Global mapping of topography on transition zone velocity discontinuities by stacking <i>SS</i> precursors

Flanagan, Megan P.; Shearer, Peter M.

doi:10.1029/97jb03212

Cited by 429 publications

(517 citation statements)

References 72 publications

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“…As the Fresnel zones on the for vertically propagating ScS discontinuity phases are intermediate in size between these other estimates, this may suggest the SS precursor studies average topography over large regions and thus underestimate the topography in the immediate vicinity of slabs. We also observe 10-15 km shallower overall depths to the 660 than Flanagan and Shearer, [1998]. This is due to our use of a slow upper mantle velocity model.…”

Section: Scss Discontinuity Depths and Impedenee Contrasts Are Determentioning

confidence: 60%

“…We compute the width of the transition zone by comparing 410 and 660 elevations for closely spaced bounce points and find the largest values concentrated near the slab (Fig. 5b) The amount of 660 peak-to-peak topography in the Tonga region determined in this study is greater than determined from long period teleseismic SS precursors [Flanagan and Shearer, 1998], but less than observed from high frequency convened body waves [Niu and Kawakatsu, 1995]. As the Fresnel zones on the for vertically propagating ScS discontinuity phases are intermediate in size between these other estimates, this may suggest the SS precursor studies average topography over large regions and thus underestimate the topography in the immediate vicinity of slabs.…”

Section: Scss Discontinuity Depths and Impedenee Contrasts Are Determentioning

confidence: 67%

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Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations

Roth¹,

Wiens²

1999

Geophysical Research Letters

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Section: Scss Discontinuity Depths and Impedenee Contrasts Are Determentioning

confidence: 60%

Section: Scss Discontinuity Depths and Impedenee Contrasts Are Determentioning

confidence: 67%

Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations

Roth¹,

Wiens²

1999

Geophysical Research Letters

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“…Chaljub and Tarantola [1997] show that 660 uplift is faithfully estimated at length scales >1500 km and that 660 depression is estimated at length scales >3000 km. Thus, for estimating discontinuity depths worldwide or in broad regions, PP and SS precursors are ideal on account of the broad coverage they afford and their spatial averaging charac-teristics [Shearer, 1991;G6ssler and Kind, 1996;Flanagan and Shearer, 1998;Gu et al, 1998]. …”

Section: Methods and Limitationsmentioning

confidence: 99%

“…Short-period characterization of the 520 suggests it is diffuse, variously 10-30 km to 50 km thick, with a 1% or less velocity contrast [Cummins et al, 1992;Jones et al, 1992;Bostock, 1996]. Where the 520 emerges is in long-period studies of underside discontinuity reflections using SdS or PdP [Shearer, 1990[Shearer, , 1991Flanagan and Shearer, 1998]. With a large, --•5.1 MPa K -1 seismic Clapeyron slope (Table 1), its position will be the most sensitive among (R1)-(R3) to any background thermal variation in the mantle.…”

Section: The 520 Visibilitymentioning

confidence: 99%

Topography of the transition zone seismic discontinuities

Helffrich¹

2000

Reviews of Geophysics

257

192

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Abstract.Phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities, nominally at 410, 520, and 660 km depth. , 1923]. This paper focuses on the mantle discontinuities in the transition zone, the region between about 400 and 800 km depth. Whereas in seismology's nascency new discontinuity discoveries progressively refined our basic knowledge of Earth structure, in its maturity the discontinuities themselves become investigative tools rather than investigative targets. The goal of this review is to show how changes in discontinuity depth have been and can be used to investigate the mantle's properties. The argument develops along two lines. The first establishes what the major discontinuities at 410, 520, and 660 km depth are. Significantly, the topography on the discontinuities gives strong evidence to discriminate among different ideas about how they arise. The second strand describes the sort of scientific craftsmanship that can be achieved using the spatial variation of discontinuity depths as a tool, a tool that turns out to be surprisingly versatile, under continual refinement, and increasingly common in a research seismologist's tool kit. Detecting DiscontinuitiesSeismic discontinuities are detected by the additional arrivals they cause in the seismic wave field by reflection and refraction of the direct wave at the discontinuity. Reflection typically leads to an earlier arrival than would be expected in a uniform medium. Its timing relative to the direct arrival yields the discontinuity's depth. A refraction usually leads to more than one arrival, where only one would be expected in a uniform medium. In this case, one deduces the discontinuity depth from the distance from the seismic source where the multiple arrivals appear. The Earth's major discontinuities were initially discovered this way, and others continue to be discovered like this [Oldham, 1906;Lehmann, 1936;Song and Helmberger, 1998].There are quite diverse methods for measuring discontinuity depths, probably best described through sketches (Figure 2) (see Shearer [1991] for more exotic ones). One way to characterize the methods is by the position where the seismic wave interacts with the discontinuity: near the source, near the receiver, or elsewhere. Near-source studies provide the best spatial resolution because there is little opportunity for the seismic wave field to spread out before interacting with the discontinuity (its Fresnel zone width is small). Spatial resolution for near-receiver studies is worse since the interaction is at least 410 km from the receiver. Interac-

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“…The amplitude of boundary topography in the upper mantle has differed by as much as 50% in recent models [Flanagan and Shearer, 1998;Gu et al, 1998], and different assumptions about mantle viscosity in dynamic models can imply widely varying models of any kind of dynamic topography [e.g., Keiffer and Kellogg, 1998]. Where structural heterogeneity is dominantly thermal in origin, vp and ρ should be highly correlated with vs, while, in the presence of chemical heterogeneity, either vp or ρ or both may be largely decorrelated from vs. A blend of chemical and thermal heterogeneity is expected in the uppermost mantle, but may also be important in the transition zone and in the lowermost mantle [Kellogg et al, 1999].…”

Section: Regularization: Sampling Plausible Priorsmentioning

confidence: 99%

Regularization uncertainty in density models estimated from normal mode data

Resovsky¹,

Ritzwoller²

1999

Geophysical Research Letters

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Abstract. Normal mode structure coefficients provide important constraints on the long-wavelength component of 3-D mantle density (ρ) structure, but inversions for independent models of vs, vp, and ρ using normal mode data alone are ill-posed even at long wavelengths. Ill-posed inversions typically are regularized by imposing a priori assumptions on the set of estimated models, but such regularization can introduce important uncertainties in the models. We characterize these uncertainties for ρ models estimated from current normal mode data using a set of 512 different "regularization schemes". These schemes sample a variety of plausible a priori assumptions about the nature and distribution of mantle heterogeneity by specifying allowable vs, vp, ρ, and boundary topography structures. The estimated ρ models are fairly robust with respect to prior constraints on vs, vp, and topography. However, the character and amplitude of the estimated ρ models depend strongly on how ρ is allowed to decorrelate from vs, and we display several models in which ρ and vs decorrelate in very different depth intervals. Because these models all result from plausible prior constraints and fit the data equally and acceptably well, inversions of current normal mode data cannot robustly locate the decorrelation of ρ from vs. It remains possible that reliable ρ models may be obtained in the future as more normal mode measurements are introduced to break the strong tradeoffs between upper and lower mantle ρ structures that characterize current inversions.

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Global mapping of topography on transition zone velocity discontinuities by stacking SS precursors

Cited by 429 publications

References 72 publications

Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations

Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations

Topography of the transition zone seismic discontinuities

Regularization uncertainty in density models estimated from normal mode data

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