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We estimate seismic azimuthal anisotropy for the Juan de Fuca ‐ Gorda plates from inversion of a new 10–80 s period Rayleigh wave dataset, resulting in a two‐layer model to 80 km depth. In the lithosphere, most anisotropy patterns reflect the kinematics of plate formation, as approximated from seafloor‐age‐based paleo‐spreading, except for regions close to propagator wakes and near plate boundaries. In the asthenosphere, the fast propagation orientations align with convective shear as inferred from the NUVEL1A plate motion model, which is indicative of a ∼3 Myr average, rather than with the more recent, ∼0.8 Myr, motions inferred from MORVEL. Regional anisotropy of this young plate system thus records convection like older plates such as the Pacific. On smaller scales, anisotropy imaging provides insights into dynamics of plate generation and can further elucidate plate reorganizations and changes in boundary loading. 

Abstract A shallow sub‐seafloor seismic model that includes well‐determined seismic velocities and clarifies sediment‐crust discontinuities is needed to characterize the physical properties of marine sediments and the oceanic crust and to serve as a reference for deeper seismic modeling endeavors. This study estimates the seismic structure of marine sediments and the shallow oceanic crust of the Alaska‐Aleutian subduction zone at the Alaska Peninsula, using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We measure seafloor compliance and Ps converted wave delays from AACSE ocean‐bottom seismometers (OBS) and seafloor pressure data and interpret these measurements using a joint Bayesian Monte Carlo inversion to produce a sub‐seafloor S‐wave velocity model beneath each available OBS station. The sediment thickness across the array varies considerably, ranging from about 50 m to 2.80 km, with the thickest sediment located in the continental slope. Lithological composition plays an important role in shaping the seismic properties of seafloor sediment. Deep‐sea deposits on the incoming plate, which contain biogenic materials, tend to have reduced S‐wave velocities, contrasting with the clay‐rich sediments in the shallow continental shelf and continental slope. A difference in S‐wave velocities is observed for upper oceanic crust formed at fast‐rate (Shumagin) and intermediate‐rate (Semidi) spreading centers. The reduced S‐wave velocities in the Semidi crust may be caused by increased faulting and possible lithological variations, related to a previous period of intermediate‐rate spreading. 

Abstract Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counterintuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfvén waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold,α= 2 as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed >600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: preflare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine thatα= 1.63 ± 0.03. This is below the critical threshold, suggesting that Alfvén waves are an important driver of coronal heating.