Seismic attenuation and mantle wedge temperatures in the Alaska subduction zone

Publication Type  Journal Article
Year of Publication  2004
Authors  Stachnik, J. C.; Abers, G. A.; Christensen, D. H.
Journal Title  Journal of Geophysical Research-Solid Earth
Volume  109
Issue  B10
Pages  -
Journal Date  Oct 8
ISBN Number  0148-0227
Accession Number  ISI:000224430800003
Key Words  subduction zone; seismic attenuation; tomography; wave attenuation; s-waves; northeastern japan; southern california; structure beneath; lg attenuation; body wave; km depth; tomography; seismograms
Abstract  

Anelastic loss of seismic wave energy, or seismic attenuation (1/Q), provides a proxy for temperature under certain conditions. The Q structure of the upper mantle beneath central Alaska is imaged here at high resolution, an active subduction zone where arc volcanism is absent, to investigate mantle thermal structure. The recent Broadband Experiment Across the Alaska Range (BEAAR) provides the first dense broadband seismic coverage of this region. The spectra of P and SH waves for regional earthquakes are inverted for path averaged attenuation operators between 0.5 and 20 Hz, along with earthquake source parameters. These measurements fit waveforms significantly better when the frequency dependence of Q is taken into account, and in the mantle, frequency dependence lies close to laboratory values. Inverting these measurements for spatial variations in Q reveals a highly attenuating wedge, with Q < 150 for S waves at 1 Hz, and a low-attenuation slab, with Q > 500, assuming frequency dependence. Comparison with P results shows that attenuation in bulk modulus is negligible within the low-Q wedge, as expected for thermally activated attenuation mechanisms. Bulk attenuation is significant in the overlying crust and subducting plate, indicating that Q must be controlled by other processes. The shallowest part of the wedge shows little attenuation, as expected for a cold viscous nose that is not involved in wedge corner flow. Overall, the spatial pattern of Q beneath Alaska is qualitatively similar to other subduction zones, although the highest wedge attenuation is about a factor of 2 lower. The Q values imply that temperatures exceed 1200degreesC in the wedge, on the basis of recent laboratory-based calibrations for dry peridotite. These temperatures are 100-150degreesC colder than we infer beneath Japan or the Andes, possibly explaining the absence of arc volcanism in central Alaska.

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861PZTimes Cited:11Cited References Count:64

URL  <Go to ISI>://000224430800003
DOI  Doi 10.1029/2004jb003018