Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.s12d..08l&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #S12D-08
Physics
7203 Body Wave Propagation, 7207 Core And Mantle, 8120 Dynamics Of Lithosphere And Mantle: General, 8124 Earth'S Interior: Composition And State (Old 8105), 8147 Planetary Interiors (5430, 5724)
Scientific paper
Shear waves grazing a small corridor in the D" region under the Pacific Ocean west of Central America (Long. -90 deg, Lat. 0-15 deg) encounter a strong lateral gradient in shear velocity in the lowermost mantle, with ScS-S differential times decreasing by 3-4 s across about 100 km, near 8 deg N. We observe this pattern using a dense set of broadband BDSN and TERRAscope stations in California for earthquakes beneath South America. Several whole mantle tomography models indicate that this gradient is part of the transition from relatively slow structure under the Pacific to the anomalously high velocity structure beneath the Caribbean and North America that underlies high velocity mid-mantle heterogeneities commonly attributed to subducted Farallon plate. Coincident with this strong lateral gradient in shear velocity structure, we find abrupt changes in the nature of shear wave reflections from the radial shear velocity structure at the top of D". In the southern part of our corridor we observe clear waveform evidence for a shear wave triplication, well modeled by previously proposed shear velocity structures with a 2.5-3% increase in velocity (either a discontinuity or strong gradient over no more than 30-50 km) about 200 km above the core-mantle boundary (CMB). At the northern end of the corridor there is evidence for a thicker high velocity D" layer, with a discontinuity perhaps 290 km above the CMB. However, in the northern half of the corridor we find a coherent region for which the raw waveforms lack evidence of any triplication arrival. In a broad survey of structure under the Caribbean that we are separately reporting, we find consistent triplication arrivals throughout the region, with this localized area providing the least indication of a triplication (the triplication may fade out off of eastern North America, but the data are sparse for that region). We utilize source wavelet deconvolution and stacking algorithms to explore the lateral variation in the triplication arrivals in this region, along with systematic investigation of receiver effects. One possibility suggested by our preliminary analysis is that the discontinuity is strongly distorted, and possibly even shallower in the mantle than in the region to the north, causing the triplication to overlap with direct S. There is also some evidence for a region with two smaller shear velocity discontinuities. Characterizing the small scale lateral variations in D" structure across this localized region provides a test of various interpretations of the D" discontinuity, including the possibilities of a phase change, a relic slab anomaly, a heterogeneous scattering layer, and a transition to anisotropic structure in the thermal boundary layer. It is clear that dense station deployments are critical for resolving heterogeneous structure in D".
Garnero Edward
Lay Thome
Russell Stuart
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