Evidence of high velocity anomalies in the transition zone associated with Southern Kurile Subduction Zone

Tajima, Fumiko; Grand, S. P.

doi:10.1029/95gl03314

Cited by 46 publications

(43 citation statements)

References 9 publications

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“…Published models, however, show variable structures at transition zone depths in subduction regimes, and there is still debate as to whether sub ducting slabs penetrate into the lower mantle [Creager and Jordan, Figure 1). However, P wave travel times from the ISC catalog have poor sampling of the deeper part of the transition zone (...550 km to the 660 km discontinuity) because no first-arriving P waves bottom in that depth range because of the triplication caused by the discontinuity [Tajima and Grand, 1995]. Accordingly, some of the features suggested by the tomographic models are still inconclusive.…”

Section: Introductionmentioning

confidence: 89%

Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu

Tajima¹,

Grand²

1998

J. Geophys. Res.

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Abstract.The seismic structure of the transition zone beneath the northwestern Pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms. By modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most P wave tomography experiments. These secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments. We used just two one-dimensional models in the waveform comparison. The iasp91(slightly modified in the shallower depths as aiasp) is used as a global average structure, and model M3.11 is used as representative of transition zone structure near a deep subducting slab. Model M3.11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper "660" km discontinuity. Our results show strong variations in the transition zone structure beneath the northwestern Pacific. Data that sample beneath the northwestern Japanese subduction zone and the southern Kuriles are well fit by model M3.11. The deep structure beneath the Bohai Sea, to the west of Korea, also appears to show slab-related velocity anomalies in the transition zone. In contrast, the seismic data sampling beneath the Yellow Sea to the East China Sea are better modeled using the global average model iasp91. Data that sampled beneath the Philippine Sea showed evidence for anomalously high velocity in the transition zone, but there is little apparent change in the depth of the 660 km discontinuity there. The overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying fiat or piling up within the transition zone, although we find that the region where this occurs, and thus the total volume of slab within the upper mantle, is considerably less than that seen in past studies. The unusual structure beneath the Philippine plate may be due to the relatively recent descent of slab beneath this region. The slab itself is cold, and thus anomalously fast, but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened.

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Section: Introductionmentioning

confidence: 89%

Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu

Tajima¹,

Grand²

1998

J. Geophys. Res.

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“…Such changes in seismic structure in the transition zone should affect triplicated waveforms at regional distances, but the depth range near the discontinuity can be strongly sampled only by secondary arrivals. Tajima and Grand (1995) thus analyzed regional broadband P waveforms in the frequency Fig. 3.…”

Section: Implication Of Anomalies By Waveform Modelingmentioning

confidence: 99%

Evaluation of slab images in the northwestern Pacific

et al. 2014

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Recent P wave travel-time tomographic studies using data from the International Seismological Centre (ISC) catalog determine a large-scale subhorizontal high velocity anomaly in the northwestern Pacific subduction zones and it has been interpreted as imaging stagnant slab in the upper mantle transition zone (∼400 to 700 km). The limited resolution of the travel time tomographic studies in this depth range, however, makes it difficult to evaluate accurately the vertical and lateral extent of a stagnant slab. A broadband waveform modeling of triplicated regional seismic waves which are very sensitive to the transition zone structure is useful to evaluate the velocity structure along the propagation paths and therefore to constrain the spatial distribution of anomalies. This study thus compares tomographic images from the model of Obayashi et al. (1997) with results of the regional waveform modeling by Tajima and Grand (1998). The ISC tomographic model shows the largest lateral extent of high velocity anomaly in the layer of 478 to 551 km depths although part of this spread is likely due to the deteriorated resolution in that depth range. The waveform modeling suggests that the strong high velocity anomaly associated with a stagnant slab exists below 525 km with its maximum intensity in the top 50 km and decreases with increasing depth to vanish at 660 km. These results along with a recent global SH velocity model SAW12D of Li and Romanowicz (1996) which has the strongest high velocity anomaly in a depth range 500-550 km may be integrated into an image of a stagnant slab. The anomalous velocity structure associated with a stagnant slab has its maximum intensity not immediately above the 660 km discontinuity but in a depth range ∼100 km above it. This feature appears to be consistent with a thermochemical model of down-going slab in which a larger velocity contrast with the surrounding mantle is expected at a shallower depth of the transition zone. The ISC tomographic model and waveform modeling consistently show that the deflected slabs are not laterally continuous but are separated into a few subregions. Beneath the northeastern China where the resolution is good, the slab related anomaly above the 660 km discontinuity is accompanied by its downward extension into the lower mantle.

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“…Analysis of triplicated waveforms has so far been mostly applied to deep events (Tajima and Grand, 1995;Tajima et al, 2009), in order to separate the influence of the discontinuities from the depth phases. We have shown that shallow events can be modelled when source parameters are carefully estimated.…”

Section: Discussionmentioning

confidence: 99%

“…Modelling of individual triplicated waveforms has been successfully used to sample localized heterogeneities in the MTZ (Tajima and Grand, 1995;Melbourne and Helmberger, 2002;Tajima et al, 2009). A few studies that included picked arrival times into ray theoretical inversions demonstrated the potential of triplicated waves for tomography (Grand, 2002), but so far they have rarely been used.…”

Section: S C Stähler Et Al: Finite-frequency Measurements Of Triplmentioning

confidence: 99%

Triplicated P-wave measurements for waveform tomography of the mantle transition zone

2012

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Abstract. Triplicated body waves sample the mantle transition zone more extensively than any other wave type, and interact strongly with the discontinuities at 410 km and 660 km. Since the seismograms bear a strong imprint of these geodynamically interesting features, it is highly desirable to invert them for structure of the transition zone. This has rarely been attempted, due to a mismatch between the complex and band-limited data and the (ray-theoretical) modelling methods. Here we present a data processing and modelling strategy to harness such broadband seismograms for finitefrequency tomography. We include triplicated P-waves (epicentral distance range between 14 and 30 • ) across their entire broadband frequency range, for both deep and shallow sources. We show that is it possible to predict the complex sequence of arrivals in these seismograms, but only after a careful effort to estimate source time functions and other source parameters from data, variables that strongly influence the waveforms. Modelled and observed waveforms then yield decent cross-correlation fits, from which we measure finite-frequency traveltime anomalies. We discuss two such data sets, for North America and Europe, and conclude that their signal quality and azimuthal coverage should be adequate for tomographic inversion. In order to compute sensitivity kernels at the pertinent high body wave frequencies, we use fully numerical forward modelling of the seismic wavefield through a spherically symmetric Earth.

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Evidence of high velocity anomalies in the transition zone associated with Southern Kurile Subduction Zone

Cited by 46 publications

References 9 publications

Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu

Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu

Evaluation of slab images in the northwestern Pacific

Triplicated P-wave measurements for waveform tomography of the mantle transition zone

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