Constraining mantle discontinuity structure beneath North America using ScS reverberations

The Earth's mantle consists of discontinuities arising from abrupt changes in mantle mineralogy, a common example of which is a mineral phase change; there are two notable mineral phase changes that occur near the depths of 410 km (α-olivine to β-spinel) and 660 km (γ-spinel to perovskite+magnesiowustite). The variable depth of these discontinuities is tied to thermal properties of the mantle as well as compositional variations. Therefore, it is possible to use depths of observed discontinuities to infer temperature using the Clapeyron slope of the phase change. Thanks to EarthScope's continent-wide coverage, it is feasible to use array methods, such as vespagrams, to detect specific seismic phases that have interacted with discontinuities. The ScS reverberative family - a group of S waves reverberating between the surface and the core-mantle boundary and interacting with mantle discontinuities along the way - offers some of the strongest constrains on Earth's deep interior. In this study, we analyze ScS reverberations from events located near or within the continental US and recorded by EarthScope arrays. We restrict our analysis to earthquakes with Mw>5.7 and source depths of 4 km to 211 km. We analyze transverse component seismograms from a total of 26 events. For each event, seismograms are hand selected, the instrument response is removed, and a 20-100 s period band-pass filter is applied to the data. The seismograms are then picked, aligned, and normalized using the S arrivals, and vespagrams are constructed for sub-arrays. These vespagrams are then probed for zeroth and first order mantle reverberations, with clear identification of arrivals out to ScS4 sometimes possible. Discontinuity depths are then computed from the relative times of the reverberations. We discuss the depths of the discontinuities in the context of temperature variations and their relationship to other constraints on the structure of the sub-continental mantle beneath North America.