We made a region-by-region examination of subducted slab images along the circum-Pacific for some of the recent global mantle tomographic models, specifically for two high-resolution P velocity models and two long-wavelength S velocity models. We extracted the slab images that are most consistent among different models. We found that subducted slabs tend to be subhorizontally deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition region (400–1000 km). The deflected or flattened slabs reside at different depths, either above or across the 660-km discontinuity as in Chile Andes, Aleutian, Southern Kurile, Japan, and Izu-Bonin; slightly below the discontinuity as in Northern Kurile, Mariana, and Philippine; or well below it as in Peru Andes, Java, and Tonga-Kermadec. There is little indication for most of these slabs to continue “directly” to greater depths well beyond the transition region. Mantle downflow associated with present slab subduction appears to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of lithospheric slabs deeply sinking through the lower mantle, typically the presumed Farallon slab beneath North and Central America and the presumed Indian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant slabs are not connected to the surface plates or to the presently subducting slabs and appear to sink independently from the latter. The presence of these deeply sinking slabs implies that the pre-Eocene subduction occurred in much the same way as in the present day to accumulate slab bodies in the transition region and that the consequent unstable downflow occurred extensively through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the reorganization of global plate motion.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RG000068

Stagnant slabs in the upper and lower mantle transition region

[1] Statistical analysis of modern oceanic subduction zone parameters, such as the age of a downgoing plate or the absolute plate motions, is performed in order to investigate which parameter controls the dip of a slab and, conversely, what the influence of slab geometry is on upper plate behavior. For that purpose, parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge/plateau/seamount subduction. On the basis of tomographic images, slabs that penetrate through, or lie on, the 670 km discontinuity are also identified. The results of the statistical analysis are as follows: (1) Back-arc stress correlates with slab dip, i.e., back-arc spreading is observed for deep dips (deeper than 125 km) larger than 50°, whereas back-arc shortening occurs only for deep dips less than 30°. (2) Slab dip correlates with absolute motion of the overriding plate. The correlation is even better when the slab lies on, or even more penetrates through, the 670 km discontinuity. (3) Slabs dip more steeply, by about 20° on average, beneath oceanic overriding plates than beneath continental ones. (4) Slabs dip more steeply on average by about 10° near edges. (5) Slab dip does not correlate with the magnitude of slab pull, the age of subducting lithosphere at the trench, the thermal regime of the subducting lithosphere, the convergence rate, or the subduction polarity (east versus west). The present study provides evidence that the upper plate absolute motion plays an important role on slab dip, as well as on upper plate strain. Retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates. One can infer that low slab dips correlate well with compression in continental advancing upper plates, whereas steep dips are often associated with extension in oceanic retreating upper plates. Excess weight of old slabs is often counterbalanced by other forces, probably asthenospheric in origin, such as lateral mantle flow near slab edges or anchor forces, to determine slab dip.

/pdf/on-the-relationships-between-slab-dip-back-arc-stress-upper-38uoq1v7uj.pdf

On the relationships between slab dip, back‐arc stress, upper plate absolute motion, and crustal nature in subduction zones

SUMMARY The reversible jump algorithm is a statistical method for Bayesian inference with a variable number of unknowns. Here, we apply this method to the seismic tomography problem. The approach lets us consider the issue of model parametrization (i.e. the way of discretizing the velocity field) as part of the inversion process. The model is parametrized using Voronoi cells with mobile geometry and number. The size, position and shape of the cells defining the velocity model are directly determined by the data. The inverse problem is tackled within a Bayesian framework and explicit regularization of model parameters is not required. The mobile position and number of cells means that global damping procedures, controlled by an optimal regularization parameter, are avoided. Many velocity models with variable numbers of cells are generated via a transdimensional Markov chain and information is extracted from the ensemble as a whole. As an aid to interpretation we visualize the expected earth model that is obtained via Monte Carlo integration in a straightforward manner. The procedure is particularly adept at imaging rapid changes or discontinuities in wave speed. While each velocity model in the final ensemble consists of many discontinuities at cell boundaries, these are smoothed out in the averaged ensemble solution while those required by the data are reinforced. The ensemble of models can also be used to produce uncertainty estimates and experiments with synthetic data suggest that they represent actual uncertainty surprisingly well. We use the fast marching method in order to iteratively update the ray geometry and account for the non-linearity of the problem. The method is tested here with synthetic data in a 2-D application and compared with a subspace method that is a more standard matrix-based inversion scheme. Preliminary results illustrate the advantages of the reversible jump algorithm. A real data example is also shown where a tomographic image of Rayleigh wave group velocity for the Australian continent is constructed together with uncertainty estimates.

/pdf/seismic-tomography-with-the-reversible-jump-algorithm-4z1h3zxpme.pdf

Seismic tomography with the reversible jump algorithm

SUMMARY The fast marching method (FMM) is a grid based numerical scheme for tracking the evolution of monotonically advancing interfaces via finite-difference solution of the eikonal equation. Like many other grid based techniques, FMM is only capable of finding the first-arriving phase in continuous media; however, it distinguishes itself by combining both unconditional stability and rapid computation, making it a truly practical scheme for velocity fields of arbitrary complexity. The aim of this paper is to investigate the potential of FMM for finding later arriving phases in layered media. In particular, we focus on reflections from smooth subhorizontal interfaces that separate regions of continuous velocity variation. The method we adopt for calculating reflected phases involves two stages: the first stage initializes FMM at the source and tracks the incident wave front to all points on the reflector surface; the second stage tracks the reflected wave front by reinitializing FMM from the interface point with minimum traveltime. Layer velocities are described by a regular grid of velocity nodes and layer boundaries are described by a set of interface nodes that may be irregularly distributed. A triangulation routine is used to locally suture interface nodes to neighbouring velocity nodes in order to facilitate the tracking of wave fronts to and from the reflector. A number of synthetic tests are carried out to assess the accuracy, speed and robustness of the new scheme. These include comparisons with analytic solutions and with solutions obtained from a shooting method of ray tracing. The convergence of traveltimes as grid spacing is reduced is also examined. Results from these tests indicate that wave fronts can be accurately tracked with minimal computational effort, even in the presence of complex velocity fields and layer boundaries with high curvature. Incident wave fronts containing gradient discontinuities or shocks also pose no difficulty. Further development of the wave front reinitialization scheme should allow other later arrivals such as multiples to be successfully located.

/pdf/wave-front-evolution-in-strongly-heterogeneous-layered-media-2e8304e9mq.pdf

Wave front evolution in strongly heterogeneous layered media using the fast marching method

A stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above, across, or below the 660 km discontinuity. This phenomenon has now been widely recognized beneath subduction zones around the circum-Pacific and in the Mediterranean. Collaboration of seismic and electromagnetic observations, mineral physics measurements, and geodynamic modeling has begun to provide a consistent picture of stagnant slab.

https://ir.lib.hiroshima-u.ac.jp/files/public/2/28632/20141016164917394655/AnnuRevEarthPlanetSci_37_19.pdf

Stagnant slab : A review

SUMMARY A 3-D ray-tracing technique was used in a global tomographic inversion in order to obtaintomographicimagesoftheNorthPacific.ThedatareportedbytheGeophysical SurveyofRussia(1955‐1997)wereusedtogetherwiththecataloguesoftheInternational SeismologicalCenter(1964‐1991)andtheUSGeologicalSurveyNationalEarthquake Information Center (1991‐1998), and the recompiled catalogue was reprocessed. The final data set, used for following the inversion, contained 523430 summary ray paths. The whole of the Earth’s mantle was parametrized by cells of 2°◊2° and 19 layers. The large and sparse system of observation equations was solved using an iterative LSQR algorithm. Asubhorizontalhigh-velocityanomalyisrevealedjustabovethe660kmdiscontinuity beneaththeAleutiansubductionzone.Thishigh-velocityfeatureisobservedatlatitudes of up to~70°N and is interpreted as a remnant of the subducted Kula plate, which disappeared through ridge subduction at about 48Ma. A further positive velocity perturbation feature can be identified beneath the Chukotka peninsula and Okhotsk Sea, extending from~300 to~660km depth and then either extending further down to~800km (Chukotka) or deflecting along the 660km discontinuity (Okhotsk Sea). This high-velocity anomaly is interpreted as a remnant slab of the Okhotsk plate accreted to Siberia at~55Ma.

/pdf/signature-of-remnant-slabs-in-the-north-pacific-from-p-wave-49x0qwg42o.pdf

Signature of remnant slabs in the North Pacific from P‐wave tomography

SUMMARY A non-linear iterative P-wave traveltime tomography has revealed a mantle plume originating at a depth of nearly 1000 km, rising across the 600 km discontinuity, and deflecting subhorizontally in the uppermost mantle presumably by shear flow due to the overlying moving plate. Data from the Geophysical Survey of Russia (1955–1997) were inverted jointly with the catalogues of International Seismological Centre and USGS National Earthquake Information Centre (1964– 1998). The result shows a 300–500 kmwide cylindrical low-velocity anomaly (yx2 per cent) that extends from a depth of greater than 900 km to shallower than 200 km. The anomaly is almost vertical at depths up to y400 km and rises obliquely to the north up to y200 km under the ocean floor near the northern end of Emperor seamounts. Above y300 km depth a subsidiary anomaly extends subhorizontally to the NW in fair agreement with the direction of movement of the Pacific Plate. The overlying seafloor is characterized by anomalously high heat flow, which may be attributed to the thermal effect of the mantle plume.

/pdf/seismic-evidence-for-a-mantle-plume-oceanwards-of-the-3fdjsa8t2f.pdf

E. Gordeev

Papers

Signature of remnant slabs in the North Pacific from P‐wave tomography

Seismic evidence for a mantle plume oceanwards of the Kamchatka–Aleutian trench junction