SH Velocity Research Articles

Upper mantle SH velocity structure beneath Qiangtang Terrane by modeling triplicated phases

We constrain SH wave velocity structure for the upper mantle beneath western Qiangtang Terrane by comparing regional distance seismic triplicated waveforms with synthetic seismograms, based on an intermediate event (∼220 km) recorded by the INDEPTH-III seismic array. The ATIP model reveals a low-velocity anomaly with up to −4% variation at the depth of 190–270 km and a relatively small velocity gradient above the depth of 410 km in the upper mantle, which is in agreement with previous results. In combination with other geological studies, we suggest that the depth of top asthenosphere is 190 km and no large-scale lithosphere thinning occurs in western Qiangtang Terrane, besides, Qiangtang Terrane has the same kind of upper mantle structure as the stable Eurasia.

Evidence of seismic anisotropy and a lower temperature condition in the D″ layer beneath Pacific Antarctic Ridge in the Antarctic Ocean

We examine shear waves of 23 deep earthquakes in south Pacific subduction zones recorded at the Syowa station in Antarctica to study the anisotropy and velocity structure of shear waves in the D″ layer beneath Pacific Antarctic Ridge (PAR) in the Antarctic Ocean. We observe earlier arrivals of transverse components (SH) than those of longitudinal components (SV) for S waves passing through the D″ layer and Scd phases due to the D″ discontinuity that has been reported beneath circum Pacific regions. We find that the optimum shear wave velocity structure in the D″ layer is transversely isotropic from waveform modeling and the differential travel time analysis among S, ScS and SKS phases. The SH velocity structure has a 2.0% velocity discontinuity at about 350 km above the core–mantle boundary. The SV structure is similar to PREM. The anisotropic D″ layer, V SV < V SH, beneath the Antarctic Ocean is significantly thicker than those beneath Alaska and the Caribbean Sea. We interpret that the amount of deposited paleo-slab material controls the variation in the thickness of the D″ layer. The difference of the thickness of the D″ layer, together with recent experimental and theoretical results of the phase transition from perovskite to post-perovskite, means that the D″ layer beneath PAR is 630–1000 K colder than that beneath Alaska. The cold mantle beneath PAR corresponds to the location of stationary patches of intense geomagnetic field in the southern hemisphere. This coincidence suggests the existence of the core–mantle interactions.

Upper mantle structure beneath central Western Europe from data on phase velocities of surface waves

The deep structure of the upper mantle is determined from data on phase velocities of Love and Rayleigh waves measured by a differential method on traces between two stations in central Western Europe. One-dimensional velocity structures are first constructed from data of each pair of stations, after which two-dimensional distributions of SH and SV velocities are calculated by the method of two-dimensional tomography from S wave velocities at fixed depths. The results are presented in the form of 2-D vertical structures of the average S wave velocity (S = (SV + SH)/2) constructed along profiles crossing the region in directions of the best resolution. The main structural features are a higher velocity zone at depths of 60–80 km in the area (48°–50°N, 9°–11°E) and a lower velocity zone in the western part of the region at depths of 100–150 km, probably extending farther beyond the studied area.

Upper mantle SH- and P-velocity structures and compositional models beneath southern Africa

We constrain SH and P wave velocity structures in the upper mantle beneath southern Africa, using triplicated phases recorded in the epicentral distance range of 11°–28° for one shallow event. We then explore thermal and compositional models appropriate for explaining the inferred seismic structures in the region. Both SH and P wave data suggest presence of a low velocity zone with velocity reductions of at least − 5% for S waves and − 2% for P waves beneath a 150 km thick high-velocity lithospheric lid. Seismic observations also suggest that the P/S ratio is larger (1.88) in the transition zone than in the lithospheric lid (1.70). The inferred P wave velocity jump across the 660-km discontinuity is small (< 4%), while the inferred SH wave velocity jump across the discontinuity is comparable to that in the Preliminary Reference Earth Model. The low velocity zone can be explained by a high temperature gradient of 6 ° C/km or presence of partial melt. Partial melt would require presence of water or other volatile elements in the depth range of the low velocity zone to lower the mantle solidus. The different P/S velocity ratios between the lithospheric lid and the transition zone can be explained by a difference in aluminum content of mantle composition, with values of 1% in the lithospheric lid and 4% in the transition zone, respectively. The inferred P and SH velocity jumps suggest a bulk sound velocity decrease of 3.4% across the 660-km discontinuity. Such a large decrease of bulk sound velocity is possible only if the bulk modulus of perovskite is similar to those of ringwoodite and garnet, a result remained to be confirmed by experimental or theoretical studies. The presence of the low velocity zone with large velocity reductions beneath southern Africa suggests existence of a low-density anomaly beneath the lithospheric lid, which may provide an explanation to the observed localized uplift in the “African Superswell”.

Whole mantle SH velocity model constrained by waveform inversion based on three-dimensional Born kernels

SUMMARY A whole mantle SH velocity model is obtained by using a unique data set and techniques. Body and surface waveforms including major and multi-orbit phases are used as a data set and are inverted by using 3-D Born kernels. The resultant model, SH18CE, reveals the different natures of the two major upwelling systems: the strong low velocity anomalies beneath Africa extend for more than 1000 km from the core–mantle boundary (CMB), whereas those beneath the Pacific are restricted to 300–400 km from the CMB. The results also show the variable natures of stagnant slabs on the 670 discontinuity around Japan: the depths of the strongest high velocity anomalies within the stagnant slabs are different region by region, which is consistent with the detailed delay time tomography model in this area.

SH velocity and compositional models near the 660‐km discontinuity beneath South America and northeast Asia

We constrain SH wave velocity structures near the 660‐km discontinuity beneath South America and northeast Asia, using triplicated phases near the discontinuity recorded in the epicentral distance range of 10°–35° for three deep events. We then explore mineralogical and compositional models appropriate for explaining the inferred seismic structures between the two regions. SH velocity structures near the 660‐km discontinuity are found to be different in the two regions. Beneath South America, the velocity gradient above the 660‐km discontinuity is larger than that of Preliminary Earth Reference Model (PREM), while the velocity jump across the discontinuity is the same as PREM. Beneath northeast Asia, the velocity gradient above the 660‐km discontinuity is the same as that of PREM, while the velocity jump across the discontinuity is larger than PREM. Both regions are characterized by a large velocity gradient extending about 80 km deep below the 660‐km discontinuity. The different velocity structures require different mineralogical models in the transition zone in the two regions. The larger velocity gradient above the 660‐km discontinuity beneath South America requires existence of the ilmenite phase in the bottom of transition zone, while that beneath northeast Asia can be explained by the temperature and pressure dependence of elastic properties of the major mantle mineral assemblages. The observed large velocity gradients in the top of the lower mantle can be explained by gradual transformation of garnet to perovskite persisting to greater depths. The velocity jump across the 660‐km discontinuity beneath South America, in the presence of the ilmenite phase in the bottom of the transition zone, requires a bulk composition of more garnet than the pyrolite model, while a larger velocity jump across the discontinuity beneath northeast Asia requires a larger fraction of garnet transforming to perovskite across the discontinuity than what is required for explaining PREM. These different mineralogical models can be caused by different mantle temperature or composition, especially the aluminum content in mantle composition. The presence of garnet 80 km below the 660‐km discontinuity in the two regions may be explained by a uniform composition in the lower mantle with an aluminum content of 3.4%. The existence of ilmenite in the bottom of the transition zone beneath South America and the absence of ilmenite beneath northeast Asia can be explained by either a difference in mantle temperature of about 100°C (with that beneath South America being lower) between the two regions assuming a uniform mantle composition or, alternatively, a difference in aluminum content of about 1% (with that beneath South America being lower) between the two regions without invoking a temperature difference between the two regions. We also discuss conditions of mantle composition and temperature that double discontinuities may occur near the 660‐km depth, as well as the depth separation and velocity jumps of the double discontinuities. For the inferred mantle temperature and composition beneath South America and northeast Asia, the maximum separation of the double discontinuities is 20 km and cannot be resolved by the SH wave data.

Strain‐induced seismic anisotropy of wadsleyite polycrystals and flow patterns in the mantle transition zone

We use forward models based on recent high‐pressure experimental data on mantle minerals to predict the seismic anisotropy produced by plastic strain of orthorhombic wadsleyite, the dominant mineral in the upper transition zone. These models predict a weak seismic anisotropy for a polycrystal of pyrolitic composition (60% wadsleyite, 40% garnet) at transition zone conditions: ∼2% for P and ∼1% for S waves for a shear strain of 1. Both P and S wave anisotropy patterns show an orthorhombic symmetry. P waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane. S wave anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction. Horizontal shearing results therefore in higher velocities for horizontally propagating P waves (PH) and horizontally polarized S waves (SH), as well as in weak azimuthal variation of SV and SH velocities. On the other hand, vertical flow leads to higher velocities for vertically propagating P waves (PV) and vertically polarized S waves (SV) and to a weak azimuthal variation of SV velocity but to a roughly constant SH velocity. Analysis of global observations of seismic anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone, in agreement with predictions of geodynamical models that explicitly introduce phase transitions.

Qtomography of the upper mantle using three-component long-period waveforms

SUMMARY We present a degree-8 3-D Q model (QRLW8) of the upper mantle, derived from three-component surface waveform data in the period range 60–400 s. The inversion procedure involves two steps. In the first step, 3-D whole-mantle velocity models are derived separately for elastic SH (transverse component) and SV (vertical and longitudinal component) velocity models, using both surface and body waveforms and the non-linear asymptotic coupling theory (NACT) approach. In the second step, the surface waveforms thus aligned in phase are inverted to obtain a 3-D Q model in the depth range 80–670 km. Various stability tests are performed to assess the quality of the resulting Q model and, in particular, to assess possible contamination from focusing effects. We find that the 3-D patterns obtained are stable, but the amplitude of the lateral variations in Q is not well constrained, because large damping is necessary to extract the weak Q signal from data. The model obtained agrees with previous results in that a strong correlation of Q with tectonics is observed in the first 250 km of the upper mantle, with high attenuation under oceanic regions and low attenuation under continental shields. It is gradually replaced by a simpler pattern at larger depth. At the depths below 400 km, the Q distribution is generally dominated by two strong minima, one under the southern Pacific and one under Africa, yielding a strong degree-2 pattern. Most hotspots are located above regions of low Q at this depth. Ridges are shallow features in both velocity and Q models.

The anisotropic seismic structure of the East European platform

We present models for the P and S velocity structure of the upper mantle beneath the East European platform including measurements of radial anisotropy. The models were obtained by full waveform inversion of 3‐component broadband seismograms from strike‐slip earthquakes located near the edge of the platform and recorded in Russia and Europe. We used direct and multiply reflected body wave arrivals (S, SS, P, PP) combined with fundamental mode Love and Rayleigh waves at source‐receiver distances between 15° and 50° to resolve mantle structure down to the 410 km discontinuity. The platform is underlain by a radially anisotropic mantle lid extending to a depth of 200 km with a largely isotropic mantle below. The model has a positive velocity gradient from 41 km to 100 km depth, and a relatively uniform velocity structure from 100 km to 200 km depth with high SH and PH velocities (4.77 km/s, 8.45 km/s). Shear anisotropy is uniform at 5% (βH &gt; βV) from 41 to 200 km depth, drops to 2% from 200 to 250 km and is isotropic below that. The average shear velocity from 100 to 250 km is also uniform at 4.65 km/s and the drop in anisotropy is matched by a drop in βH to 4.70 km/s combined with an increase in βV to 4.60 km/s. Below 250 km there is a positive velocity gradient in both P and S velocity down to 410 km. P anisotropy is not well resolved, but P structure mimics the SH velocity structure, suggesting that P is also anisotropic within the lid.

Experimental verification of the anisotropic Gassmann model for fluid substitution in fractured reservoirs

Porous reservoirs with aligned fractures exhibit frequency dependent seismic anisotropy due to wave induced fluid flow between pores and fractures. We model this frequency dependent anisotropy by combining the low-frequency anisotropic Gassmann model with the Hudson et al. model for frequency dependent properties of a porous material with penny-shaped cracks. The predictions of the anisotropic Gassmann model are compared with experimental measurements of elastic wave velocities as function of angle for a synthetic sample with aligned disc-like cracks. The properties of the host rock and fracture compliances are obtained from P, SV, and SH velocities measured on the dry sample. The dry fractured porous rock properties serve as input to anisotropic Gassmann fluid substitution model, which is used to compute the saturated rock properties. The stiffnesses of the saturated fractured porous rock are used to calculate the angular dependent compressional and shear wave velocities. The predicted velocities are then compared to the measured velocities. The agreement is reasonably good for both S-wave velocities, but P-wave anisotropy is overestimated by about 25%. This discrepancy can be explained by the fact that the low frequency assumption of the anisotropic Gassmann model does not account for the fluid diffusion effects occurring at relatively high frequencies used in the experiment (100 KHz). A combination of the low frequency anisotropic Gassmann model with the Hudson et al. frequency dependency accounts for fluid diffusion effects and results in an excellent agreement for P- as well as S- waves.

A seismic cross-section through the east European continent

SUMMARY A 2-D pro¢le for the shear wave velocity and anisotropybetween Egypt and Spitsbergen is presented. The pro¢le is constructed using fundamental- and higher-mode Love and Rayleigh waves recorded by stations of the NARS-DEEP, IRIS and GEOFON networks. The surface wave data have been inverted for shear velocity and anisotropy using a waveform inversion. In the eastern Mediterranean lithosphere we observe a large diierence (7 per cent) between SH and SV velocities. We interpret this anomaly as an anisotropic oceanic lithosphere beneath the eastern Mediterranean, an interpretation which is consistent with tectonic reconstructions of the region. The east European continent is imaged as a high-velocity body whose thickness increases with the estimated age of the lithosphere. The continental root of the Ukrainian and Baltic shields and east European platform extends to a depth of 200 km. This is in contrast to the surrounding younger continental regions which appear to be less than 100 km thick. We further studied the structure of the continental lithosphere by investigating a possible relation between seismic velocities and tectonic age. Both a logarithmic and a square root relation have been ¢tted to the average seismic velocities in each tectonic region. The data slightly favour a logarithmic relation but a square root relation cannot be excluded.

Evaluation of slab images in the northwestern Pacific

Recent P wave travel-time tomographic studies using data from the International Seismological Centre (ISC) catalog determine a large-scale subhorizontal high velocity anomaly in the northwestern Pacific subduction zones and it has been interpreted as imaging stagnant slab in the upper mantle transition zone (∼400 to 700 km). The limited resolution of the travel time tomographic studies in this depth range, however, makes it difficult to evaluate accurately the vertical and lateral extent of a stagnant slab. A broadband waveform modeling of triplicated regional seismic waves which are very sensitive to the transition zone structure is useful to evaluate the velocity structure along the propagation paths and therefore to constrain the spatial distribution of anomalies. This study thus compares tomographic images from the model of Obayashi et al. (1997) with results of the regional waveform modeling by Tajima and Grand (1998). The ISC tomographic model shows the largest lateral extent of high velocity anomaly in the layer of 478 to 551 km depths although part of this spread is likely due to the deteriorated resolution in that depth range. The waveform modeling suggests that the strong high velocity anomaly associated with a stagnant slab exists below 525 km with its maximum intensity in the top 50 km and decreases with increasing depth to vanish at 660 km. These results along with a recent global SH velocity model SAW12D of Li and Romanowicz (1996) which has the strongest high velocity anomaly in a depth range 500–550 km may be integrated into an image of a stagnant slab. The anomalous velocity structure associated with a stagnant slab has its maximum intensity not immediately above the 660 km discontinuity but in a depth range ∼100 km above it. This feature appears to be consistent with a thermo-chemical model of down-going slab in which a larger velocity contrast with the surrounding mantle is expected at a shallower depth of the transition zone. The ISC tomographic model and waveform modeling consistently show that the deflected slabs are not laterally continuous but are separated into a few subregions. Beneath the northeastern China where the resolution is good, the slab related anomaly above the 660 km discontinuity is accompanied by its downward extension into the lower mantle.

Full-wavefield inversion of wide-apertureSHand Love wave data

Reverse-time linearized inversion is implemented for synthetic wide-aperture SH and Love wave data, including multiple reflections, to estimate the S-wave velocity in a two-dimensionally inhomogeneous medium. Complete wide-aperture wave-fields, in which triplications, pre- and post-critical reflections, and surface waves (Love waves) are present, can be imaged by reverse-time inversion to estimate the SH velocity distribution. The algorithm operates on common-source gathers and involves cross-correlation of the source and recorded wavefields to define the steepest descent direction to update the velocity at each point on a finite-difference grid. Effects of noise can be reduced by stacking information from different sources.

Three-dimensional modelling of upper mantle structure under the Pacific Ocean and surrounding area

SUMMARY Long-period (75-250 s) fundamental-mode Love and Rayleigh waves are inverted for the seismic velocity variations in the upper mantle under the Pacific Ocean and surrounding area. The data are inverted to map the regional variation of phase velocity, and then the velocity structure of the upper mantle. No a priori regionalization is used in this study, and the lateral heterogeneity is expanded horizontally into blocks (lo' X 10'). To estimate the reliability of the velocity distribution, we have calculated the resolving kernels. In the central, NW, NE and SW regions, the resolving kernels have fairly sharp peaks. In the SE region, however, resolution is not so good. The results show good correlation with surface tectonics. In the velocity map, shields and ridges are the major features in the depth interval 50-100km. Anomalies associated with the East Pacific Rise and the shields persist to 300 km. At 400 and 500 km, high-velocity anomalies appear under the eastern and southern Pacific ridges, and a low-velocity anomaly appears beneath Australia. The present study provides better resolution than previous studies because of the better path coverage. For example, a large low-velocity anomaly emerges under Hawaii down to the depth of 200 km for SH velocity. The horizontal extent of this anomaly is comparable to the sue of the Hawaiian swell. This feature supports that the Hawaii hotspot is an isolated, truly anomalous feature in the Pacific Ocean.

The Backus-Gilbert approach to the 3-D structure in the upper mantle - II. SH and SV velocity

Phase velocity variations obtained in the previous paper are inverted by the Backus–Gilbert method for the velocity structure of the upper mantle. Spheroidal modes and toroidal modes in the period range of 125–260 s are used in the inversion. The data cannot constrain all six parameters in a transversely isotropic medium and we chose to perturb only two parameters, SH and SV velocities. SV velocities are resolved between the depths of about 200 and 400 km and SH velocities between 0 and 200 km. Resolution kernels have half-peak widths of about 200–300 km in depth, becoming broader for deeper target depths. SV velocity kernels show secondary peaks near the surface of the Earth, with widths varying from 50 to 100 km. The deeper the target depths, the wider the secondary peaks near the surface. SH velocity kernels do not possess such secondary peaks. The trade-off between SV and SH velocities is small. SV velocity is essentially determined by spheroidal modes and SH velocity by toroidal modes. Because of the broad width of the resolution kernels, the structure in the resolved region is difficult to detect from our data set; for example the differences in SV velocity structure between 250 and 350 km or the differences in SH velocity between 100 and 200 km are difficult to distinguish. Considering the horizontal resolution of about 2000 km, obtained in the previous paper, averaging kernels for 3-D structure are quite elongated in the horizontal dimension.

Seismic anisotropy in the upper oceanic crust

Seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at Deep Sea Drilling Project (DSDP) site 504B on DSDP leg 92. Particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis. There are four diagnostic patterns: (1) Along symmetry axes, where vertically polarized shear waves (SV) are generated but horizontally polarized shear waves (SH) are not generated, the particle motions are purely vertical, (2) for azimuths at which both SV and SH are generated and the SH velocity is significantly faster than SV, a cruciform pattern with horizontal first motion is observed, (3) for azimuths at which both are generated and the SV velocity is significantly faster than SH, a cruciform pattern with vertical first motion is observed, and (4) for azimuths at which both are generated and SV and SH velocities are similar elliptical particle motions are observed. The shear wave particle motions and compressional wave travel times (from a 2‐km radius circle) are consistent with an anisotropic model with hexagonal symmetry. The compressional wave velocity has a two theta azimuthal variation between 4.0 and 5.0 km/s. The symmetry axis is horizontal with an azimuth of N20°W±10°. The spreading direction at the site (6 m.y. age) is north‐south. The observed seismic anisotropy is most probably caused by the preferred orientation of large‐scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting.

S velocities in D'' from diffracted SH-waves at the core boundary

Summary. The slowness of the diffracted SH-wave around the core boundary is re-examined. The criticism that measurements from the Iranian earthquake of 1968 August 31 are affected by the focal mechanism is shown to be unjustified theoretically and by comparison with analogous recordings from the Imperial Valley earthquake of 1979 October 15. Evidence is presented that an SH ray parameter estimated from first impetus SH diffracted onsets does not represent a shear velocity averaged over a hundred kilometres depth but rather the velocity within a few tens of kilometres of the core–mantle boundary. In addition to a re-examination of the seismograms previously used for the 1968 August 3 1 Iran earthquake, long-period seismograms for 80° < ▵ < 150° have been studied for two Tonga earthquakes, 1967 October 9 and 1977 August 11. Exclusion of readings with ▵ < 105° from the Iranian earthquake increases the gradient from 8.68 ± 0.13 to 9.48 ± 0.26s deg−1. The exceptionally large ray parameter is not fully explained, but a plausible hypothesis is anisotropy in shear velocity near the core–mantle boundary. When observations are restricted to clear arrivals at distances exceeding a few degrees from the shadow boundary, reliable new estimates are obtained for the SH slowness, namely 8.41 ± 0.07s deg−1 for Tonga 1967 and 8.39 ± 0.06 for Tonga 1977. The statistical uncertainty of the estimates indicates no significant average change in SH velocity at the base of the mantle along two easterly profiles 3000 km apart under the east Pacific and North America. For a core radius of 3485 km and a slowness of 8.4s dec−1, the average velocity near the core–mantle boundary is 7.24km s−1. This value is in agreement with estimates in D″ by Mula & Muller. The observed attenuation of the diffracted SH ground motion amplitudes at periods of 15–20s is similar for the three profiles considered, with a reduction of an order of magnitude within a distance of 25° in the shadow zone. This corresponds to an equivalent decay parameter γ= 0.10 at the core boundary.

On the anisotropy of sedimentary rocks from shear-wave analysis

Summary. The paper describes some results of experimental seismic prospecting investigations of the anisotropic properties of sedimentary rocks at depths of less than 2.5 km. Shear and converted PS-waves were recorded. Examples of SV and SH velocity distributions and shear-wave polarization are given. The main conclusion is that the medium essentially differs from that usually adopted for sedimentary rocks in that the symmetry axis is not normal to the layering.

P-wave complexity and fault asperities: The Borrego Mountain, California, earthquake of 1968

AbstractResults from a synthetic seismogram analysis of the short-period P waves from the Borrego Mountain earthquake of 9 April 1968 (ML = 6.4) are used to model the strong-motion recording at El Centro. A short-period-long-period deconvolution analysis of the teleseismic P waves suggested that a two-source model would fit the data much better than the single-source model presented by Burdick and Mellman (1976). Forward and inverse modeling of the data demonstrated that two sources, each of less than 2-sec duration, the second occurring 2.2 sec after the first and both being at about 8-km depth, best fit the short-period waveforms. From this model, long-period synthetics were generated which were found to be quite compatible with the data. This source model was also used to synthesize the strong-motion SH displacement, velocity, and acceleration records from El Centro, California. The close match of synthetics and data is used to argue that short-period waveforms contain much information about asperities which play a crucial role in the near-source strong motions from an earthquake. The Borrego Mountain event probably began with the failure of a fault asperity. The evidence for this is the several-hundred-bars stress drops of the two short-period sources and the probable location of these sources in a place where there was almost no aftershock activity or postseismic creep on the fault.

Interpretation of synthetic common‐depth‐point gathers for a single anisotropic layer

Analysis of synthetic traveltime gathers shows that anisotropy may have a large enough effect on P, SH, and SV propagation to alter significantly the interpretation of the subsurface below the anisotropic layers. Consequently, if anisotropy exists below a seismic line, it is important to estimate the anisotropic parameters correctly. We discuss the effects of anisotropy on seismic waves and present a method for estimating the elastic constants of a transversely isotropic layer from P and SH arrival‐time gathers. The technique may be extended to more general anisotropic symmetries by analyzing gathers from several azimuths. To illustrate the possible effect of anisotropy on exploration surveys, P, SH, and SV velocity variations are calculated for several types of anisotropic sedimentary fabrics. Alignments due to bedding, shale lithology, and dry parallel cracks may have similar velocity variations. Fabrics with other configurations of cracks may still possess overall transversely isotropic symmetry, but they have a wide range of angular velocity variations with different polarities and periodicities. Synthetic gather curves are generated for a range of models with an anisotropic layer over an isotropic substrate. They show departures from hyperbolas, and erroneous depth determinations, that depend upon the elastic constants of the anisotropic layer. The elastic constants of the anisotropic layers are estimated from the synthetic gather curves by means of approximate equations for the angular velocity variations, which are linear in the elastic constants. Formulas are developed which relate tangents to the gather curves directly in terms of the elastic constants. These are tested for single‐layer transversely isotropic models and allow the five elastic constants to be estimated by drawing three tangents to P and SH synthetic arrival‐time gathers in [Formula: see text] space. Comparisons of estimated with original elastic constants are good for a number of different types of transversely isotropic fabrics. Gathers are also calculated at two azimuths in an anisotropic layer with orthorhombic symmetry and are analyzed with some success.