Evidence of a laterally variable lower mantle structure fromP- andS-waves

Abstract

S U M M A R YExamples of Grafenberg-array data showing anomalous P-waves which typically arrive 3-5s after the direct P-wave and which have a slowness 0.7-0.8 sdeg-' smaller than direct P are presented. This additional phase is most frequently observed for events located in the NE portion of the southern Kurile Island subduction zone 73"-80" from Grafenberg, but systematically disappears for events in the SW portion of this zone.Because of the magnitude of the slowness difference, these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab. Likewise, they may not be accounted for by near-receiver structure because these phases are not seen for all Kurile events. If present they appear at all stations of the array but they follow direct P too closely to be a multiple from the Moho. Therefore, we conclude they are very likely caused by lower mantle velocity structure.The most likely explanation is the presence of a P velocity jump of about 3 per cent approximately 290 km above the core-mantle boundary, since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional P phase. The distribution of bounce points on this reflector for the Kurile events indicates a lateral extension of this velocity anomaly under northern Siberia of about 150 km by at least 200 km. The best fitting S-wave model has a reflector in the same depth, but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the P and S velocity in D".Few events from other regions in this distance range are suitable for a definitive analysis of this kind. From among this group some observations indicate a lower mantle anomaly under the Lomonosow Ridge and under northern Greenland; but since the lower mantle under western Siberia, northern Novaya Zemlya, the Azores Islands region and the USSR-Afghanistan border region does not produce an additional phase in the Grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature.

Scale lengths of shear velocity heterogeneity at the base of the mantle from S wave differential travel times

“…The same conclusion can be drawn for the paths under Eurasia (Figure 17), where there is somewhat better ray path crossing coverage and a clearer separation of fast and slow regions. The strong lateral fluctuations near the northern coastline of Eurasia correspond to a region with intermittent observations of short-period P wave reflections from D" [Weber and Davis, 1990;Weber, 1993], and it may be that the S wave structure is somewhat more coherent or that our long-period data are averaging over a structure that is truly intermittent spatially. It is important to note that such boundary may be worth pursuing as the data coverage expands, but we feel that this should be performed with broadband data, for which small waveform details may help to guide the modeling, rather than the long-period data used in this study.…”

Seismic Detection of Post-perovskite Inside the Earth

“…Lay and Helmberger 1983;Weber and Kornig 1990;Weber and Davis 1990;Davis and Weber 1990;Young and Lay 1990;Nataf 1992, 1993;Weber 1993;Weber and Wicks 1996;Kendall and Nangini 1996;Thomas and Weber 1997;Reasoner and Revenaugh 1999;Russell et al 2001;Thomas et al 2002Thomas et al , 2004aWallace and Thomas 2005;Lay et al 2006;Avants et al 2006;van der Hilst et al 2007;Kito et al 2007;Hutko et al 2008Hutko et al , 2009Chaloner et al 2009). This discontinuity, usually referred to as "the D″ discontinuity", appears to have a complex structure, with both small and long wavelength lateral variations in its strength and depth.…”

Evidence for an S‐velocity discontinuity in the lowermost mantle beneath the South Eastern Pacific Basin

“…The effective Fresnel zone at the core-mantle boundary for an S wave with dominant period of about 6 seconds observed at a distance of about 80 ø is an ellipse with axes 3.5 ø x 7 ø [Weber and Davis, 1990;Weber, 1993]. This is the area that contributes constructively to the formation of the reflected wave, [1994] --would only yield a 40 km deeper discontinuity, a difference which could be considered as the error for our depth determination; while upper mantle heterogeneity would equally influence S, SdS, and ScS not affecting our results.…”