next up previous contents
Home: Berkeley Seismological Laboratory
Next: About this document ... Up: Ongoing Research - Global Previous: Inversion of Normal Mode

Seismological Study of the Upper Mantle Transition Zone Using Broadband Waveform Modeling

Fumiko Tajima


Anomalous P Wave Speed Models M2.0 and M3.11

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 $\sim$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 $\sim$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.


  
Figure 25.1: P waveforms observed at HIA station in China for events from the southern Kurile (left) and Izu-Bonin (right) subduction zones (top bold traces) and synthetics which are modeled well by M3.11 or M2.0, respectively (thin traces immediately below the observed waves). Below those are the synthetics with other models (iasp for iasp91) for comparison.
\begin{figure*}
\begin{center}
\epsfig{file=fumiko99_2_1.ps, width=7cm}\end{center}\end{figure*}

Evaluation of Slab Images in the northwestern Pacific

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 $\sim$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).


  
Figure 25.2: Cross projection profiles of slowness along some of the ray paths for the preferred model of M3.11. The peak of high velocity anomalies is in the depth range 478-551 km in the tomographic model, while the waveform modeling suggests the strongest anomaly in the deeper part of the transition zone, but the agreement between the tomographic image and the waveform modeling is reasonable along tomographic image and the waveform modeling is reasonable along these profiles.
\begin{figure*}
\begin{center}
\epsfig{file=fumiko99_2_2.ps, width=10cm}\end{center}\end{figure*}

References

Fukao, Y., M. Obayashi, H. Inoue, and M. Nenbai, Subducting slabs stagnant in the mantle transition zone, J. Geophys. Res., 97, 4809-4822, 1922.

Li, B., R. C. Liebermann, and D. J. Weidner, Elastic moduli of waldsleyite ($\beta $-Mg2SiO4) to 7 Gigapasscals and 873 Kelvin, Science, 281, 675-677, 1998.

Shearer, P.M., and T. G. Masters, Global mapping of topography on the 660-km discontinuity, Nature, 355, 791-796, 1992.

Tajima, F., and S. P. Grand, Evidence of high velocity anomalies in the transition zone associated with southern Kurile subduction zone, Geophys. Res. Lett., 22, 3139-3142, 1995.

Tajima, F., and S. P. Grand, Variation of transition zone high velocity anomalies and depression of the "660"km discontinuity associated with subduction zones from the southern Kuriles to Izu-Bonin, J. Geophys. Res., 103, B7, 15015-15036, 1998.

Tajima, F., Y. Fukao, M. Obayashi, and T. Sakurai, Evaluation of slab images in the northwestern Pacific, Special issue of the Ocean Hemisphere Project symposium, Earth Planet. Space, 50, no. 11 and 12, 953-964, 1998.

van der Hilst, R.D., R. Engdahl, W. Sparkman, and G. Nolet, Tomographic imaging of subducted lithosphere below northwest Pacific islands arcs, Nature, 353, 37-43, 1991.


next up previous contents
Next: About this document ... Up: Ongoing Research - Global Previous: Inversion of Normal Mode

The Berkeley Seismological Laboratory, 202 McCone Hall, UC Berkeley, Berkeley CA 94720
Questions and comments to www@seismo.berkeley.edu
Copyright 1999, The Regents of the University of California.