HR: 16:15h
AN: S52K-02    [PDF]
TI: Very Deep Earthquakes Occur Above the Depressed Top of the Lower Mantle
AU: * Brudzinski, M R
EM: brudzins@geology.wisc.edu
AF: University of Wisconsin, Department of Geology and Geophysics, Madison, WI 53706 United States
AU: Pillet, R
EM: Robert.Pillet@noumea.ird.nc
AF: Institut de Recherche pour le Developpement, Geosciences Azur, Noumea, 98848 New Caledonia
AU: Chen, W
EM: wpchen@uiuc.edu
AF: University of Illinois, Department of Geology, Urbana, IL 61801 United States
AB: We present unequivocal evidence that very deep earthquakes near 690 km still occur above the "660-km" discontinuity which separates the lower mantle and the transition zone. The strongest evidence comes from two recent earthquakes that occurred at great depths of 688$\pm$10 km and 693$\pm$10 km along the Tonga and the Kurile-Kamchatka subduction zones, respectively. The focal depths are constrained by relative timing (with respect to direct {\it P} and {\it S} arrivals) of {\it pP}, {\it sP}, and {\it sS} phases along paths unaffected by subduction or {\it pwP}. Then we directly determine the {\bf relative} position of the foci and the "660-km" discontinuity based on the principle that triplicate arrivals are strong from a source located above a discontinuity, but such arrivals would be completely absent for a source located just below the discontinuity. We observe strongly triplicate {\it P} and {\it S} waveforms that exhibit distinct move-outs across broadband (high-resolution), permanent seismic arrays at regional distances (7$\deg$-20$\deg$). Thus these earthquakes at depths near 690 km must have occurred above a strong discontinuity. We matched the observed waveforms with synthetic seismograms and the results show the prominent contrast in seismic wave speeds across the discontinuity is consistent with that across the "660-km" discontinuity of the average Earth. Thus the "660-km" discontinuity is apparently deflected downward by about 5 km farther below the hypocenters, reaching a depth of about 695 km. Downward deflection of the "660-km" discontinuity below earthquakes as deep as 690 km implies that the Clapeyron slope associated with the "660-km" discontinuity is large and the necessary conditions to trigger deep earthquakes exist only above the lower mantle.
DE: 1025 Composition of the mantle
DE: 7203 Body wave propagation
DE: 7209 Earthquake dynamics and mechanics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting