Constraining lower mantle anomalies using USArray
Abstract
Tomographic images of P and S wave velocities of the lowermost mantle are dominated by two regions of significantly reduced seismic velocities, the Large Low Shear Velocity Provinces (LLSVPs). LLSVPs are often thought of as being both thermally and chemically anomalous relative to the surrounding mantle, although purely thermal models have also been proposed. This is based on both observations of the sharpness and magnitude of the velocity contrasts at the boundaries, and dynamical calculations of the magnitude of a chemical or thermal anomaly required to fit the observations. Knowing the 3-dimensional shape of these anomalies will give constraints on their density and material properties which, in turn, could help to clarify their origin and composition and could help to resolve the debate regarding thermal and chemical contributions. We use direct P and Pdiff waves from earthquakes recorded at USArray, taking advantage of the huge range of directions and distances sampled for each event, to constrain the location and nature of the margins of the Pacific LLSVP. By observing waves at increasing source-receiver distances, and therefore turning depths, we map the vertical extent of the LLSVP. Over the range of azimuths from the event to USArray stations we also map the lateral extent of the LLSVP thus showing the shape of the boundary. The uniformity of stations and station spacing in the array, and the higher frequency of P waves relative to S waves, gives higher resolution than possible before. By analyzing a range of frequencies we probe the sharpness of the LLSVP boundary and find further evidence for Ultra Low Velocity Zones (ULVZs) at the edges of LLSVPs. By analyzing travel-time anomalies and waveforms along paths outside, grazing, and inside the LLSVP at various depths, we delineate the boundary of the low velocity mantle.The selected source and receiver locations allow good sampling of the northern and eastern boundaries of the Pacific LLSVP. These new observations of LLSVP shape will help guide models of mantle dynamics and mineral physics to develop a better understanding of the interaction between 'normal' and 'anomalous' mantle, and their physical nature.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMDI51A2266R
- Keywords:
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- 7208 SEISMOLOGY Mantle;
- 7207 SEISMOLOGY Core;
- 7203 SEISMOLOGY Body waves