Wave Velocity Gradient Research Articles

Thoracic aorta injury detected by 4D flow MRI predicts subsequent main adverse cardiovascular events in breast cancer patients receiving anthracyclines: A longitudinal study

PurposeTo investigate longitudinal thoracic aorta injury using 3-dimensional phase-contrast magnetic resonance imaging (4D flow MRI) parameters and to evaluate their value for predicting the subsequent main adverse cardiovascular events (MACEs) in breast cancer patients receiving anthracyclines. MethodsBetween July 2020 and July 2021, eighty-eight female participants with breast cancer scheduled to receive anthracyclines with or without trastuzumab prospectively enrolled. Each subjects underwent 4D flow MRI at baseline, 3 and 6 months in relation to baseline. The diameter, peak velocity (Vpeak), wall shear stress (WSS), pulse wave velocity (PWV), energy loss (EL) and pressure gradient (PG) of thoracic aorta were measured. The association between these parameters and subsequent MACEs was performed by Cox proportional hazard models. ResultsTen participants had subsequently MACEs. The Vpeak and PG gradually decreased and the WSS, PWV and EL progressively increased at 3 and 6 months compared with baseline. Adjusted multivariable analysis showed that the WSS of the proximal, mid- and distal ascending aorta [HR, 1.314 (95% confidence interval (CI): 1.003, 1.898)], [HR, 1.320 (95% CI: 1.002, 1.801)] and [HR, 1.322 (95% CI: 1.001, 1.805)] and PWV of ascending aorta [HR, 2.223 (95% CI: 1.010, 4.653)] at 3 months were associated with subsequent MACEs. Combined WSS and PWV of ascending aorta at 3 months yielded the highest AUC (0.912) for predicting subsequent MACEs. ConclusionCombined WSS and PWV of ascending aorta at 3 months is helpful for predicting the subsequent MACEs in breast cancer patients treated by anthracyclines.

The apparent focal depth, emergence angle, and take-off angle of seismic wave measured by YRY-4-type borehole strainmeter as one kind of strain seismograph

Introduction: In theory, the observation objects and principles of strain seismograph and traditional pendulum seismograph are different, and the characteristics of observed signals should also be dissimilar. The observation results of pendulum seismograph show that seismic waves in inhomogeneous media will undergo refraction, reflection, and attenuation. Then, what signal characteristics can be detected by strain seismograph is great significance for understanding and explaining the observation results.Methods: Using YRY-4 type four-gauge borehole strainmeter as one kind of strain seismograph to detect the strain tensor change of the plane seismic wave emitted from the surface, a five-site strain seismograph observation network was built in Shanxi Province, with continuous observation for 2 years at a sampling rate of 100 Hz. In this paper, two local events occurring in the area covered by the strain seismograph observation network are taken as examples. We systematically studied the characteristics of seismic wave signals recorded by strain seismographs at five sites, inverted for the focal depth of the two local earthquakes and the relationship between the wave velocity and the wave velocity gradient of the focal depth, and calculated the apparent focal depth, the emergence angle and the take-off angle of seismic waves.Results: These results show stable uniqueness and apparent regularity, especially since the inverted focal depths are basically consistent with the seismic solutions based on those traditional pendulum seismographs. The observations from this study show that the strain seismograph can be used as an effective supplement to the pendulum seismograph.Discussion: In the future, we will continue to study the rupture process and focal mechanism of moderate-strong earthquakes and teleseismic earthquakes by combining two kinds of observations.

Influence of the variability of soil profile properties on weak and strong seismic response

Characterizing the potential effect of local site conditions on the amplification of ground motions is a critical aspect of seismic hazard and risk assessment. The aim of this study is to investigate the reliability and the limit of using the average shear wave velocity in the upper 30m of the soil profile vs,30, as single proxy, to characterize seismic site effects for weak and strong events.To this regard, a dataset of 300 one-dimensional soil profiles with a given vs,30 are generated through a Monte Carlo approach. Their seismic responses are computed for a set of 40 real accelerograms, with different seismic features. The vertical propagation from the bottom of the generated columns is modeled using a finite element spatial discretization, accounting for both linear and nonlinear soil behavior.The site dominant frequency f0 and the shear wave velocity gradient in the profile B30 are proposed as proxies to characterize seismic site effects and the variability of the response spectra for the numerical signals, at the free surface of the set of columns, is discussed. Correlations between site-specific amplification factors deduced using the numerical response spectra and the proposed site proxies are analyzed for different sub-ranges of periods. The obtained amplification factors are then compared to those proposed by different international and national design codes. The results, obtained under assumption of linear and nonlinear behavior of soil, emphasize the need to introduce complementary site parameters proxies, in addition to vs,30, to characterize the expected site effects in design response spectra.

Across-Fault Velocity Gradients and Slip Behavior of the San Andreas Fault Near Parkfield.

A long‐lasting question in earthquake physics is why slip on faults occurs as creep or dynamic rupture. We compute passive measurements of the seismic P wave velocity gradient across the San Andreas Fault near Parkfield, where this transition of slip mode occurs at a scale of a few kilometers. Unbiased measurements are obtained through the application of a new Bayesian local earthquake tomographic code that avoids the imposition of any user‐defined, initial velocity‐contrast across the fault, or any damping scheme that may cause biased amplitude in retrieved seismic velocities. We observe that across‐fault velocity gradients correlate with the slip behavior of the fault. The P wave velocity contrast decays from 20% in the fault section that experience dynamic rupture to 4% in the creeping section, suggesting that rapid change of material properties and attitude to sustain supra‐hydrostatic fluid pressure are conditions for development of dynamic rupture. Low Vp and high Vp/Vs suggest that fault rheology at shallow depth is conversely controlled by low frictional strength material.

Sexual Function Is an Indicator of Central Arterial Stiffness and Arterial Stiffness Gradient in Japanese Adult Men.

BackgroundAs arterial stiffness increases in the absence of subjective symptoms, a personal indicator that reflects increased risk of cardiovascular disease is necessary. Penile erection is regulated by vascular function, and atherosclerosis affects the penile artery earlier than it affects the coronary and carotid arteries. Therefore, we hypothesized that deterioration of erectile function could be a marker of increased risk for cardiovascular disease. To test our hypothesis, we assessed erectile function and arterial stiffness in a cross‐sectional study.Methods and ResultsCarotid‐femoral pulse wave velocity (PWV), brachial‐ankle PWV, femoral‐ankle PWV, and arterial stiffness gradient (PWV ratio: carotid‐femoral PWV/femoral‐ankle PWV) were measured as indexes of central, systemic, and peripheral arterial stiffness and peripheral organ damage, respectively, in 317 adult men. In addition, erectile function was assessed by using the questionnaire International Index of Erectile Function 5 (a descending score indicates worsening of erectile function). The scores of male sexual function were inversely correlated with carotid‐femoral PWV (r s=−0.41), brachial‐ankle PWV (r s=−0.35), femoral‐ankle PWV (r s=−0.19), and PWV ratio (r s=−0.33). Furthermore, multivariate linear regression analyses revealed that International Index of Erectile Function 5 scores were significantly associated with carotid‐femoral PWV (β=−0.22) and PWV ratio (β=−0.25), but not with brachial‐ankle PWV and femoral‐ankle PWV.ConclusionsOur results indicated that erectile function is independently associated with central arterial stiffness and peripheral organ damage. These findings suggest that male sexual function could be an easily identifiable and independent marker of increased central arterial stiffness and peripheral organ damage.

The lateral variation of Pn velocity gradient under Eurasia

AbstractMantle lid P wave velocity gradient, or Pn velocity gradient, reflects the depth and lateral variations of thermal and rheological state of the uppermost mantle. Mapping the Pn velocity gradient and its lateral variation helps us gain insight into the temperature, composition, and dynamics of the uppermost mantle. In addition, because Pn velocity gradient has profound influence on Pn propagation behavior, an accurate mapping of Pn velocity gradient also improves the modeling and prediction of Pn travel times and amplitudes. In this study, I used measured Pn travel times to derive path‐specific Pn velocity gradients. I then inverted these velocity gradients for two‐dimensional (2‐D) Pn velocity‐gradient models for Eurasia based on the assumption that a path‐specific Pn velocity gradient is the mean of laterally varying Pn velocity gradients along the Pn path. Result from a Monte Carlo simulation indicates that the assumption is appropriate. The 2‐D velocity‐gradient models show that most of Eurasia has positive velocity gradients. High velocity gradients exist mainly in tectonically active regions. Most tectonically stable regions show low and more uniform velocity gradients. Strong velocity‐gradient variations occur largely along convergent plate boundaries, particularly under overriding plates.

Evaluation of 1‐D and 3‐D seismic models of the Pacific lower mantle with S, SKS, and SKKS traveltimes and amplitudes

AbstractIn this study, we analyzed the seismic phases S, SKS, and SKKS from 31 deep‐focus earthquakes in the Tonga‐Fiji region recorded in North America between epicentral distances of 85° and 120°. The differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one‐dimensional (1‐D) reference Earth models. The increase of the S/SKS amplitude ratio up to a factor of 10 is accompanied by an increase of the S‐SKS differential traveltime of up to 10 s. SKKS‐SKS differential traveltimes of 2–3 s and SKKS/SKS amplitude ratios of a factor of 2–4 across the epicentral range have maxima near 107°. We examined these observations using full (1‐D and 3‐D) waveforms for three 1‐D seismic velocity profiles for the central Pacific region and for the tomographic model S40RTS including modifications: different regularization parameters, great‐circle path azimuthal variation, strength of S wave velocity perturbations, S wave velocity gradients in the lower mantle, and ultra–low velocity zones. To explain these data, we constructed a hybrid model that combines both features of S40RTS and short‐wavelength features from the 1‐D profiles. The large‐scale seismic structure is represented by S40RTS. Embedded within S40RTS are a 20 km thick ultra–low velocity zone at the core‐mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths. Our analysis demonstrates that the S wave velocity structure of the Pacific large low shear‐velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other.

Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity

Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense ‘piles’, which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical ‘piles’. A prescribed core-mantle-boundary temperature of 4000K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical ‘piles’ at the base of Earth's mantle, where ‘piles’ correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wave-speed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anti-correlation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent ‘piles’ are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.

Cross-hole seismic technique for assessing in situ rock mass conditions around a tunnel

The cross-hole seismic technique was used to identify the extent of the Excavation Damage Zone (EDZ) is in a test tunnel and in a tunnel under construction. For the cross-hole test, a new electro-mechanical impact-based transmitter was developed and tested. The transmitter was specially designed to generate appropriate impact energy and fit into a 45-mm rock bolt borehole. In the test tunnel, 79P-wave rays were measured. The interval between the boreholes was 1.5m. From the measured P-wave velocity, 2D tomography was constructed and the average velocity gradient curves at the offsets of 0.21, 0.57, 0.93, and 1.29m were drawn. Given that the wave velocity gradient curves show spatial variation due to the quality of the rock mass, upper and lower bounds that delimit the P-wave velocity were drawn. The extent of the EDZ was determined as the depth at which the average velocity is lower than the lower bound. By applying this approach, the depth of the EDZ at the test tunnel was determined to be about 1.1m from the tunnel surface. The cross-hole seismic measurement was compared with the condition of the extracted rock cores. A gray-scale image of the rock core was compared to the P-wave velocity profile. According to the image, disintegrated rock cores were present up to a depth of 1.1m. From the seismic data and a visual inspection, a similar result was found. The same approach was applied in a tunnel under construction. From the site, the observed depth of the EDZ was about 0.9m. In addition, because of the developed transmitter can generate S-waves, P- and S-wave velocities were measured and compared. The results show that S-waves can be successfully applied to identify an EDZ. The S-wave profile shows less scatter compared to a P-wave measurement.

A model for pressure loss, film thickness, and entrained fraction for gas–liquid annular flow

A two-component (air–water) annular flow model is presented requiring only flow rates, absolute pressure, temperature, and tube diameter. Film thicknesses (base film and wave height) are calculated from a critical film thickness model. Modeled pressure gradient is weighted by wave intermittency to compute average pressure gradient. Film flow rate and wave velocity are estimated using the universal velocity profile in the waves and a piecewise linear profile in the base film. For vertical flow, mean absolute errors for film thickness, wave velocity, and pressure gradient are 9%, 9%, and 19%, respectively. In horizontal flow, mean absolute errors for pressure gradient, base film thickness, and disturbance wave velocity are 17%, 10%, and 14%, respectively, on par with those from single-behavior models that require additional film thickness or other data as inputs.

Constraints on the deep thermal structure of the Dharwar craton, India, from heat flow, shear wave velocities, and mantle xenoliths

[1] We have used constraints from seismic shear wave vertical velocity profiles, geothermobarometry estimates on mantle xenoliths, and surface heat flux and heat production measurements to analyze the thermal regime of the deep lithosphere beneath India. In the Dharwar craton of southern India, the shear wave velocity gradient in the mantle, as well as xenolith geothermobarometry data, suggests a low mantle heat flux, 14–20 mW m−2, consistent with surface heat flux measurements. However, for standard cratonic mantle composition, seismic velocities require Moho and mantle temperatures to be about 300 K higher than inferred from heat flux and xenolith data. This discrepancy can be only resolved by changing the mantle composition, specifically by increasing the Fe number. The shear wave velocities are highest beneath north central India, where calculated S wave travel times are 2 s shorter than in the Dharwar craton. These differences in traveltime and the very steep gradient in the shear wave velocity profiles in north central India cannot be explained by variations in mantle temperature but require differences in mantle composition.

Tomographic P wave velocity and vertical velocity gradient structure across the geothermal site Groß Schönebeck (NE German Basin): Relationship to lithology, salt tectonics, and thermal regime

Seismic wide‐angle data were collected along a 40‐km‐long profile centered at the geothermal research well GrSk 3/90 in the Northeast German Basin. Tomographic inversion of travel time data provided a velocity and a vertical velocity gradient model, indicative of Cenozoic to Pre‐Permian sediments. Wide‐angle reflections are modeled and interpreted as top Zechstein and top Pre‐Permian. Changes in velocity gradients are interpreted as the transition from mechanical to chemical compaction at 2–3 km depth, and localized salt structures are imaged, suggesting a previously unknown salt pillow in the southern part of the seismic profile. The Zechstein salt shows decreased velocities in the adjacent salt pillows compared to the salt lows, which is confirmed by sonic log data. This decrease in velocity could be explained by the mobilization of less dense salt, which moved and formed the salt pillows, whereas the denser salt remained in place at the salt lows. We interpret a narrow subvertical low‐velocity zone under the salt pillow at GrSk 3/90 as a fault in the deep Permian to Pre‐Permian. This WNW‐ESE trending fault influenced the location of the salt tectonics and led to the formation of a fault‐bounded graben in the Rotliegend sandstones with optimal mechanical conditions for geothermal production. Thermal modeling showed that salt pillows are related to chimney effects, a decrease in temperature, and increasing velocity. The assumed variations in salt lithology, density, and strain must thus be even higher to compensate for the temperature effect.

Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle

SUMMARY There is a clear correlation between downward projected large igneous province (LIP) eruption sites of the past 200 Myr and the margins of the large low-velocity provinces (LLVPs) at the base of the mantle. We established this correlation by using palaeomagnetic as well as fixed and moving hotspot reference frames. Our finding indicates that the majority of the LIPs have been generated by plumes that rose from the D �� zone at the edges of the LLVPs. Most LIP eruption sites project radially downwards to the core‐mantle boundary (CMB) within ±10 ◦ of the 1 per cent slow shear wave velocity contour in the SMEAN tomographic model. Steep shear wave velocity gradients have been mapped near the CMB along much of the lengths of the LLVP margins close to that contour which marks a faster/slower boundary (FSB) within the D �� zone. The observation that eruption sites of LIPs as old as 200 Myr can be linked to this prominent present day seismic structure shows that the FSBs of the two LLVPs have occupied their current positions for at least as long and that the process that leads to the generation of deep-seated plumes has been localized on the FSBs at the margins of the African and Pacific LLVPs for the same interval. The persistence of the LLVPs over 200 Myr is consistent with independent evidence that they are compositionally distinct and are not just simply hotter than the material making up the rest of the D �� zone.

S velocity reversal in the mantle transition zone

The transition zone differs from the rest of the mantle by high positive P and S wave velocity gradients. However, by applying S receiver function technique to recordings of about 50 globally distributed stations, in 7 regions we obtain evidence of negative discontinuity at a depth around 500 km. Four of these regions correspond to the well known hotspots. No other hotspots are sampled by our observations at this depth. Our modeling suggests that the negative discontinuity is hardly an effect of anisotropy of wadsleyite. More likely, it is caused by isotropic S velocity reduction of around 0.2 km/s. Origin of the low velocity can be related to enhanced water content in wadsleyite.

Mineralogy and elasticity of the oceanic upper mantle: Origin of the low‐velocity zone

We use a new method to construct an upper mantle model based on self‐consistent computation of phase equilibria and physical properties. Computation of the isotropic elastic wave velocities of a pyrolytic bulk composition in thermodynamic equilibrium shows a distinct low‐velocity zone with a minimum velocity VS = 4.47 km s−1 along the 100 Ma geotherm. In the vicinity of the low‐velocity zone the velocity of this null hypothesis is approximated along oceanic geotherms by VS = 4.77 + 0.0380(P, z/29.80) − 0.000378(T − 300), with pressure P in GPa, depth z in km, temperature T in K, and velocity VS in km s−1. The null hypothesis predicts a minimum VS 0.1–0.2 km s−1 higher than that in seismological models of 100 Ma Pacific. We find that dispersion, estimated solely on the basis of seismological attenuation models, can account for this residual velocity deficit. Except in the immediate vicinity of the ridge (t < 5 Ma), a solid‐state low‐velocity zone provides a satisfactory quantitative explanation of seismic observations. We do not find a satisfactory explanation for the magnitude of the Gutenberg discontinuity or for the high shear wave velocity gradient zone.

Effects of initial bubble size on flow pattern transition in a 28.9 mm diameter column

The experimental results described in this paper were carried out in a 28.9 mm diameter column at a constant water velocity of 0.356 m/s for four different bubble sizes. The void fraction waves were measured with impedance void fraction meters. It has been found that the initial bubble size has strong effects on the flow pattern transition and the instabilities of void fraction waves. The critical void fraction at which the flow pattern transition happens decreases with increasing bubble size. At a constant liquid velocity with increasing gas flow rate, the point at which the system gain factor becomes larger than 1, and the point where the wave velocity gradient first becomes negative, also decrease with increasing bubble size. This study has confirmed that the instability of the void fraction wave is not the factor that causes the bubble-to-slug flow pattern transition in gas–liquid verticle flow.

Upper mantle S velocities beneath Afar and Western Saudi Arabia from Rayleigh wave dispersion

Two‐station methods are employed to determine Rayleigh wave phase velocities along three paths crossing the Afro‐Arabian dome. Phase velocities are modeled using a grid‐search technique, and the range of successful models indicate that the uppermost mantle beneath Afar and western Saudi Arabia is characterized by pronounced negative S wave velocity gradients and S wave velocities that are 0.2–0.8 km/s lower than PREM at 100 km depth. These results are consistent with a thermal plume model for Afar, provided that there is a few percent of partial melt present within the mantle. The low velocity anomaly beneath western Saudi Arabia could be caused by migrating plume material from Afar, upwelling mantle material associated with Red Sea rifting, or isolated small‐scale convection.

Ground motion at mountains and sedimentary basins with vertical seismic velocity gradient

SUMMARY The boundary integral-Gaussian beam method (Benites & Aki 1989) is applied to study the ground motion in 2-D structures that exhibit irregular topography and interface, and whose shear wave velocity varies linearly with depth, for incident plane SH waves. In our first example of application the model is a half-space whose free-surface topography is a ridge of cosine shape, with vertical shear wave velocity gradient. In the second, the model is a semi-cylindrical sedimentary basin in a homogeneous half-space, in which the shear wave velocity of the sediments increases with depth. Our results for the case of the mountain show that the amplification on its top, predicted by the 2-D modelling when the velocity is constant, is enhanced when the velocity gradient is present, for all frequencies and by a factor up to 3. In the case of the basin, results show that the velocity gradient; (1) enhances the amplification at the edges of the valley, (2) makes the reverberations due to 2-D resonance have larger amplitudes and shorter intervals between arrivals, (3) shortens the total duration of the seismograms at all stations within the basin.

Upper mantle structure of southern Africa from Pnl waves

We modeled Pnl waveforms from two moderate‐sized normal‐fault earthquakes in Zambia to determine an upper mantle P wave velocity model for southern Africa. The event parameters are May 15, 1968, mb = 5.7, depth = 28 km; and December 12, 1968, mb = 5.9, depth = 6 km. Teleseismic body wave inversions by previous researchers constrain the focal parameters for these events. We compute synthetic seismograms for six upper mantle velocity models using a wave number integration algorithm until an acceptable fit to the data is obtained. These models have different mantle P wave velocity gradients and low‐velocity zone (LVZ) depths. Quality of fit is measured primarily by the Pn/PL amplitude ratio. Source‐station geometry allows for the independent sampling of the upper mantle beneath the Kaapvaal‐Zimbabwe craton and mobile belt structural provinces. Synthetics from a constant velocity mantle model do not contain prominent precursor arrivals seen in the data; these are interpreted as P waves turning in the upper mantle. These synthetics also produce a too low Pn/PL amplitude ratio. Synthetics for a model with a mantle P wave velocity gradient of 0.0033 s−1 and a LVZ no shallower than 170 km fit the cratonic path data adequately. A slightly lower gradient, with a LVZ no shallower than 150 km, is indicated for the mobile belt regions. All the data preclude a mantle lid velocity gradient as high as 0.0066 s−1. Different velocity gradients between the two provinces implies different thermal gradients, which supports the hypothesis that a deep, cool lithospheric root exists beneath the Kaapvaal‐Zimbabwe craton. Cratonic thermal gradients derived from our velocity gradients agree closely with those derived from thermal modeling by Ballard and Pollack [1987].

Numerical modeling of water waves recorded by a subbottom seismometer in the southwest Pacific

The amplitudes of water waves generated by shallow explosive sources and recorded by a subbottom seismometer provide constraints on velocity and attenuation parameters of oceanic sediments and upper oceanic crust. Analysis of data collected during the 1983 Ngendei Seismic Experiment in the southwest Pacific Ocean illustrates this method. The data were recorded by a triaxial geophone emplaced 54 m into the basaltic basement. Synthetic seismograms are computed using wave number integration for several crustal models to determine the sensitivity of water wave amplitudes to specific parameters of the sediments and oceanic crust. The synthetic seismograms fit the recorded data to better than a factor of 2 for frequencies as high as 15 Hz and ranges out to 30 km. The amplitude of the direct water wave beyond critical incidence (“direct wave root”) indicates that the sediment P wave velocity is near 1.8 km/s. Modeling of the attenuation of shear wave reverberations in the sediments indicates that sediment Qβ is between 30 and 50. Modulations in water wave amplitude as a function of receiver offset are related to the velocity of the upper oceanic crust. The distances at which amplitude maxima occur indicate that the shear wave velocity of the upper crust at the Ngendei site is 2.4±0.1 km/s. This is an important result since reliable estimates of the shear wave velocity of upper oceanic crust are rare. The distances at which amplitude minima occur are used to constrain the compressional wave velocity of the uppermost crust. However, the modulations do not resolve the compressional wave velocity of the crust as well as the shear wave velocity. The relative amplitudes of successive water waves indicate that the reflection coefficient at the sediment‐basement interface is 0.66. This further constrains the compressional wave velocity of the upper crust. Using estimates of the sediment properties and crustal shear wave velocity, the compressional wave velocity of the upper crust at the Ngendei site is estimated from this reflection coefficient to be 4.3–4.4 km/s. A method for estimating the velocity gradient in the upper kilometer of the crust from the interference of water waves with upgoing crustal refractions is presented. Application of the method to these data indicate that the shear wave velocity gradient is about 1.0 s−1, which is indistinguishable from the previous determination by Shearer and Orcutt (1986) using a quite different methodology. The velocity of the upper crust at the Ngendei site is consistent with velocities reported for other regions of similar crustal age (>100 m.y.) and is much greater than velocities typically found for young oceanic crust (<5 m.y.). This lends support to current theories regarding the evolution of the uppermost oceanic crust that attribute higher velocities in older crust to the closing of cracks and pores.