Shear Wave Travel Time Research Articles

MODAL ANALYSIS OF SOFT-SOIL SITES INCLUDING RADIATION DAMPING

For the one-dimensional analysis of soft-soil layers on an elastic half-space, a general form of analytical solution is developed for converting radiation damping due to energy leaking back to the half-space into equivalent modal damping, allowing the modal analysis technique to be extended to a site where radiation damping has to be accounted for. Closed-form solutions for equivalent modal damping ratios and effective modal participation factors are developed for a single layer with a shear wave velocity distribution varying from constant to linearly increasing with depth. Compact and recursive forms of solutions for equivalent modal damping ratios are developed for a system with an arbitrary number of homogeneous layers on an elastic half-space. Comparisons with numerical solutions show that the modal solutions are accurate. The nominal frequency of a site, i.e. the inverse of four times the total shear wave travel time through the layers, is an important parameter for estimating the high mode frequencies. A parameter study shows that for the same impedance ratio of the bottom layer to the elastic half-space, a system of soil layers with an increasing soil rigidity with depth has, in general, larger peak modal amplifications at the ground surface than does a single homogeneous layer on an elastic half-space, while a system with a decreasing soil rigidity with depth has smaller modal peak amplifications. © 1997 by John Wiley & Sons, Ltd.

Measurement of stiffness of soils using small strain triaxial testing and bender elements

Abstract With the growing appreciation of the value of measuring very small strain stiffness, G max , by determining shear wave velocities, the authors have installed piezoceramic bender elements into a conventional triaxial cell. Whilst the installation process and high speed data acquisition requirements presented no serious problems, interpretation of bender element data was not as straightforward as was first thought. This paper describes the tests carried out and the problems encountered. Analytical techniques which remove the subjective nature of shear wave travel time determinations are discussed along with ways to discriminate between near field compressional-wave components and genuine shear waves.

Upper mantle shear structure beneath the Colorado Rocky Mountains

A tomographic inversion of teleseismic shear wave delays has been performed using data collected in the 1992 Program for Array Seismic Studies of the Continental Lithosphere Rocky Mountain Front experiment. The shear wave residuals used as data were corrected for known crustal, sediment and topographic differences and thus were sensitive to mantle shear velocity variations beneath the Colorado array. Large differences in shear wave travel time are observed across Colorado and into western Kansas with a maximum difference of near 5 sec between the central Rockies in Colorado and a station in western Kansas. The shear wave residuals were inverted for shear wave velocity variations in the upper 350 km of the mantle beneath Colorado and Kansas. Large differences in mantle shear velocity are found between the Rockies and the Kansas Great Plains. The average shear velocity in the upper 200 km is about 9% greater beneath Kansas than beneath the Rockies. At greater depths, lateral variations are far smaller and not as well correlated with surface tectonics. In a qualitative sense, there appear to be three mantle structures present beneath the study area. Beneath the Rockies the upper mantle is extremely slow. Both to the west and east of the Rockies we find intermediate mantle shear velocity. There is then an abrupt increase in mantle shear velocity near the Colorado‐Kansas border. We interpret the abrupt increase in mantle shear velocity near the Colorado‐Kansas border to mark the boundary of cratonic lithosphere. The slow velocity zone beneath the Rockies appears to be upwelled asthenosphere as is seen beneath the Rio Grande Rift to the south. To match the seismic contrast between the mantle beneath Kansas and the Colorado Rockies requires a temperature contrast of at least 350° C as well as partial melt beneath the Rockies. The hot mantle beneath the Rockies implies that the mantle density is low and the high topography of the Rockies is at least partially due to buoyant mantle.

Geomagnetic induction in a heterogenous sphere: Azimuthally symmetric test computations and the response of an undulating 660‐km discontinuity

A finite element numerical method is presented for computing electromagnetic induction in a heterogeneous conducting sphere by external source current excitation. The numerical model has applicability in the problem of determining the three‐dimensional electrical conductivity structure of Earth's mantle. The formulation is in terms of vector and scalar potentials. Boundary value problems are derived for fully three‐dimensional (3‐D) and azimuthally symmetric geometries. To validate the code, we check against a quasi‐analytic solution for an azimuthally symmetric configuration of eccentrically nested spheres and against an integral equation solution for an azimuthally symmetric, buried thin spherical shell model. To illustrate the capability of the code for modeling the electromagnetic response of realistic mantle structural variations, we compute solutions for an azimuthally symmetric electrical model whose lateral heterogeneity corresponds to zonal averages of the topographic relief on the 660‐km seismic discontinuity. The latter is derived from differential shear wave travel times. The anomalous responses are small but either spatially correlated or anticorrelated with the relief, depending on the period. The behavior can be reproduced by simple one‐dimensional (1‐D) modeling. The volumetric heterogeneity associated with recent seismic tomographic models should yield a greater surface anomaly, since electromagnetic data are inherently more sensitive to bulk electrical properties than sharp internal interfaces. The finite element method presented here is general and can account for galvanic, oceanic, and non‐P10 source effects. The present implementation does not include these complications, but the code is designed so that they can be added easily. The major obstacle to their inclusion at present is the scarcity of other 3‐D solutions to validate our calculated results.

Shear-Wave Porosity Logging in Sands

Summary We investigated the correlation between porosity and shear-wave velocity from acoustic logs in cored sand intervals. Data were collected in 13 wells with >5, 000 ft of core representing worldwide locations. Core porosities ranged from 0.02 to 0.38 at depths between 2, 400 and 11, 600 ft. Shear-wave velocities were obtained with Mobil's dipole logging tool, and porosities were obtained from standard core analysis. Our correlation data were obtained by averaging porosities and shear-wave travel times over intervals ≥4 ft, the approximate resolution of our shear-wave tool. We excluded intervals in which the porosity or shear-wave travel time varied widely. We find a linear correlation between porosity and shear-wave travel time, provided we exclude clay-bonded sands such as those found along the U.S. gulf coast. The corelation coefficient is 0.91, about the same as that for compressional-wave data in the same intervals. These results are consistent with shear-wave data reported for ultrasonic laboratory measurements. Further comparison of our data with ultrasonic measurements shows evidence of dispersion effects. The velocity of shear waves is more sensitive than that of compressional waves to porosity and is insensitive to gas effects. Also, shear-wave logs, unlike compressional-wave logs, can be obtained through casing with the dipole tool, regardless of cement-bond quality. These advantages make shear-wave logging attractive for evaluating porosity in old wells where porosity logs were never run or were of poor quality. The method fails in slow formations because the shear-wave arrival falls behind a strong tube-wave arrival generated by the bender tool in cased wells. Other applications of shear-wave logging also are discussed.

Spectra of mantle shear wave velocity structure

SUMMARY We applied the stochastic method of Gudmundsson, Davies & Clayton (1990) (which was applied to ISC P-wave data) to teleseismic ISC S-wave data to obtain an independent estimate of mantle structure. We inverted the variance of S-wave traveltime residuals of bundles of rays to obtain a description of the spectrum of lateral heterogeneity as a function of depth through the mantle. The technique yields robust estimates of the traveltime scattering power (the product of a characteristic scalelength of heterogeneity and the mean square of slowness perturbations). We can estimate the characteristic scalelength (half-width), from the autocovariance; which can be reconstructed from the spectra. Hence by division, we can estimate the root mean square slowness. By extrapolating the variance of bundles of rays to bundles of zero cross-sectional area we can also estimate the scale-incoherent signal (which is a plausible estimate of the noise in the data), which is removed from the data. We find that most of the structure generating shear wave traveltime residuals is located in the uppermost mantle. About half of the structure is short scale (harmonic degree 1 > 50). The large-scale structure (1 < 50) has a half-width of about 500 km in the upper half of the mantle. This S-wave half-width is consistent with the P-wave half-widths determined by Gudmundsson et al. (1990). The S-wave half-width in the lower half of the mantle is poorly constrained. It varies from 500 to 3000km, which spans the better constrained value of 1200 km found by Gudmundsson et al. (1990) for P-waves. The incoherent scatter suggests that the signal-to-noise ratio of the S-wave data set is around 1.5. Assuming that the compressional and shear wave velocity variations are correlated then the signal weighted value of the ratio d In (y.)ld In (y,) is =2, as also found in normal mode studies. This is much larger than the value of ~0.8-1.4 suggested by laboratory experiments undertaken at atmospheric pressure. There is no evidence of periodicity in the traveltime autocovariance; this suggests little or no periodicity in the underlying convection. The short half-width through most of the mantle suggests high Rayleigh number convection, with its attendant small-scale structures. The power decreases by an order of magnitude or more in going from the upper mantle to the lower mantle, the same as found by Gudmundsson et al. (1990) for P-waves. This large difference suggests either a change in convective regime and/or a difference in the temperature sensitivity of elastic constants in both layers. The increased short-scale structure at the top of the mantle suggests that a large part of the seismic signature at this boundary is compositional, since one would expect a red spectrum for a thermal boundary layer. The derived spectra between 1 = 10 and I = 50 are similar in shape to spectra from the mantle convection simulations of Glatzmaier (1988) with a Rayleigh number of 106-107, which would suggest layered convection, if the comparison is valid.

Joint inversion of shear wave travel time residuals and geoid and depth anomalies for long‐wavelength variations in upper mantle temperature and composition along the Mid‐Atlantic Ridge

We report measurements of SS‐S differential travel time residuals for nearly 500 paths crossing the northern Mid‐Atlantic Ridge. Differential travel times of such phases as SS and S with identical source and receiver have the advantage that residuals are likely to be dominated by contributions from the upper mantle near the surface bounce point of the reflected phase (SS). Under this assumption, differential SS‐S travel time residuals are mapped at the SS bounce points as a means of delineating lateral variations in mantle structure. After removing the signature of lithosphere age, we find evidence for variations in SS‐S residuals along the ridge at wavelengths of 1000–7000 km. These travel time anomalies correlate qualitatively with along‐axis variations in bathymetry and geoid height. We formulate a joint inversion of travel time residual, geoid height, and bathymetry under the assumption that all arise from variations in upper mantle temperature or bulk composition (parameterized in terms of Mg #). The inversion employs geoid and topography kernels which depend on the mantle viscosity structure. Inversion for thermal perturbations alone provides good fits to travel time and geoid data. The fit to topography, which is likely dominated by unmodeled crustal thickness variations, is not as good. The inversions for temperature favor the presence of a thin low‐viscosity layer in the upper mantle and temperature perturbations concentrated at depths less than 300 km. Compositional variations alone are unable to match the travel time and geoid or bathymetry data simultaneously. A joint inversion for temperature and composition provides good fits to both geoid and travel time anomalies. Temperature variations are ±50 K and compositional variations are ±0.5–3% Mg # for models with the temperature variations uniformly distributed over the uppermost 300 km and the compositional variations either distributed uniformly over the same interval or concentrated at shallower depths. The magnitudes of these variations are consistent with the chemistry and geothermometry of dredged peridotites along the Mid‐Atlantic Ridge.

Investigation of deep slab structure using long‐period S waves

Travel times and amplitudes of long‐period SH, ScSH, sSH, and sScSH phases from several deep focus earthquakes in the northwest Pacific are analyzed for evidence of lithospheric slab penetration into the lower mantle. Inclusion of amplitude observations in the analysis provides constraints on lateral velocity gradients present in the deep slabs which are not resolvable using travel times alone. Travel time and amplitude residual spheres are presented for two deep focus events with good azimuthal coverage, but while interesting patterns are present, quantitative analysis is precluded by the lack of accurate methods for calculating synthetic long‐period seismograms for three‐dimensional slab models. Therefore, we focus our analysis on a two‐dimensional approximation to the downdip geometry of the Kurile and Japan slabs, which allows comparison of a much larger data set with accurate two‐dimensional synthetics, as well as the use of sS and sScS travel time and amplitude patterns as empirical corrections for deep mantle structure. While the limited azimuth range required for this approximation reduces the diagnostic capability of the data, it also increases our confidence that the corrected data are most sensitive to the near‐source region. The sS and sScS observations indicate that the downdip shear wave travel time pattern previously attributed to a deep slab extension is primarily caused by broad‐scale lower mantle heterogeneity. Once corrected for this structure, the S wave observations do not support a simple, undeformed slab steepening in dip and penetrating deep into the lower mantle beneath the Kurile and Japan Islands, as proposed by Jordan (1977) and Creager and Jordan (1984, 1986). Rather, the observations support shorter and/or broader slab models man those previously hypothesized: models which can probably be reconciled with P wave data. Further analysis of the complete three‐dimensional patterns will be required for more precise resolution of the penetration depths of these slabs, if, indeed, any deep slab heterogeneity is actually seismically detectable.

Shear wave travel time, amplitude, and waveform analysis for earthquakes in the kurile slab: Constraints on deep slab structure and mantle heterogeneity

Shear wave travel times, amplitudes and broadband waveforms from 16 events in the Kurile slab are analyzed to place further constraints on the seismic velocity heterogeneity associated with the subducted oceanic lithosphere. Several previous studies have proposed that the Kurile slab penetrates into the lower mantle with little deformation other than a steepening of the dip angle, while other studies suggest that strong slab deformations occur with perhaps negligible lower mantle penetration. In an effort to detect any deep slab effects, we compare anomaly patterns covering a 60° azimuthal range straddling the strike of the slab for events in three different depth intervals (&gt; 500 km, 350–500 km, and 100–200 km). S and ScS pulse broadening is used as a measure of waveform complexity. Complexity of broadband shear waves does not vary systematically across our azimuthal range as the propagation direction changes with respect to the postulated lower mantle slab model for this region. Strong residual sphere travel time variations are found for shear waves from events at all depths; however, these variations do not display the depth dependence apparent in previous P wave analysis that has been interpreted as requiring a change in slab geometry and lower mantle penetration. The same S wave residual pattern is apparent in sS depth phases, and is well predicted by a recent model of shear velocity structure beneath North America. A combination of deep mantle and near‐receiver aspherical structure can explain most of our observed shear wave travel time patterns, with only a minor source depth‐dependent component remaining after correction. Individual amplitude and travel time measurements are poorly correlated; however, ScS/S amplitude ratios and ScS‐S differential times are significantly correlated, consistent with geometric focussing and defocussing effects somewhere along the raypaths. The travel time behavior suggests that the responsible velocity heterogeneity is not located near the source. The absence of a strong deep slab effect on the shear waves indicates that deep slab heterogeneity is less pronounced for shear waves than expected on the basis of existing P wave models derived from travel time residual sphere analysis. Various interpretations include: estimates of δVs/δT and compositional effects previously used to predict shear velocity structure of the deep Kurile slab are in error, with actual lower mantle slab anomalies being much weaker than proposed; the Kurile slab is so distorted in the lower mantle that no source depth dependent trends are produced for our limited focal sphere coverage; or the Kurile slab does not penetrate into the lower mantle.

Inversion of full seismogram envelopes based on the parabolic approximation: Estimation of randomness and attenuation in southeast Honshu, Japan

The envelope shape around the arrival of the direct waves of seismic signals traveling in a randomly inhomogeneous medium has been predicted, based on the parabolic approximation, to be a measure of the long‐wavelength components of the randomness as well as the attenuation properties of the medium. We use a nonlinear Marquardt‐Levenberg inversion technique in order to model the SH wave envelopes of 119 earthquakes in the frequency band 2–6 Hz for lapse times less than 1.5 times the shear wave travel time. We attempted to obtain the ratio of the mean square fractional velocity fluctuation to the correlation length (ε2V/a) estimates as well as estimates of attenuation Qs−1. For the majority of the events we found a good correlation between the envelope shape and the hypocentral distances. The resultant ε2V/a of 10−3.27±0.32 km−1 is independent of frequency. It agrees well with the choice of the Gaussian autocorrelation function for the long‐wavelength components of the random velocity fluctuations. The resultant attenuation Qs−1 is roughly proportional to the reciprocal of frequency. We may interpret it as either the scattering loss due to short‐wavelength components of randomness or intrinsic loss. We have performed numerical simulations of the inversion process to quantify the model parameter uncertainties and to obtain a better understanding of the model parameter resolution. By modeling the wave envelopes as a superposition of noise‐free wave envelopes and band‐pass‐filtered Gaussian noise we were able to reproduce the visual appearance of the observed envelopes as well as the observed features in the model parameter dependency. We find that for long hypocentral distances the envelope shape is controlled by the attenuation coefficient, while for short hypocentral distances the velocity fluctuations contribute dominantly.

A study of mantle layering beneath the western Pacific

The reverberative interval of the seismogram, defined as the portion following the surface wave train propagating along the minor arc and ending with the first body wave arrivals from the major arc, provides an excellent window in which to observe subcritical reflections from mantle discontinuities. On an SH‐polarized seismogram the intervals between ScSn and sScSn wave groups (zeroth‐order reverberations) are approximately 15 min long and consist almost entirely of waves reflected one or more times from mantle discontinuities (first‐ and higher‐order reverberations). We have developed signal enhancement and waveform inversion schemes to progressively extract information pertaining to mantle layering from the waveforms of zeroth‐ and first‐order reverberations and have applied them to a data set of long‐period digital seismograms drawn from a number of tectonic regions in and around the western Pacific. The results place important constraints on the nature of the transition zone discontinuities. Measurements of the two‐way, vertical shear wave travel time from the free surface to the 400‐km and 650‐km discontinuities (τ400 and τ650 respectively) yield estimates of transition zone travel time, τTZ, largely uncontaminated by outside heterogeneity. Variations in the path‐averaged estimates of τTZ are of the order of 5.4 s, indicating significant lateral heterogeneity in the transition zone. Correlations of τTZ with τ400 and τLM (travel time through the lower mantle) are consistent with, but do not require, the thermal deflection of an exothermic phase transition near 400 km and an endothermic transition at 650 km, with ∼12 km of long‐wavelength (500–5000 km) topography on each. The amount of topography estimated for the 650‐km discontinuity is significantly less than predicted for a rigorously stratified mantle (&gt;100 km). In a search for other mantle discontinuities, we find evidence for a strong reflector possibly associated with the onset of partial melting in the upper mantle (average depth of 86 km). Due to ambiguity inherent to the method, this feature can also be interpreted as an impedance increase in the D″ region of the lower mantle (70–170 km above the core‐mantle boundary). No evidence is found for a 220‐km discontinuity or the D″ discontinuity proposed by Lay and Helmberger (1983), suggesting that if such features are present in the western Pacific, they are either small (normal incidence, SH reflection coefficient R &lt; 1.5%) or intermittent or have lateral depth variations well in excess of 50 km.

Three‐dimensional Vp/Vs variations for the Coso Region, California

Recent seismological studies of the Coso region of southeastern California document both low P wave velocities and abnormal SV attenuation in Indian Wells Valley, south of the Pleistocene volcanics of the Coso Range. In order to learn more about the physical nature of these colocated anomalies, a tomographic inversion for the three‐dimensional variations of Vp/Vs the ratio of compressional to shear velocity was performed. Iterative back projection of 2966 shear and compressional wave travel time residuals from local earthquakes recorded on vertical instruments reveals that Vp/Vs is generally high at the surface and decreases systematically to 10 km depth. Superimposed on this trend are several large anomalies. Near Devil's Kitchen in the Coso Geothermal Area, Vp/Vs values are very low near the surface, consistent with measured values for steam‐dominated geothermal systems. Abnormally high values of Vp/Vs are observed in portions of Indian Wells Valley and also below the Cactus Peak rhyolite dome from 2 to 5 km in depth. In Indian Wells Valley the co‐occurrence of low P velocities, low S velocities, high Vp/Vs, and anomalous SV attenuation are indicative of subsurface partial melt. Because the inversion is based on vertical data only, however, the results cannot be considered conclusive, only suggestive.

Observations of first-order mantle reverberations

AbstractWe have observed first-order mantle reverberations, specifically SH-polarized ScSn and sScSn phases reflected at near-normal incidence from upper mantle discontinuities, as discrete phases on long-period digital seismograms of the HGLP and SRO networks. Such arrivals correspond to an n or n + 1 member dynamic ray family denoted by {ScSn, Sd±S}, where n is the multiple number of the parent phase, “d+” signifies a reflection from the top of an internal discontinuity at depth d, and “d−” signifies a reflection from the bottom. The travel times and attenuation of these phases place important constraints on the nature of the transition zone. We have employed the phase equalization and stacking algorithm of Jordan and Sipkin (1977) to obtain the differential attenuation operators of {sScSn, S650+S} − sScSn and {ScSn, S650−S} − ScSn phase pairs from deep-focus Tonga events recorded by the HGLP station KIP on the island of Oahu. The apparent Q's of the zeroth-order and first-order reverberations over the frequency band of 10 to 30 mHz have been inverted for the average quality factors of the upper mantle (QUM) and lower mantle (QLM) and the reflection coefficient of the 650 km discontinuity (R650). At 20 mHz, the results are: QUM = 82 ± 18, QLM = 231 ± 60, and |R650| = 0.080 ± 0.004 (at normal incidence). The QLM estimate is significantly less than recent normal mode solutions, suggesting that lower mantle attenuation structure is frequency-dependent at very low frequencies. We have measured the differential travel times between the first-order reverberations {sScSn, S400+S} and {sScSn, S650+S}, which yields a direct estimate of the vertical shear-wave travel time between the discontinuities along the Tonga-to-KIP path. The observed two-way time, 92.2 ± 1.5 sec, is less than predicted by some recent oceanic upper mantle models, indicating either a smaller separation of the discontinuities and/or greater shear velocity in the transition zone. Measurements of differential travel times between first-order reverberations and multiple ScS phases suggest that the former is more likely and are consistent with depths of 400 and 650 km for the transition zone discontinuities. Observations of first-order mantle reverberations can potentially provide unique constraints on lateral heterogeneity of the earth's mantle, especially within the transition zone.

System for measuring shear wave travel times

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Shear wave observations in the region north of the San Francisco Bay

abstract We have examined local earthquake shear wave travel times from a four-station, intermediate-period, three-component seismograph array located in the Coast Ranges north of San Francisco Bay. Our preferred interpretation of the data indicates an upper crustal layer with a shear velocity of 3.05 ± 0.10 km/sec over a lower crustal layer having a shear velocity of 3.87 ± 0.15 km/sec. We estimate the thickness of the upper crustal layer to be 13.0 ± 3.0 km. The shear velocity of the lower crustal layer, 3.87 km/sec, is higher than crustal shear velocities reported elsewhere in the Coast Ranges and corresponds to the shear velocity of layer three of the oceanic crust.

Comments on “Shear-wave travel times fromSS” by Rhett Butler

Letter| December 01, 1980 Comments on “Shear-wave travel times from SS” by Rhett Butler G. G. R. Buchbinder G. G. R. Buchbinder Earth Physics Branch Division of Seismology and Geothermal Studies1 Observatory CrescentOttawa, Ontario, CanadaK1A 0Y3 Search for other works by this author on: GSW Google Scholar Author and Article Information G. G. R. Buchbinder Earth Physics Branch Division of Seismology and Geothermal Studies1 Observatory CrescentOttawa, Ontario, CanadaK1A 0Y3 Publisher: Seismological Society of America Received: 28 Feb 1980 First Online: 03 Mar 2017 Online Issn: 1943-3573 Print Issn: 0037-1106 Copyright © 1980, by the Seismological Society of America Bulletin of the Seismological Society of America (1980) 70 (6): 2299–2300. https://doi.org/10.1785/BSSA0700062299 Article history Received: 28 Feb 1980 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. G. R. Buchbinder; Comments on “Shear-wave travel times from SS” by Rhett Butler. Bulletin of the Seismological Society of America 1980;; 70 (6): 2299–2300. doi: https://doi.org/10.1785/BSSA0700062299 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

S-wave travel times for a spherically averaged earth

Summary. Shear-wave travel times in a spherically averaged earth are estimated using ‘differential’S minus P (S–P) travel-time measurements and detailed statistical procedures. Fourteen earthquakes and 48 stations are specially selected, yielding 302 S - P times for 6° < Δ < 111°. Analysis of variance techniques are used to estimate simultaneously azimuthally varying source and station adjustments while constructing an S–P travel-time model. A method of weighting the equations of condition based on the distribution of stations and epicentres is developed to reduce the effects of systematic errors due to non-random sampling of the Earth. The resulting S - P travel times are added to the 1939 Jeffreys–Bullen and the 1968 Herrin P travel times as a function of distance to obtain shear-wave travel-time models. Confidence intervals for the models are estimated from the variance of the observed S–P travel times. The standard error for a single observed S–P travel time (6° < Δ < 111°) is 2.1 s and the residual distribution is not significantly different from a normal distribution at the 95 per cent confidence level. For 30° < Δ < 80° the mean S travel time is 1.3 s later than the corresponding mean for Jeffreys–Bullen tables, which is significant at the 95 per cent confidence level.

Shear-wave travel times and amplitudes for two well-constrained earthquakes

abstract The wave-form correlation technique (Hart, 1975) for determining precise teleseismic shear-wave travel times is extended to two large earthquakes with well-constrained source mechanisms, the 1968 Borrego Mountain, California earthquake and the 1973 Hawaii earthquake. A total of 87 SH travel times in the distance range of 30° to 92° were obtained through analysis of WWSSN and Canadian Network seismograms from these two events. Major features of the travel-time data include a trend toward faster travel times at a distance of about 40° (previously noted by Ibrahim and Nuttli, 1967; Hart, 1975); another somewhat less pronounced trend toward faster times at about 75°; a +6 sec base line shift, with respect to the Jeffreys-Bullen Table, for the Borrego Mountain data; and large azimuthally-dependent scatter for the Hawaiian data, probably reflecting dramatic lateral variations in the near-source region. When azimuthal variations in the Hawaii data are removed, the travel times from both events show very low scatter. The correlations were also used to investigate SH amplitudes for possible distance dependence in the data and variations in tβ*. The Borrego Mountain data show very low scatter and no discernible distance dependence. All of the data are compatible with a value of tβ* = 5.2 ± 0.5. The amplitudes from the Hawaii earthquake show the same azimuthal variations found in the travel-time data. When those effects are removed, the Hawaii data satisfies a value of tβ* equal to 4.0 ± 0.5 and, as with the other data set, no distance dependence is apparent.

Shear wave travel time residuals from oceanic earthquakes and the evolution of oceanic lithosphere

Shear wave travel time residuals have been measured for 25 oceanic earthquakes, including normal faulting events on ridge crests and intraplate events. When the residuals are plotted against the age of the lithosphere in which the event occurred, they show a decrease of 6±1 s from ridge crest to oceanic lithosphere 100 m.y. old. This decrease is caused principally by an increase in shear velocity within the upper mantle as a function of lithospheric age. Study of depth phases from oceanic earthquakes indicates that all such events are shallower than 10 km below the sea floor, so focal depth variations are not an important contributor to the travel time residual versus age relationship. Modeling of seismic velocities in the upper mantle, using a standard thermal model for the spreading lithosphere and published phase diagrams for dry and wet gabbro-eclogite and peridotite, was conducted in an attempt to produce a residual curve similar to that observed. Comparison of the predicted and observed residuals indicates that the mantle cannot be dry but is unable to distinguish between the wet systems, principally because of the large number of assumptions involved in modeling velocity changes across phase boundaries. However, the wet gabbro-eclogite model is unable to match local shear velocities in the oceanic lithosphere obtained from Sn wave velocities and inversion of surface wave phase velocity, and a peridotitic upper mantle is favored. A strong azimuthal variation of residual exists for an event along the Gibbs fracture zone, in approximate agreement with the velocity versus age models. Shear wave residuals for events on the Indian plate are systematically less by 2 s than residuals for earthquakes in lithosphere of comparable age in the Atlantic or Pacific; this offset may result from the rapid northward motion of the Carlsberg and southeast Indian ridges with respect to the mesosphere.

Lateral heterogeneity of the upper mantle determined from the travel times of multipleScS

We have extended our investigation of lateral variations in upper mantle shear velocity structure by employing multiple ScS phases. An analysis of 108 multiple ScS differential travel times indicates that to a first approximation, upper mantle heterogeneity as expressed in vertical, travel time differences can be correlated with surface tectonic features. Travel times for continental regions with Phanerozoic orogenic histories are significantly greater than those from Precambrian continental regions and correlate with heat flow. Oceanic regions have large delays relative to stable continental regions, and the travel time residuals show a consistent decay as a function of crustal age. The average one-way vertical shear wave travel time difference between oceans and stable continental regions is estimated to be +4.0±0.5 s, not significantly different from the value obtained in our previous study. These results support the hypothesis that shield ocean heterogeneity persists to great depths, probably exceeding 400 km. For shear waves with characteristic frequencies of 0.1 Hz, the two-way vertical travel time through the mantle of the spherically averaged earth is 937.3±1.0 s, about 3 s less than the value derived from eigenfrequency data and 1.6 s greater than the Jeffreys-Bullen value. This result is inconsistent with the hypothesis that the base line discrepancy between free oscillation models and classical body wave travel time models is because of continental bias in the travel times, but it is consistent with the hypothesis that the base line discrepancy is an expression of anelastic dispersion.