Approximate 3D Body-Wave Synthetics for Tomographic Models

Helmberger, Donald V.; Ni, Sidao

doi:10.1785/0120040004

Cited by 23 publications

(24 citation statements)

References 34 publications

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“…Again, the complexity in waveforms seems to be caused by the interference of two arrivals, indicated by the two thick lines, indicative of multipathing. For this event it is quite clear that the first arrivals denoted by the first thick line diminish in amplitude for larger azimuths (from 214° to 216°), which is typical of diffraction effects (Helmberger & Ni 2005; Ni et al 2002).…”

Section: Data and Analysismentioning

confidence: 88%

“…Fortunately, advances in computational and theoretical seismology have facilitated fully 3‐D waveform modelling (Komatitsch et al 2002). Additionally, ray‐based techniques involving a combination of analytical approximations (DWKM) lead to much faster but less accurate 3‐D algorithms (Helmberger & Ni 2005). Such approximate algorithms provide intuition about how seismic waveforms behave for different structures, and the added speed makes it possible to investigate a wide variety of structures before performing a more costly spectral‐element calculation.…”

Section: Introductionmentioning

confidence: 99%

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Three-dimensional structure of the African superplume from waveform modelling

Helmberger

Tromp

2005

Geophysical Journal International

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S U M M A R YPrevious 2-D modelling of seismic waveforms and traveltimes has revealed a large-scale ridge-like velocity anomaly beneath Africa, which is usually referred to as the African superplume. The structure starts from the southern Indian Ocean and extends northwestwards into the Atlantic Ocean, with its base on the core-mantle boundary. The structure has relatively sharp lateral boundaries with the shear velocity inside 3 per cent lower than the ambient mantle, while the compressional velocity is almost normal. The 3-D structure is best illustrated by seismic waveforms recorded by the South Africa Array generated by earthquakes in the western Pacific Ocean. The diffracted S phase travels along the axis of the structure for over 60 • , with the northern-and southernmost paths sampling its edges. The S waveforms are simple but delayed by up to 20 s for paths sampling the middle of the structure, whereas they display two arrivals for paths along the boundaries. These complex waveforms can be explained by 3-D multipathing due to rapid lateral variations in shear-wave velocity along the edges of the structure. These sharp features are confirmed by modelling broadband records associated with the proposed ridge structure with two independent methods: the spectral element method and a ray-based 3-D code (DWKM).

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Section: Data and Analysismentioning

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Three-dimensional structure of the African superplume from waveform modelling

Helmberger

Tromp

2005

Geophysical Journal International

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“…These synthetics were generated with a new code called DWKM [Heimberger and Ni, 2004]. The method uses WKM for generation of2D synthetics and adds out-of-the great-circle-plane arrivals by applying a diffraction operator.…”

Section: Structural Definition Of the Eastern Provincementioning

confidence: 99%

Seismic modeling constraints on the South African super plume

Helmberger¹,

Ni²

2005

Earth's Deep Mantle: Structure, Composition, and Evolution

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Tomographic studies of the structure of the lower mantle beneath South Africa reveal large-scale low velocities above the core-mantle boundary. Predicted SKS delay patterns (up to 3 s) for some of these models fit observations (Kaapvaal Array data) quite well except for magnitude level, explaining less than one-halfthe observed anomaly. Moreover, the sharpness in travel-time offsets and waveform complications require that nearly vertical walls separate the anomalous structure from the normal preliminary reference Earth model (PREM) mantle. We present numerous record sections along with 2D and 3D synthetics displaying multipathing of arrivals (Sd' SKS, SKKS, S, and ScS), based on a large-scale 3D structure. This kidney-shaped structure has one apex beneath the Indian Ocean (Kerguelen) and the other extending beneath the Mid-Atlantic (Cape Verde). The structure is about 1200 km wide beneath South Africa and extends upward to at least 1000 km through the lower mantle, similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM. We have not found any evidences for ultra-low-velocity zones (ULVZ) beneath the main structure but ample evidence at some locations near the edges. We also analyzed Pd and the differentials between PcP travel times and P travel times (PcP-P) along the same great circle paths from the same events. The P-velocity is not very anomalous, perhaps -0.5%. The sharpness of the lateral boundaries (walls) and the large contrast in P and S velocities can be used in arguments for a thermochemical origin.

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“…(C) displays the 2D and 3D synthetics along profile AA' in Figure 4A. 3D synthetics are generated by DWKM method (Helmberger and Ni, 2005), which approximates 3D wave propagation by summing the contribution from the Lit and the Diffraction zones. Here we only considered two Lit zones.…”

mentioning

confidence: 99%