Seismic imaging offers critical insight into mantle thermal structure and melting in subduction zones. While seismic velocities provide first-order constraints, alone they suffer from ambiguities in resolving melt from temperature (e.g., Hammond & Humphreys, 2000) and are sensitive to composition and crustal geology. Seismic attenuation (parameterized by its reciprocal, the quality factor Q) can provide powerful constraints with greater sensitivity to temperature, potentially to melt, and relatively less to composition than velocity measurements (

Plain Language Summary Seismic waves lose more energy passing through hot and partly molten volumes than cold regions. As a result, measurements of variation in their amplitudes, or attenuation, provides a tool for mapping out the upper mantle, complementing more traditional measurements of their variation in travel time. New high-quality arrays across southern Alaska, along with recent methodological developments, now allow this measurement to be made systematically across the entire region. They show consistently low attenuation where subducting plates are near the surface or along paths that follow the cold subducting plates. These regions are in southernmost Alaska. By contrast, signals traveling beneath volcanic regions or north of them, where hot mantle flows toward subduction zones, show high attenuation. The attenuation patterns resemble those from travel time, but seem to show more sensitivity to the upper 150 km of the Earth while travel time delays more uniformly sample deeper. Sedimentary basins show confusing signals, with travel time delays as expected for low-wavespeed sediments, but high amplitudes that are difficult to explain. These signals allow quantitative mapping of temperature and melt variations in the upper mantle, even in regions as complex as subduction zones where both properties vary rapidly over short distances.SOTO CASTANEDA ET AL.