Fine seismic velocity structure of the 410 km and 660 km discontinuities beneath eastern Asia
Abstract
The fine seismic structure of the Earth’s upper mantle discontinuities is one of the key indicators of mantle composition, temperature, mineralogy and dynamics. Theoretical and high-pressure experimental studies suggested that, at some temperature and compositional conditions, perovskite-forming phase transformations should produce double discontinuities near 660 km depth with one resulted from the phase transformation from ringwoodite to perovskite plus magnesiowustite, and the other from garnet to perovskite. Some dynamic considerations hypothesized, and many seismic studies supported, existence of a partially molten thin layer about the 410-km discontinuity. We study these two issues on the basis of waveform modeling of seismic data recorded in the triplication distance range by Hi-net, a dense seismic array in Japan with about 600 high-sensitivity stations. We selected three events occurring in the Sea of Okhotsk, with two located just above the 660-km discontinuity and one just above the 410-km discontinuity. The waveforms of these deep events are mostly sensitive to the fine seismic features near the 660-km and 410-km discontinuities, respectively. We stack seismic data for every half degree in the distance range of 10 - 30 degrees. The stacked data for the events close to 660 km depth show clear triplication of phases with one turning below the discontinuity and one diffracted along the discontinuity. However, an additional strong phase is observed after the diffracted phases with a consistent move-out. These waveform features can be explained by a model of double discontinuities near 660 km depth separated by 30 km in depth, with a relatively smaller shear velocity jump across the shallower discontinuity and a smaller velocity gradient above it. We present detailed compositional and thermal models that are consistent with the characteristics of the double discontinuities we observed based on mineral physics modeling of two perovskite-forming transformations: ringwoodite to perovskite + magnesiowustite and garnet to perovskite. The stacked seismic data for the shallow event exhibit similar features as the synthetics predicted by the Preliminary Reference Earth Model. They are inconsistent with a model with a low velocity zone above the 410-km discontinuity as the low velocity zone would generate strong reverberations inside the layer which are not observed in the seismic data. Therefore, we suggest no-existence of a low velocity zone above the 410-km discontinuity in this region.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMDI51C1887C
- Keywords:
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- 8124 TECTONOPHYSICS / Earth's interior: composition and state;
- 3621 MINERALOGY AND PETROLOGY / Mantle processes;
- 7203 SEISMOLOGY / Body waves;
- 7208 SEISMOLOGY / Mantle