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Earth, Planets and Space

, Volume 50, Issue 11–12, pp 953–964

First Online: 06 June 2014Received: 21 July 1998Revised: 11 September 1998Accepted: 06 October 1998


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 thermo-chemical 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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Author: Fumiko Tajima - Yoshio Fukao - Masayuki Obayashi - Taro Sakurai

Source: https://link.springer.com/

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