Detection of a tall ridge at the core–mantle boundary from scattered PKP energy

Abstract

The seismic wavefield mainly contains reflected, refracted and direct waves but energy related to elastic scattering can also be identified at frequencies of 1 Hz and higher. The scattered, highfrequency seismic wavefield contains information on the small-scale structure of the Earth's crust, mantle and core. Due to the high thermal conductivity of mantle materials causing rapid dissipation of thermal anomalies, the Earth's small-scale structure most likely reveals details of the composition of the interior, and, is therefore essential for our understanding of the dynamics and evolution of the Earth. Using specific ray configurations we can identify scattered energy originating in the lower mantle and under certain circumstances locate its point of origin in the Earth allowing further insight into the structure of the lowermost mantle. Here we present evidence, from scattered PKP waves, for a heterogeneous structure at the core-mantle boundary (CMB) beneath southern Africa. The structure rises approximately 80 km above the CMB and is located at the eastern edge of the African LLSVP. Mining-related and tectonic seismic events in South Africa, with m b from 3.2 to 6.0 recorded at epicentral distances of 119.3 • to 138.8 • from Yellowknife Array (YKA) (Canada), show large amplitude precursors to PKP df arriving 3-15 s prior to the main phase. We use array processing to measure slowness and backazimuth of the scattered energy and determine the scatterer location in the deep Earth. To improve the resolution of the slowness vector at the medium aperture YKA we present a new application of the F-statistic. The high-resolution slowness and backazimuth measurements indicate scattering from a structure up to 80 km tall at the CMB with lateral dimensions of at least 1200 km by 300 km, at the edge of the African Large Low Shear Velocity Province. The forward scattering nature of the PKP probe indicates that this is velocity-type scattering resulting primarily from changes in elastic parameters. The PKP scattering data are in agreement with dynamically supported dense material related to the Large Low Shear Velocity Province.

Seismic Detections of Small-Scale Heterogeneities in the Deep Earth

“…Another LLSVP can be found beneath the Pacific, and combined, they form a strong degree 2 pattern of low velocities in most tomographic models (Garnero and McNamara 2008). Strong scattering from the edge of the LLSVP has been reported earlier (Wen 2000;Rost and Earle 2010;Frost et al 2013). On the other hand, we can relocate the near-surface scattering to a region beneath South America (scattering region 2 in Fig.…”

“…Alternatively, the scattering has been attributed to CMB topography with RMS height of ~300 m (Bataille et al 1990). A second group of studies located discrete scatterers at the CMB rather than treating the scattering as a statistical process (Thomas et al 1999;Frost et al 2013). Resolving heterogeneities high above the CMB with PKP is challenging, but this gap can be closed using other phases as described below.…”

“…Further testing will see whether smaller arrays than YKA (with apertures of 3-10 km, which have been built as part of the Comprehensive Test Ban Treaty) are able to detect the weak scattered energy related to P′•P′. To enhance detection of coherent signal power in the stacked array data and increase resolution in slowness space, we apply the F-statistic to the array beams (Selby 2011;Frost et al 2013). To detect the weak scattered energy related to P′•P,′ we perform a grid search over slowness (ranging from 0 to 13 s/°) and backazimuth (from 0° to 360°).…”

“…The maximum amplitude of this F-trace is then measured in a time window representing a specific scattering depth as discussed below. The resulting slowness/backazimuth F-trace beampower maps (F-packs) indicate the slowness and backazimuth of the incoming energy with high precision (Frost et al 2013). The use of the F-statistic penalizes energy traveling with incidence angles other than the one the beam is calculated for, therefore sharpening the array response function and the wave number resolution of the array, while improving the signal-to-noise ratio of the energy traveling along the correct slowness vector.…”

Strong seismic scatterers near the core–mantle boundary north of the Pacific Anomaly

“…Particular advantages of using PKP precursors are that they are sensitive to heterogeneity at a scale as small as $10 km, and are not contaminated by the coda of other phases due to their earlier arrival times. There have been successful applications of PKP precursors studies on global scales (e.g., Hedlin et al, 1997;Cormier, 1999;Margerin and Nolet, 2003b;Mancinelli and Shearer, 2013), as well as regional scales (e.g., Vidale and Hedlin, 1998;Wen and Helmberger, 1998;Niu and Wen, 2001;Miller and Niu, 2008;Thomas et al, 2009;Frost et al, 2013). In regional studies that incorporate well-sited arrays of seismographs, greater information about the anomalies, such as their boundary locations and sharpness, may be resolvable.…”