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The 2011 Mw9.0 Tohoku-Oki earthquake may be representative of “maximum”earthquakes: it ruptured the entire seismogenic depth range of the Japan megathrust, including the shallowest segment that reaches the trench where the displacement grew to 60 m and spawned a catastrophic tsunami. Models and direct seafloor measurements imply a comparably large initial relative motion and sustained long-period oscillations between sediment and water at the seafloor above the shallowest megathrust segment. This motion may develop enough shear to re-suspend sediment, but exclusively for the maximum earthquakes. This new co-seismic sediment-entrainment process should leave a recognizable sedimentary fingerprint of these earthquakes. Our physical experiments are testing effects of this shear between sediment and water and its interaction with high-frequency vertical shaking. We also investigate the impact of sediment properties and slope on the entrainment. We worked on several synthetic mixtures, defined according to the grain size distribution, clay mineralogy and water content with either freshwater or sea water. The grain size distribution is simplified but matches those of sediment cores from different subduction zones. For each mixture, we built matrices of the erosion rates according to the flow velocities, which shows the role of water content and vertical shaking. We have also identifi ed different mechanism during the runs:grain-by-grain or clasts entrainment, stripping, motion of the sediment interface, and formation of a dense sediment layer above the surface. These observations maybe recorded in the associated deposit, suggesting different fingerprinting by the tsunamigenic earthquakes depending on the characteristics of each subduction zone. 

Abstract Successful management of flooding and erosion hazards on floodplains depends on our ability to predict a river channel's shape and the lifespan during which it will continue to flow. Recent progress has improved our understanding of what sets the lifespan and width of single‐thread channels; the next challenge is to extend this knowledge to braided channels and their interwoven sub‐channels (threads). In this study, we investigate the lifespan and width of braided channel threads in a large experimental data set, coupled with particle‐image velocimetry‐derived measurements of riverbank erosion and accretion. We find that, unlike single‐thread channels, braided channels in the experiment do not exhibit an equilibrium between bank erosion and accretion. Instead, bank erosion outpaces lateral accretion, causing individual threads to widen and infill until they are abandoned. Thread lifespan is limited to the time it takes for threads to triple their width: tripling of the width yields enough bank material to aggrade more than half the channel depth, at which point flow is rerouted to a narrower thread. In consequence the width of active threads is limited to three times their initial width. Threshold channel theory accurately predicts the median thread width, which is roughly double the initial width and two‐thirds the limiting width. The results are consistent with existing field data and suggest that differential bank migration is sufficient to explain why braided channels show greater width variability and higher width‐to‐depth ratios than their single‐thread counterparts. 

Abstract Deltaic river networks naturally reorganize as interconnected channels move to redistribute water, sediment, and nutrients across the delta plain. Network change is documented in decades of satellite imagery and laboratory experiments, but our ability to measure and understand channel movements is limited: existing methods are difficult to employ efficiently and struggle to distinguish between gradual movements (channel migration) and abrupt shifts in river course (channel avulsions). Here, we present a method to extract channel migration from plan‐view imagery using particle image velocimetry (PIV). Although originally designed to track particles moving in a fluid, PIV can be adapted to track channels moving on the delta surface, based on input estimates of channel width, migration timescale, and maps of the wet‐dry interface. Results for a delta experiment show that PIV‐derived vector fields accurately capture channel‐bank movements, as compared to manually drawn maps and an independent image‐registration technique. Unlike other methods, PIV targets the process of channel migration, excluding changes associated with channel avulsions and overbank flow. PIV‐derived migration rates from the experiment span an order of magnitude and are reduced under lower sediment supply and during sea‐level rise, supporting recent models. Together, results indicate that PIV offers a fast and reliable way to measure channel migration in river networks, that channel migration rates under non‐cohesive conditions can displace channels a distance comparable to their width in the time needed to aggrade ∼10% of the channel depth, and that migration direction is ∼60% orthogonal to mean flow direction and ∼40% flow‐parallel overall.