Evaluation of slab images in the northwestern Pacific

Abstract

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.

The three-dimensional shear velocity structure of the mantle from the inversion of body, surface and higher-mode waveforms

“…The anomaly penetrates into the lower mantle at its eastern end. The amplitude of this feature is weak and may not be well resolved but it agrees qualitatively with the results of Tajima et al (1998).…”

Determination of the absolute depths of the mantle transition zone discontinuities beneath China: Effect of stagnant slabs on transition zone discontinuities

“…Our results therefore indicate that the mantle transition zone beneath BJI is colder than average, suggesting that the sub- ducted Pacific plate exists east of Beijing in the Bohai sea region. Independent evidence that subducted Pacific plate exists in the same region is presented by the recent waveform study of Tajima et al (1998).…”

“…The HVAs, which extend several thousands of kilometers, are usually attributed to the stagnant slabs originating at the western Pacific subduction zones. Recent waveform studies by Tajima et al (1998), however, suggests that the real lateral scale of HVAs may be smaller. The HVAs are generally believed to be colder than the surrounding mantle, and the '660-km' discontinuity is anticipated to be depressed by tens of kilometers within the cold interior of the HVAs.…”

Whole mantle P wave tomography using P and PP‐P data

“…The theoretical PP‐P times are calculated for a spherically averaged version of WEPP2. WEPP2 is a 3‐D mantle P velocity model derived from 2 million the first arrival times reported to the ISC [ Obayashi et al , 1997; Tajima et al , 1998; Fukao et al , 2001]. Plotted are 3° radius cap smoothings of Δ T vel and Δ T dis on the bounce points.…”