Several well-known tomographic studies published in the 1990's (van der Hilst et al., 1991; Fukao et al., 1992) captured a large-scale subhorizontal high velocity anomaly at the bottom of the upper mantle in the northwestern Pacific. These models suggest that a large volume of subducted slab is stagnant in the transition zone. To support the hypothesis, there are also some observational studies for depression of the 660 km discontinuity due to the effects of stagnant cold slab in the same region (Shearer and Masters, 1992; Tajima and Grand, 1995).
Using broadband waveform modeling P wave speed model M3.11 was derived for the transition zone associated with the southern Kurile subduction zone where a large volume of stagnant slab may exist. This model is characterized by a high speed anomaly (relative to a standard model iasp91) below 525 km with its maximum intensity not immediately above the 660 km discontinuity but in a depth range 100 km above it. On the other hand the transition zone structure beneath the northern Philippine Sea Plate is represented by model M2.0 which has high speed anomaly in the deeper part of the transition zone like M3.11 but with little apparent change in the depth of the 660 km discontinuity (see the agreement between the observed and synthetic waveforms calculated with the optimal models in Figure 25.1). The lack of depression of the discontinuity depth may suggest that the temperature beneath the flattened slab in this area is normal.
An interesting finding is that whether the discontinuity depth is depressed or not, the P wave speed at the phase transition depth is about the same. Models M3.11 and M2.0 in the depth range from 525 to 660 km are remarkably similar to the P wave speed model derived from bulk moduli measured under high pressure and temperature using a pyrolite composition (Li et al., 1998). Both of the seismic models and the pyrolite model have smaller gradient of P wave speed in the transition zone than iasp91 and converge to the standard model right above the "660" discontinuity.
The P wave travel-time tomographic images (Obayashi et al., 1997) were examined using waveform modeling which incorporate the travel-time data of secondary waves. Here note that most P wave tomography experiments do not use travel-time data of secondary waves. Those secondary waves are highly sensitive to the velocity structure 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. Our results show strong variations in the transition zone structure beneath the northwestern Pacific. The overall results are in agreement with past work indicating fairly broad regions where the subducting plate is lying flat 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 (Tajima and Grand, 1998). Results also suggest that the anomalous structure associated with a stagnant slab has its maximum intensity not immediately above the 660 km discontinuity but in the depth range 100 km above it (Tajima et al., 1998) (see the cross projection profiles of the tomographic images by Obayashi et al. (1997) and the overlain seismic rays for which M3.11 is a suitable model in Figure 25.2).
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