Imaging Upper‐Mantle Structure Under USArray Using Long‐Period Reflection Seismology

Abstract

Topside reverberations off mantle discontinuities are commonly observed at long periods, but their interpretation is complicated because they include both near‐source and near‐receiver reflections. We have developed a method to isolate the stationside reflectors in large data sets with many sources and receivers. Analysis of USArray transverse‐component data from 3,200 earthquakes, using direct S as a reference phase, shows clear reflections off the 410‐ and 660‐km discontinuities, which can be used to map the depth and brightness of these features. Because our results are sensitive to the impedance contrast (velocity and density), they provide a useful complement to receiver‐function studies, which are primarily sensitive to the S velocity jump alone. In addition, reflectors in our images are more spread out in time than in receiver functions, providing good depth resolution. Our images show strong discontinuities near 410 and 660 km across the entire USArray footprint, with intriguing reflectors at shallower depths in many regions. Overall, the discontinuities in the east appear simpler and more monotonous with a uniform transition zone thickness of 250 km compared to the western United States. In the west, we observe more complex discontinuity topography and small‐scale changes below the Great Basin and the Rocky Mountains, and a decrease in transition‐zone thickness along the western coast. We also observe a dipping reflector in the west that aligns with the top of the high‐velocity Farallon slab anomaly seen in some tomography models, but which also may be an artifact caused by near‐surface scattering of incoming S waves.

Continental Tectonics Inferred From High‐Resolution Imaging of the Mantle Beneath the United States, Through the Combination of USArray Data Types

“…The high velocities are caused by thermal differences between the lithospheric slab and the surrounding lower mantle material. Our images of the subducted Farallon plate are largely consistent with previous tomographic studies (Burdick et al, 2008(Burdick et al, , 2012Grand, 2002;James et al, 2011;Obrebski et al, 2010Obrebski et al, , 2011Schmandt & Humphreys, 2010;Shearer & Buehler, 2019;Sigloch et al, 2008;Tian et al, 2009Tian et al, , 2011Van der Lee & Nolet, 1997a).…”

Complicated Lithospheric Structure Beneath the Contiguous US Revealed by Teleseismic S‐Reflections

“…To further verify our data quality, we plot a record section with the 50,904 traces that passed our selection criteria (Figure 4a). The record section shows clear direct S and ScS , which closely resemble these two phases in Figure 1 of Shearer and Buehler (2019).…”

“…To reduce contamination from source-side structure, here we analyze S-reflections of earthquakes deeper than 150 km to image receiver-side structures shallower than the event focal depths. In this case, direct topside reflections from a layer above 150 km occur only at the receiver side (blue rays in Figure 2), which eliminates the need for the inversion procedure in Shearer and Buehler (2019). Note that underside reflections from interfaces shallower than the event focal depths may be generated near the source, but these arrivals will not stack coherently over varying source depths.…”

Global Patterns in Cratonic Mid‐Lithospheric Discontinuities From Sp Receiver Functions

“…A secondary goal of our paper is to reevaluate the impacts of different data processing steps on the arrivals observed in stacks of S-to-P receiver functions. However, we also note that a variety of seismic phases have indicated the presence of NVGs and/or low velocity layers in the shallow cratonic lithosphere, including Rayleigh-waves (e.g., Dalton et al, 2017;Eeken et al, 2018), topside reflections of direct S phases (Shearer & Buehler, 2019), SS precursors (Tharimena et al, 2017), ScS reverberations (Revenaugh & Jordan, 1991), P reflectivity (Sun & Kennett, 2017), and longrange refraction surveys (Thybo & Perchuc, 1997).…”