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Ponko, S.C., 1995

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Bibliographic Reference

Ponko, S.C., 1995, Attenuation and velocity tomography and thermal modeling of crustal and upper-mantle fource regions: Tempe, Arizona, Arizona State University, Ph.D. dissertation, 267 p., illust.

Abstract

The composition and thermal structure of two volcanic regions has been investigated using a combination of seismic imaging and thermal modeling techniques. P- and S-wave amplitude data from digital, three-component seismograms of local earthquakes were used to tomographically invert for the three-dimensional attenuation structure in the upper crust of Long Valley caldera, California. High P-wave attenuation is observed at a depth of 4-5 km beneath Mammoth Mountain and at 6-8 km beneath the resurgent dome. High S-wave attenuation is seen at 7-8 km beneath the resurgent dome. At 6-7 km, the S-wave results are near average. High S-wave attenuation at 7-8 km beneath the resurgent dome may indicate the roof of a magmatic system. High P-wave attenuation at 4-5 km beneath Mammoth Mountain and 6-8 km beneath the resurgent dome may be due to the presence of supercritical water. These zones may represent some of the source regions for the Long Valley geothermal system. Travel time residuals, S-P lag times and S/P-amplitude ratios from 4,118 high quality, vertical component digital seismograms recorded by the Alaska seismic network were inverted to image the velocity, Vp/Vs and S-wave attenuation structure of the southern Alaska subduction zone. A kinematic, numerical thermal model of the southern Alaska subduction zone was developed to help constrain interpretations of the tomographic images. First-order inversion and thermal modeling results are consistent with a high-temperature mantle wedge and locally higher temperatures or partial melting beneath the volcanic arc. Low Vp/Vs-low attenuation anomalies in the subducting oceanic slab at depths of 80-130 km may represent fluid release due to dehydration reactions. High Vp/Vs-low attenuation anomalies observed at depths of 40-80 km in the mantle wedge and directly above the subducting slab may represent regions of extensive serpentinization of mantle wedge peridotite.

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