Constructing synthetics from deep earth tomographic models

Ni, Sidao; Ding, Xiaoming; Helmberger, Donald V.

doi:10.1046/j.1365-246x.2000.00982.x

Cited by 40 publications

(35 citation statements)

References 29 publications

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“…Synthetics generated for tomographic models such as those presented in Fig. 1 do not produce significant Scd arrivals unless the velocity gradients and contrasts are enhanced (21). The profile of seismograms labeled 2D in Fig.…”

Section: Circum-pacific High-velocity Provincesmentioning

confidence: 97%

Deep mantle structure and the postperovskite phase transition

Helmberger

Lay

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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Seismologists have known for many years that the lowermost mantle of the Earth is complex. Models based on observed seismic phases sampling this region include relatively sharp horizontal discontinuities with strong zones of anisotropy, nearly vertical contrasts in structure, and small pockets of ultralow velocity zones (ULVZs). This diversity of structures is beginning to be understood in terms of geodynamics and mineral physics, with dense partial melts causing the ULVZs and a postperovskite solid-solid phase transition producing regional layering, with the possibility of large-scale variations in chemistry. This strong heterogeneity has significant implications on heat transport out of core, the evolution of the magnetic field, and magnetic field polarity reversals.core-mantle boundary ͉ ultralow velocity zone

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Section: Circum-pacific High-velocity Provincesmentioning

confidence: 97%

Deep mantle structure and the postperovskite phase transition

Helmberger

Lay

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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“…1b) with the HBMS structure (Fig. 3a), while assuming the same geometry and then generate 2D synthetics (15). Then, we compare the predicted synthetic seismograms with the data.…”

Section: Predicting Seismogramsmentioning

confidence: 99%

Seismological support for the metastable superplume model, sharp features, and phase changes within the lower mantle

Sun

Tan

Helmberger

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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Recently, a metastable thermal-chemical convection model was proposed to explain the African Superplume. Its bulk tabular shape remains relatively stable while its interior undergoes significant stirring with low-velocity conduits along its edges and downwelling near the middle. Here, we perform a mapping of chemistry and temperature into P and S velocity variations and replace a seismically derived structure with this hybrid model. Synthetic seismogram sections generated for this 2D model are then compared directly with corresponding seismic observations of P (P, P CP, and PKP) and S (S, S CS, and SKS) phases. These results explain the anticorrelation between the bulk velocity and shear velocity and the sharpness and level of SKS travel time delays. In addition, we present evidence for the existence of a D'' triplication (a putative phase change) beneath the down-welling structure.core-mantle boundary ͉ DЉ T he large-scale structure of the lower mantle has been well resolved by global tomography, with a belt of high seismic velocity along the circum-Pacific and two large low velocity provinces (LLVPs) beneath South Africa and the mid-Pacific. The fastest regions appear to contain a sharp positive velocity jump associated with a phase-change from perovskite (PV) to postperovskite (PPV) (1), whereas the slowest regions contain a ␦V S /␦V P ratio Ͼ2.5 and an anticorrelated bulk sound velocity V and shear velocity V S (2, 3). Although both LLVPs show these properties, their interior structures appear to differ, with the Pacific anomaly showing more complexity compared with the apparently monolithic African anomaly (4, 5). Tomographic studies of the African structure reveal a large-scale feature that extends throughout the lower mantle. Predicted SKS delay patterns up to 3 s for some of these tomographic models fit the observations at the South African seismic array well except for magnitude and sharpness (Fig. 1), where the data require Ͼ6-s offsets (5, 6). Note that the SKS ray paths cross the core-mantle boundary (CMB) interface at relatively steep angles and their abrupt change in delays require nearly vertical walls to separate the normal Preliminary Reference Earth Model (PREM) from the anomalous structure denoted by the heavy green lines in Fig. 1b (reviewed in ref. 5). Such a structure with its sharp sides is suggestive of thermo-chemical convection containing a density increase (7). Metastable SuperplumeThe fate of a dense chemical basal layer in a convecting mantle has a well developed history. The results from Christensen (8) and McNamara and Zhong (9), involving dense piles, look similar to the LLVPs in tomographic locations and appear compatible with the history of subduction. Stabilized by an intrinsically larger density (⌬ ch ), the pile will remain at the CMB until exceeded by a thermal density with opposite sign (⌬ th ). However, if there is a difference in compressibility between the material within the plume compared with ambient mantle, then metastable conditions are possible. Tan and Gurnis (10...

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“…A drawback to the hybrid approach is the considerable effort in smoothing and interfacing grids, which must be customized and tested for each specific structure. An advantage of the WKM method used here is that a relatively automatic and systematic parameterization can be applied to existing to- mographic-type models (Ni et al, 2000). The original theory in one dimension is based on Chapman (1976), which derives an approximate solution for a layered model containing ray parameters, p i , and travel times, t i , reflecting off each interface.…”

Section: Wkmmentioning

confidence: 99%

“…Note that they become shorter by moving toward the right, with the extra paths accommodated on the faster velocities on the left side (not shown). The process could be repeated for higher resolution, but typically the correction is small (Ni et al, 2000). The travel-time difference between paths bracketing the boundary has distinctly different travel times and, thus, a jump in the (p ‫מ‬ t) curve (Fig.…”

Section: Wkmmentioning

confidence: 99%

“…The pseudospectral modeling parameters were identical to those used for the SV profile, but for a transverse vector point force observed on the transverse component of motion. The WKM synthetics were generated with a Cagniard-deHoop version of the code, which treats diffracted S more accurately (Ni et al, 2000). Following this approach, we use the ray parameters derived for each reflecting interface p i and define the travel time t i as…”

Section: Shmentioning

confidence: 99%

See 1 more Smart Citation

A Comparison of Synthetic Seismograms for 2D Structures: Semianalytical versus Numerical

Ni¹

2003

Bulletin of the Seismological Society of America

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Teleseismic wave fields occasionally exhibit rapid changes in travel times and waveforms over distances less than several great-circle degrees when observed at broadband arrays. These rapid changes in wave field suggest the existence of significant structural transitions occurring over scales of several hundred kilometers or less in the mid-and deep mantle. Although approximate analytical methods based on raytracing can be readily adapted to structures having arbitrarily small scale lengths, it is important to validate their accuracy against the predictions of numerical methods. Here we compare synthetics from an approximate ray-based method WKBJ modified (WKM) against the pseudospectral method for a 2D model of the S-velocity anomaly associated with the South African plume. This model consists of a uniform 3% decrease in S velocity over a broad (Ͼ10Њ) region of the mid-and deep mantle beneath South Africa, contiguous at its bottom with a thin (100-to 200-km-thick) zone of low velocity extending 30Њ westward toward South America along the coremantle boundary. Transitions between anomalous and radially symmetric structures of the test model are sharp, occurring over l0 km or less. SV and SH wave fields synthesized by the WKM and pseudospectral methods in this model generally agree with each other well. Slight mismatches in the two methods can be understood as the result of either differences in model parameterization or the effects of asymptotic approximations in the ray-based WKM method.

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Constructing synthetics from deep earth tomographic models

Cited by 40 publications

References 29 publications

Deep mantle structure and the postperovskite phase transition

Deep mantle structure and the postperovskite phase transition

Seismological support for the metastable superplume model, sharp features, and phase changes within the lower mantle

A Comparison of Synthetic Seismograms for 2D Structures: Semianalytical versus Numerical

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