Interferometric Imaging of Deep Mantle Reflectors Beneath the Western United States

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Recent seismological investigations of the upper-mantle located directly beneath the western United States have resolved a remarkable degree of structure and heterogeneity. These efforts have revealed previously unknown structures, such as large-scale mantle drips. How these upper-mantle structures are related to structure and processes in the mid- and lower-mantle is currently unresolved. A key step in deciphering this interaction between upper-mantle structures with deeper mantle layers is to use high resolution mapping of seismic discontinuities. In particular, discontinuities located approximately 400 to 700 km in depth (the mantle transition zone) are expected to be strongly affected by their interaction with upper mantle structures and are key target areas being investigated in this study.This work has developed a new technique based on seismic interferometry to investigate the nature of seismic reflectors in the upper- to mid-mantle region. Seismic processing using an interferometry approach has been extensively utilized in small-scale oil exploration efforts in the last decade, yet has not been applied to deeper targets as techniques of this kind require dense spacing of seismic recorders. Prior to the significant increase in seismic instruments made possible by the USArray project, this technique would not have been possible to adapt.Past efforts for looking at mantle discontinuity structure have been focused on studying seismic wave reflections off of the underside of seismic discontinuities. Many of these efforts have used underside reflections from the seismic phase SS (termed SS-precursors). However, using SS-precursors presents challenges in determining discontinuity depth, as knowledge of the seismic wave velocity on both the source- and receiver-side of the SS path is needed. The technique used in this study applies cross-correlation interferometry to migrate the SS-precursor data with the direct S-wave arrival passing through the discontinuity. This eliminates the need for detailed velocity structure on both source- and receiver-paths and is not dependent on any prior knowledge of where the seismic source is located. This technique provides better spatial resolution on mantle discontinuity structure and is elucidating the link between upper- and lower-mantle processes.