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Topography on the D” reflector inferred from localized double-array stacks. The discontinuity is shallower in the region to the southwest.
Small-scale variability of the D” shear velocity discontinuity beneath the central Pacific is being imaged using 363 broadband tangential component S waveforms recorded by GSN, BDSN, and TRInet stations in western North America for 39 deep focus events in the Tonga-Fiji region. Double-array stacking of spatially binned subsets of data demonstrates variations in timing and relative amplitude of reflection from the deep mantle discontinuity on scale lengths of about 130 km across the 6°x8° region of D” sampled. Waveform modeling using localized one-dimensional structures indicates variations of discontinuity shear velocity increase ranging from 0.3% to 2.4% and discontinuity depths ranging from 2568 to 2730 km. The discontinuity tends to deepen and weaken from southwest to northeast across the study area, a trend that correlates with local spatial gradients in shear velocity anomalies and ScS splitting measurements. The existence of a shear velocity discontinuity in this region is somewhat surprising. While the discontinuity is more variable than in circum-Pacific regions, it has about the same general characteristics, thus it is likely to originate from a similar fundamental cause. It is not reasonable to invoke a slab-related explanation as the region is far removed from subduction, and there is no major depth variation as predicted by a high Clapeyron slope phase transition. Chemical heterogeneity of the D” layer may provide the answer to the D” discontinuity after all.
Avants, M., T. Lay, and S. A.. Russell, Lateral variation of the D” shear velocity discontinuity beneath the central Pacific, J. Geophys. Res., submitted, 2005.
Supported by NSF grants EAR-0125595.
Date Taken: January 29, 2009 Photographer / Contributor: Megan Avants, Thorne Lay • University of California, Santa Cruz; Sara A. Russell • Weston Geophysical
