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

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.