Evaluation of slab images in the northwestern Pacific

Recent P wave travel-time tomographic studies using data from the Internati onal Seismological Centre (ISC) catalog determine a large-scale subhorizont al high velocity anomaly in the northwestern Pacific subduction zones and i t has been interpreted as imaging stagnant slab in the upper mantle transit ion zone (similar to 400 to 700 km). The limited resolution of the travel t ime 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 t he spatial distribution of anomalies. This study thus compares tomographic images from the model of Obayashi et al. (1997) with results of the regiona l waveform modeling by Tajima and Grand (1998). The ISC tomographic model s hows the largest lateral extent of high velocity anomaly in the layer of 47 8 to 551 km depths although part of this spread is likely due to the deteri orated resolution in that depth range. The waveform modeling suggests that the strong high velocity anomaly associated with a stagnant slab exists bel ow 525 km with its maximum intensity in the top 50 km and decreases with in creasing depth to vanish at 660 km. These results along with a recent globa l SH velocity model SAW12D of Li and Romanowicz (1996) which has the strong est high velocity anomaly in a depth range 500-550 km may be integrated int o an image of a stagnant slab. The anomalous velocity structure associated with a stagnant slab has its maximum intensity not immediately above the 66 0 km discontinuity but in a depth range similar to 100 km above it. This fe ature appears to be consistent with a thermochemical model of down-going sl ab in which a larger velocity contrast with the surrounding mantle is expec ted at a shallower depth of the transition zone. The ISC tomographic model and waveform modeling consistently show that the deflected slabs are not la terally continuous but are separated into a few subregions. Beneath the nor theastern China where the resolution is good, the slab related anomaly abov e the 660 km discontinuity is accompanied by its downward extension into th e lower mantle.