Teleseismic attenuation, temperature, and melt of the upper mantle in
the Alaska subduction zone
Abstract
Seismic deployments in the Alaska subduction zone provide dense sampling
of the seismic wavefield that constrains thermal structure and
subduction geometry. We measure P and S attenuation from
pairwise amplitude and phase spectral ratios for teleseismic body waves
at 206 stations from regional and short-term arrays. Parallel
teleseismic travel-time measurements provide information on seismic
velocities at the same scale. These data show consistently low
attenuation over the forearc of subduction systems and high attenuation
over the arc and backarc, similar to local-earthquake attenuation
studies but at 10´ lower frequencies. The pattern is seen both across
the area of normal Pacific subduction in the Cook Inlet, and across the
Wrangell Volcanic Field where subduction has been debated. These
observations confirm subduction-dominated thermal regime beneath the
latter. Travel times show evidence for subducting lithosphere much
deeper than seismicity, while attenuation measurements appear mostly
reflective of mantle temperature less than 150 km deep, depths where the
mantle is closest to its solidus and where subduction-related melting
may take place. Travel times show strong delays over thick sedimentary
basins. Attenuation signals show no evidence of absorption by basins,
although some basins show signals anomalously rich in high-frequency
energy, with consequent negative apparent attenuation. Outside of
basins, these data are consistent with mantle attenuation in the upper
220 km that is quantitatively similar to observations from surface waves
and local-earthquake body waves. Differences between P and
S attenuation suggest primarily shear-modulus relaxation. Overall
the attenuation measurements show consistent, coherent
subduction-related structure, complementary to travel times.