Using Topside Reflections to Image Upper Mantle Discontinuity Structure Beneath North America via the Earthscope Transportable Array

With EarthScope’s USArray Transportable Array (TA), an unprecedented access to information about the crust and mantle beneath the North American Continent (NA) is possible. The depth and sharpness of upper mantle seismic discontinuities, parameters sensitive to chemical and thermal properties of the mantle, are indicative of the dynamics under NA. Using the high spatial sampling offered by TA, we analyze upper mantle discontinuity structure via topside reflections of seismic waves. This array-based approach allows us to investigate the discontinuity structure beneath NA at lateral scale lengths of several hundred kilometers or less.

The topside reflections are labeled as SdsS or SSds, where d represents the depth from which the wave reflects. Topside reflections arrive as postcursory phases of the primary S arrival, delayed up to several hundreds of seconds. Our analysis relies on the use of SH waves, the horizontal component of transverse polarization. Similar to other secondary seismic phases, SdsS amplitudes are roughly 5-15% of the main S-arrival amplitude. Stacking is then used to isolate the topside related phase components from the background noise.

Topside reflections may occur at either the receiver-side or source-side of the path; lifting this ambiguity is made possible through use of multiple-array analysis. Using a dataset of ~251 earthquakes recorded during 2005-2014 by the large moving configuration of TA, with events originating in the Tonga-Fiji subduction zone, we address this ambiguity by implementing an array-based approach to image station-side heterogeneity beneath NA. Events are selected from a relatively compact region in the vicinity of Tonga, while the receiver-side observations are extensive across NA. Our new approach to analyzing the topside reflections allows inferences about the receiver side structure of the upper mantle discontinuities, a result that we relate to thermal and chemical heterogeneity beneath NA.