SP12RTS: a degree-12 model of shear- and compressional-wave velocity for Earth's mantle (original) (raw)
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Anisotropic shear‐wave velocity structure of the Earth's mantle: A global model
Journal of Geophysical Research: Solid Earth, 2008
We combine a new, large data set of surface wave phase anomalies, long‐period waveforms, and body wave travel times to construct a three‐dimensional model of the anisotropic shear wave velocity in the Earth's mantle. Our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model, simultaneous inversion for velocity and anisotropy, as well as discontinuity topographies, and implementation of nonlinear crustal corrections for waveforms. A comparison of our new three‐dimensional model, S362ANI, with two other models derived from comparable data sets but using different techniques reveals persistent features: (1) strong, ∼200‐km‐thick, high‐velocity anomalies beneath cratons, likely representing the continental lithosphere, underlain by weaker, fast anomalies extending below 250 km, which may represent continental roots, (2) weak velocity heterogeneity between 250...
Radial variation of compressional and shear velocities in the Earth's lower mantle
Geophysical Journal International, 1978
This paper extends an earlier study (Sengupta & Julian) of travel times of P waves of deep-focus earthquakes to include shear waves. Primary advantage of deep-focus earthquakes is the reduction of anomalies caused by complex structures near the source. The standard deviations of travel times and station anomalies of this study are about half as large as those determined from the data of shallow-focus earthquakes (e.g. Herrin et ul.; Hales & Roberts). Spherically-symmetric velocity models derived from the travel times by a linearized inverse technique have resolving lengths of about 70 km for standard errors in velocity of about 0.02 km/s. No pronounced reversal of either compressional or shear velocity was required at the base of the mantle to satisfy the data, though a small velocity decrease could not be entirely ruled out. Some anomalous rapid changes in compressional velocity gradient were, however, found centred around the depths of 2400 and 2600 km. The models derived in this study agree most closely with that of Herrin er ul. for compressional velocity and the model 1066B of Gilbert & Dziewonski for shear velocity.
The shear-wave velocity gradient at the base of the mantle
Journal of Geophysical Research, 1983
The relative amplitudes and travel have been directed toward obtaining global times of ScS and S phases are utilized to place averages, and the degree of lateral variation in constraints on the shear-wave velocity gradient D" properties remains an open question. above the core-mantle boundary. A previously A conflicting result was found by Mitchell and reported long-period ScSH/SH amplitude ratio Helmberger [1973], who utilized the relative minimum in the distance range 65 ø to 70 ø is shown amplitudes and timing of long-period ScS and S to be a localized feature, apparently produced by phases to constrain the S-wave velocity gradient an amplitude anomaly in the direct S phase, and in D". They found a minimum in the ScSH/SH therefore need not reflect the velocity gradient amplitude ratio near 68 ø , which was attributed to at the base of the mantle. The amplitude ratios low amplitudes of the ScS arrivals. Unable to that are free of this anomaly are consistent with explain this feature by models with negative or calculations for the JB model or models with mild near-zero shear velocity gradients in D", they positive or negative velocity gradients in the proposed models with positive S-wave velocity lowermost 200 km of the mantle. ScSV arrivals gradients above the CMB. These positive are particularly sensitive to the shear velocity gradients extended over 40 to 70 km above the structure just above the core-mantle boundary. core, reaching velocities at the CMB as high as The apparent arrival time of the peak of ScSV is 7.6 to 7.8 km/s. These models can explain the as much as 4 s •reater than that of ScSH in the observed amplitude ratio behavior, as well as an distance range 75 v to 80 ø for Sea of Okhotsk apparent difference observed in the arrival times events recorded in North America. This can be of transversely and radially polarized ScS. explained by interference effects produced by a Mitchell and Helmberger also proposed a low Q$ localized high velocity layer or strong positive zone in D", or finite outer core rigidity, to S wave velocity gradient in the lowermost 20 km explain the baseline of the ScSH/SH amplitude of the mantle. A velocity increase of about 5% ratios. While the majority of their data was for is required to explain the observed shift between deep South American events recorded in North ScSV and ScSH. This thin, high velocity layer America, they did analyze one deep Sea of Okhotsk varies laterally, as it is not observed in event for which the radial and transverse ScS similar data from Argentine events. Refined arrival times were not different, which suggested estimates of the outermost core P velocity lateral variations in the D" velocity structure. structure are obtained by modeling SKS signals in In this paper we extend the analysis of ScS the distance range 75 ø to 85 ø ß and S phases using an enlarged data set in order to understand the discrepancy between the ß SES LHC BOZ ß SCB ON • •RCD AAM• DUG• •GOLFLO• N45*W ß WWSSN Stations ß CSN Stotions X Deep Argentine Events WES SCP
We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on~43,000,000 surface wave and~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts. Preliminary Reference Earth Model (PREM) [Dziewoński and Anderson, 1981], one of the most widely used 1-D reference models, incorporates radial anisotropy from the Moho (24.4 km depth) to the 220 km discontinuity. It is mainly constrained by joint analysis of Rayleigh and Love wave data. Montagner and Anderson [1989] and Montagner and Kennett [1996] modified PREM using additional normal-mode and travel time data, respectively. Moreover, Beghein et al. [2006] also modified PREM using normal-mode data in a wide frequency range. Fully 3-D radially anisotropic models for both the upper and whole mantle have been developed since the 1980s [e.g.,
Shear wave speeds at the base of the mantle
Journal of Geophysical Research, 2000
We inverted 4864 ScS-S and 1671 S¡ £ ¢¤-SKS residual travel times for shear wave speed anomalies at the base of the Earth's mantle. We applied ellipticity corrections, accounted for mantle structure outside of the basal layer using mantle tomography models, and used finite size sensitivity kernels. The basal layer thickness was set to 290 km; however, the data allow thicknesses between 200 and 500 km. The residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small Euclidean norm, which limited spectral leakage of higher-order structures into low-order wavelengths. Hotspots dominantly overlay slow wave speed regions. Nonsightings of ultralow-velocity zones (ULVZs) most frequently appear in fast regions, suggesting that slow regions at the base of the mantle are associated with ULVZs. However, ULVZ sightings appear in both slow and fast regions. Recently active subduction zones do not correlate with velocity anomalies; however, the locations of subduction zones active prior to 90 Ma correlate extremely well with fast anomalies, implying that slabs descend as fast as 2 cm yr¥ through the lower mantle. The correlation continues through the historical subduction record to 180 Ma, suggesting that slabs remain in the deep mantle at least 90 Myr. Fast anomalies reach +2%, while slow anomalies extend to-5%. If we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed/thermal anomaly conversion, the temperature deviations would be over-500 ¦ K (cold) in fastest regions and over +1000 ¦ K (hot) in the slowest regions, which would initiate plumes much hotter than those observed at the surface. Alternative explanations for the large anomalies are widespread partial melt or compositional differences in the lowermost mantle.
Lateral Variations in Compressional/Shear Velocities at the Base of the Mantle
Science, 1999
Observations of core-diffracted P ( P diff ) and SH ( SH diff ) waves recorded by the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional ( P ) seismic velocities to horizontal shear ( SH ) velocities at the base of the mantle changes abruptly from beneath the mid-Pacific ( V P / V S = 1.88, also the value predicted by reference Earth models) to beneath Alaska ( V P / V S = 1.83). This change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin. A textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern Pacific paleotrench.
Journal of Geophysical Research, 2012
1] We present DR2012, a global SV-wave tomographic model of the upper mantle. We use an extension of the automated waveform inversion approach of which improves our mapping of the transition zone with extraction of fundamental and higher-mode information. The new approach is fully automated and has been successfully used to match approximately 375,000 Rayleigh waveforms. For each seismogram, we obtain a path average shear velocity and quality factor model, and a set of fundamental and higher-mode dispersion and attenuation curves. We incorporate the resulting set of path average shear velocity models into a tomographic inversion. In the uppermost 200 km of the mantle, SV wave heterogeneities correlate with surface tectonics. The high velocity signature of cratons is slightly shallower (≈200 km) than in other seismic models. Thicker continental roots are not required by our data, but can be produced by imposing a priori a smoother model in the vertical direction. Regions deeper than 200 km show no velocity contrasts larger than AE1% at large scale, except for high velocity slabs within the transition zone. Comparisons with other seismic models show that current surface wave datasets allow to build consistent models up to degrees 40 in the upper 200 km of the mantle. The agreement is poorer in the transition zone and confined to low harmonic degrees (≤10).
Geophysical Journal International, 2002
We investigate the correlation of large-scale P-and S-velocity heterogeneity in the mantle by determining how well 106,000 compressional P, PP, PPP, and PKPab traveltimes can be explained by S-wave velocity model S20RTS (scaled using a depth dependent factor) and by a model in which the lateral P-velocity variations are different. We first assess the assumption that P-wave traveltimes can be explained by a model in which lateral P-velocity variations (δv P) are identical to S-velocity variations (δv S) in model S20RTS. For a given depth, we project δv S from S20RTS into model S2P using a depth-dependent scaling factor R defined as: δv S = R(z) × δv P. We find, by grid search, that the highest reduction of data variance is obtained when R increases linearly from 1.25 at the surface to 3.0 at the core-mantle boundary. A comparison of S-wave (+SS) and P-wave (+P P) traveltimes for identical source-receiver pairs also indicates that R increases with depth. Significantly higher variance reduction is not obtained when R is parametrized with an additional degree of freedom. Therefore, the precise shape of R cannot be constrained by our data. P-and PP-wave traveltime anomalies with respect to the scaled model S2P yield coherent geographic variations. This indicates that there are large-scale lateral P-velocity variations in the mantle that are different from those in model S2P. We estimate these variations by inverting P-wave traveltime anomalies with respect to model S2P for a degree 12 model of P-velocity heterogeneity. This model, P12 s2p , indicates where in the well-sampled mantle regions we need to modify model S2P to further improve the fit to the traveltime data. Anomalies in P12 s2p exist throughout the mantle. It is, therefore, not obvious that compositional heterogeneity is prominent in the lower 1000 km of the mantle only, as suggested previously. Low P-wave velocities in the upper mantle beneath oceans are the strongest anomalies in P12 s2p and explain better the delayed traveltimes of PP-wave phases with oceanic surface refection points. Lower mantle anomalies include high and low P-velocity structures beneath eastern Asia and North America, respectively. The high P-velocity anomaly in the lower mantle beneath the central Pacific is consistent with the assertion made by other researchers that large-scale lower mantle upwellings are not purely thermal in origin.
Earth and Planetary Science Letters, 2012
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 300 Myr 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 4000 K, 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 wavespeed 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 anticorrelation 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.