A little more than a decade ago, a whole mantle convection or a layered convection was still a matter of debate. Mineral physics predicted an eventual whole mantle convection, but seismic modeling was not able to constrain such a feature even using similar data sets. In the 1990's seismic tomography models captured a large-scale subhorizontal high velocity anomaly that suggested a large volume of stagnant cold slab as well as down going slab images into lower mantle at the bottom of the upper mantle. While such tomographic approaches are robust for modeling the Earth's deep structure, the gap between the conjecture based on large scale tomographic images and the reality of mantle property measurements in mineral physics is enormous. In an effort to reduce such a gap we have carried out broadband waveform modeling using regional body waveform data with support from NSF (EAR-9526678, EAR-9805006, EAR-9996301). The seismic models developed have shown some correspondences to mineral physics experimental results or modeling [Tajima, 2000].
We have shown various features associated with flattened cold slab in the back arc of southern Kuriles, central Japan and Izu-Bonin subduction zones. The velocity models M3.11 and M2.0 developed for the structure with stagnant slab in the transition zone are characterized by high velocity anomaly in the deeper part of the transition zone (525 to 660 km) with its maximum intensity in a depth range 100 km above the 660 km discontinuity. The volume of stagnant slab estimated from the waveform modeling is much smaller than that in the tomographic images, and comparable to the subducted slab in the past 17 Ma. There is variation of the discontinuity depth in the range between 660 and 690 km from the Kuriles to Izu-Bonin subduction zones where the subduction and back arc opening history is more or less similar to each other in the past 17 Ma. Whether the discontinuity depth is depressed or not suggests the temperature condition right beneath the flattened slab, i.e., the temperature may be normal possibly due to the higher thermal conductivity at the bottom of the flattened slab, or the supply of cold slab over the flattened slab is less in the northern Philippine Sea than in the Kuriles. While the subducting slabs in the southern Kuril-Japan-Izu Bonin arcs are bent to subhorizontal above the "660" km discontinuity, the slab-like high velocity zone penetrates this discontinuity with considerable spreading in the Java arc. We are also intrigued by the image of high velocity anomalies that extend into lower mantle from a stagnant zone beneath the transition zone from place to place. This image may indicate that a stagnant slab eventually descends into lower mantle and is favorable for a mantle-wide convective model.
If so, what controls the slab behavior for stagnation and how much of the subducted slab can stagnate in the transition zone? These issues are being explored. Tajima et al.  identified early high-frequency phases which are multiple arrivals of P waves at some stations on the Eurasian continent and the surrounding regions from deep focus events in the Java Sea to Flores Sea depending on the relative locations of the ray paths to the high velocity zone beneath Kalimantan (formerly Borneo) (see event and station locations Fig. 32.1).
Figure 32.2 shows an example of waveform sets from Events #18 (depth=596 km, M=5.9, 08/30/94) and #19 (depth=638 km, M=5.9, 09/28/94) that took place in the Java Sea region. Event #19 is located southwest off Borneo and its ray paths to QIZ, CHTO and LSA are outside the high velocity zone beneath Kalimantan. The waveforms at these stations show simple P-wave onsets while those at stations in the direction to Kalimantan show substantial high frequency arrivals (see the caption for Event #18 in Fig. 32.2). Comparison of the waveforms recorded in different directions show structural effects specific for propagation path such as apparent multi-paths waves that propagated beneath Kalimantan in which a low velocity zone outside the slab of high velocity anomaly may need to be accounted for.
Generally we often encounter an equivocal situation for deep-focus events that the waveforms could be modeled either with structural effects associated with the deep slab or with a source of multiple subevents. Waveforms need to be analyzed accounting for both effects. In addition to the previously used reflectivity code we are using a 3-D finite difference code [Larsen, 1995] to model structure along profiles between earthquake and station locations focusing on the boundary structure (see the illustration in Fig. 32.1).
The issues of "mid-mantle discontinuities" at around 920 to 1100 km are also being investigated. Most of the resulting images suggest the existence of a possible mid-mantle seismic discontinuity ("920" km discontinuity) with about 3 % of velocity increase associated with the descending slabs into the lower mantle (see also Kawakatsu and Niu, 1994). Niu and Kawakatsu  identified some near-source S-P converted waves that were interpreted to have occurred at the mantle transition discontinuities in the depth range between 900 and 1100 km. The depth variation of the mid mantle discontinuity beneath the Indonesia arc appears to be well correlated with the location of the high velocity anomalies in the recent tomographic models [Kawakatsu and Niu, 1997]. The "discontinuities" may not be flat as previously interpreted (e.g., Niu and Kawakatsu ) but the anomalous arrivals could be due to S-P conversions from more vertical structure associated with the deep anomaly seen in the tomographic models. Using the 3-D finite difference modeling of wave propagation we are investigating other possibilities for the "mid-mantle discontinuities" not associated with flat layering. If the complicated arrivals are due to structure, they should provide constraints on the amplitude and gradients associated with this lower mantle (or "mid-mantle") anomaly.
We will eventually compare the results between the regions concerning slab behaviours with a certain degree of flattening in the transition zone or descending into lower mantle and attempt to understand the apparent difference. At present the 3-D modeling is especially important for the Java region as our preliminary check of the waveforms indicate the substantial effects of the deep subduction zone below the "660" km discontinuity. Our starting models along the cross section between a hypocenter and a given station based on the images from the ISC P tomographic model [Obayashi et al., 1997] (see the illustration of cross sections in Fig. 32.1).
We thank S. P. Grand for useful suggestions for this work, S. Larsen for the use of his 3-D finite difference code, Y. Fukao and M. Obayashi for the use of their P-wave tomographic model, and H. Kawakatsu for discussion.
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