffen cemented soil was created due to the reaction between the lime and the carbon dioxide in the pore spaces of the partially saturated soil. Since the clay was saturated below the water table, there was no carbon dioxide in the pore spaces available to react with the lime, so there was no observed increase in stiffness due to cementation. As a result of these observations, an alternative strategy for the application of lime-slurry conditioning of drilled shafts in the construction of pier foundations has been conceived.

Nonlinear soil response—A reality?

Abstract Geotechnical models consistently indicate that the stress-strain relationship of soils is nonlinear and hysteretic, especially at shear strains larger than ∼10−5 to 10−4. Nonlinear effects, such as an increase in damping and reduction in shear-wave velocity as excitation strength increases, are commonly recognized in the dynamic loading of soils. On the other hand, these effects are usually ignored in seismological models of ground-motion prediction because of the lack of compelling corroborative evidence from strong-motion observations. The situation is being changed by recently obtained data. Explicit evidence of strong-motion deamplification, accompanied by changes in resonant frequencies, are found in the data from the 1985 Michoacan, Mexico, and the 1989 Loma Prieta, California, earthquakes, the events recorded by the vertical and surface accelerograph arrays in Taiwan, as well as a number of other events throughout the world. Evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of ∼100 to 200 gal. Nonlinearity is considerable in cohesionless soil but may be negligible in stiff soils. The findings of recent years indicate that nonlinear site effects are more common than previously recognized in strong-motion seismology.

Analysis for vertical velocity of seismic waves

Abstract Presented here are the results of seismic wave velocities analyzed using the acceleration time‐histories recorded at the downhole strong motion array of the Lotung large scale seismic test site during the May 20, 1986 Hualien earthquake. The spectral ratios between the surface and downhole accelerograms were used to identify the frequencies of maximum amplification of soil layers, and an elastic 1‐D wave propagation model was proposed to estimate the wave velocities of the ground during the earthquake. The results obtained indicate that significant reduction of shear wave velocity as well as soil modulus was induced by earthquake excitation as compared to the values obtained from low strain level soil testings.

1. The response of soils to dynamic loadings. Report No. 24: Wave velocities through partially saturated sand-clay mixtures: H.A. Balakrishna Rao. Massachussetts Inst. of Tech., Cambridge. Dept. of Civil Engineering

Abstract : Investigations concerning the velocity of dilatational and shear waves through compacted specimens of kaolinite and kaolinite-sand mixtures at different moisture contents have been described. The delay time between the input and output signals from piezoelectric crystals located at the two ends of compacted samples has been utilized to evaluate the wave velocity. The results reveal that there are two important parameters involved in the propagation of dilatational waves: confining stress and moisture content. An increase in confining stress tends to increase the dilatational wave velocities. An increase in the moisture content also tends to increase the dilatational wave velocity, particularly at high saturation. The magnitude of wave velocity measured in various mixtures is similar to those in dense sand. The results of shear wave tests show a reduction of shear wave velocity with increase in moisture content. The decrease is attributed partly to the decrease in the rigidity of the fabric and partly to addition of mass which contributes to the inertia force. For a constant dry density, the presence of sand tends to decrease the wave velocity, apparently because differing amounts of sand required different compactive efforts. (Author)