Seismology, dT/d  and Deep Mantle Convection

Abstract

dT/dA and azimuth measurements at the Yellowknife Seismic Array (YKA) for P and pP phases that have penetrated to depths greater than 1850 km show systematic bias relative to those values calculated from spherically symmetric Earth models. Moreover, the corresponding arrival vectors for the phases PcP, ScP, PnKP and SnKP (n > 2) approaching YKA from within the same narrow azimuth ranges as the P andpP phases show related systematic errors. From an analysis of combinations of all these phases, we show that most of the observed bias for P and pP is produced along the upgoing portion of the ray path, and that any heterogeneities and velocity anomalies near the base of the mantle cannot be clearly elucidated by a dT/dA study of the P phase alone. P wave data must be supplemented by other phases that traverse similar paths within the upper mantle beneath arrays. Evidence for regional variations in structure at depths greater than 1850 km is presented. An abrupt drop in dT/dA between distances of 87" and 90" is reported for two different regions of the Earth and possibly a third; this drop represents a fairly rapid increase in velocity gradients at about 2700 km depth that may be a world-wide phenomenon. There are also strong lateral variations in the upper mantle to the north-east and east of YKA. Array observations of precursors to PP, 1 ' ' P' and PKP indicate small-scale lateral variations in the crust and upper mantle and in the lowest 200 km of the mantle that give rise to extensive scattered P wave energy. We initially try to relate our data on lateral variations to regional variations in travel times and mantle convection. Our main concern is the problem of motion within the lower mantle, and how dT/dA measurements in combination with theoretical studies of the properties of the scattering regions might indicate flow. The combined P wave data on small-scale and large-scale lateral variations suggest that the mantle is most homogeneous between depths of 800 km and 2500 km; consequently, we propose that mantle convection is confined to the upper mantle and the lowest 400 km of the mantle, and that convection plumes, if they exist, do not originate at depths greater than 800 km. * Contribution from the Earth Physics Branch No.: 533 115 c c h,

Characteristics of Lower Mantle Inhomogeneities from Scattered Waves

“…Such a model would provide a heat sink required to maintain an appropriate temperature gradient in the outer core. The present seismic evidence suggests that partial melting, if it occurs, can only be highly localized; for example short-period body waves which pass the deep mantle as an S wave have been widely observed (Wright & Lyons 1975).…”

“…Lateral heterogeneity in the lower mantle has been suggested on the basis of a variety of body wave studies. Anomalies were inferred from diffracted P wave patterns (Phinney & Alexander 1966), travel times (Julian & Sengupta 1973;Jordan & Lynn 1974), ray parameters (Johnson 1969;Husebye, Kanestrom & Rud 1971), slowness vectors (Davies & Sheppard 1972;Kanasewich et al 1973;Wright & Lyons 1975) and amplitudes (Needham & Davies 1973). Despite the accumulated evidence a consistent picture has not yet emerged; Okal & Kuster (1975) discussed some of the inconsistencies.…”

Deep Earth Structure: Lower Mantle and D″

“…The velocity increases at the top of D 00 are primarily detected by reflections and triplications, which arrive ahead of the corereflected PcP and ScS phases (e.g., Avants et al, 2006b;Chaloner et al, 2009;Ding and Helmberger, 1997;Gaherty and Lay, 1992;Houard and Nataf, 1993;Hutko et al, 2008Hutko et al, , 2009Kendall and Nangini, 1996;Kendall and Shearer, 1994;Kito and Krü ger, 2001;Kito et al, 2004Kito et al, , 2007aKohler et al, 1997;Lay and Helmberger, 1983a;Lay et al, 2004aLay et al, , 2006Reasoner and Revenaugh, 1999;Sun and Helmberger, 2008;Sun et al, 2006Sun et al, , 2009Takeuchi and Obara, 2010;Thomas and Weber, 1997;Thomas et al, , 2004aThorne et al, 2007;Weber, 1993;Weber and Davis, 1990;Wright and Lyons, 1975;Wright et al, 1985;Lay, 1987a, 1990). The clearest observations of deep mantle triplications are from distances of 65 to 95 , where the critical angle interaction with the velocity increase greatly enhances the amplitudes of the reflected signals relative to precritical distance ranges.…”

Whole-mantle convection and plate tectonics

“…(1975). Other studies have indicated the presence of very large-scale seismic velocity anomalies throughout the mantle, the largest amplitude anomalies being in the upper mantle and near the core-mantle boundary (Niazi 1973;Julian & Sengupta 1973;Wright & Lyons 1975;Dziewonski, Hager & O'Connell 1977). These largescale anomalies correlate with long-wavelength components of the gravity field (Dziewonski et al 1977), which suggests that they are directly associated with motion of the mantle (O'Connell 1977).…”