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Seismic investigations of the Marian...

Washington University in St. Louis.

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Seismic investigations of the Mariana subduction system: Anisotropy, attenuation, and volcano seismology.

Record Type:	Language materials, printed : Monograph/item
Title/Author:	Seismic investigations of the Mariana subduction system: Anisotropy, attenuation, and volcano seismology./
Author:	Pozgay, Sara Harriet.
Description:	208 p.
Notes:	Adviser: Douglas A. Wiens.
Contained By:	Dissertation Abstracts International69-02B.
Subject:	Geology. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=3299972
ISBN:	9780549465775

Seismic investigations of the Mariana subduction system: Anisotropy, attenuation, and volcano seismology.
Pozgay, Sara Harriet.

Seismic investigations of the Mariana subduction system: Anisotropy, attenuation, and volcano seismology. - 208 p.

Adviser: Douglas A. Wiens.

Thesis (Ph.D.)--Washington University in St. Louis, 2007.

With an unprecedented dataset for studying questions regarding subduction dynamics and backarc spreading, I describe two seismic investigations of the structure and dynamics of the Mariana subduction system (seismic anisotropy and seismic attenuation) and an investigation of the seismic characteristics of an unexpected volcanic eruption. Results include: (1) The fortuitous timing of the Anatahan Island seismograph installation recorded most of the May 2003 eruption of the historically inactive volcano. Temporal and spatial analyses of three different types of volcano-related earthquakes, volcanic tremor, and a tilt signal together provide an accurate picture of the seismotectonics related to the eruption. We observe very little precursory seismic activity until &sim;6 hours before the estimated eruption onset. A prominent tilt signal, indicative of inflation and deflation of the magma chamber, is coincident with the onset of nearly continuous earthquake activity &sim;1 hour before the estimated eruption time. (2) Tomographic inversion of &sim;2800 path-averaged seismic attenuation t* measurements for 2-D QP-1 and QP/QS structures and subsequent calculation of QS structure illuminate a low attenuation slab and high attenuation regions beneath the arc and the backarc spreading center and in the core of the mantle wedge. A quasi-columnar-shaped high QP-1 region persists beneath the backarc spreading center to &sim;100-125 km depth and is distinctly separate at shallow depths (< &sim;50 km) from the high attenuation island arc anomaly. In addition, we observe high Q P-1 and QS-1 beneath forearc seamounts, likely indicative of serpentinization. (3) Results from &sim;300 shear wave splitting measurements show dominantly arc-parallel fast directions near the island arc and between the arc and the trough. Only from deep (>250 km) earthquakes and at stations located in the far backarc are fast splitting directions concurrent with a corner flow regime. These results suggest that the typical interpretation of mantle wedge flow strongly coupled to the downgoing slab is valid only at depths greater than &sim;250 km and large distances from the trench. We suggest that shallower than &sim;250 km depth, the arc-parallel fast splitting orientations are likely the result of physical arc-parallel mantle flow, which precludes the typical subduction zone corner flow regime.

ISBN: 9780549465775Subjects--Topical Terms:

516570
Geology.

Seismic investigations of the Mariana subduction system: Anisotropy, attenuation, and volcano seismology.
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020 $a 9780549465775
035 $a (UMI)AAI3299972
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100 1 $a Pozgay, Sara Harriet. $3 1020492
245 1 0 $a Seismic investigations of the Mariana subduction system: Anisotropy, attenuation, and volcano seismology.
300 $a 208 p.
500 $a Adviser: Douglas A. Wiens.
500 $a Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 0896.
502 $a Thesis (Ph.D.)--Washington University in St. Louis, 2007.
520 $a With an unprecedented dataset for studying questions regarding subduction dynamics and backarc spreading, I describe two seismic investigations of the structure and dynamics of the Mariana subduction system (seismic anisotropy and seismic attenuation) and an investigation of the seismic characteristics of an unexpected volcanic eruption. Results include: (1) The fortuitous timing of the Anatahan Island seismograph installation recorded most of the May 2003 eruption of the historically inactive volcano. Temporal and spatial analyses of three different types of volcano-related earthquakes, volcanic tremor, and a tilt signal together provide an accurate picture of the seismotectonics related to the eruption. We observe very little precursory seismic activity until &sim;6 hours before the estimated eruption onset. A prominent tilt signal, indicative of inflation and deflation of the magma chamber, is coincident with the onset of nearly continuous earthquake activity &sim;1 hour before the estimated eruption time. (2) Tomographic inversion of &sim;2800 path-averaged seismic attenuation t* measurements for 2-D QP-1 and QP/QS structures and subsequent calculation of QS structure illuminate a low attenuation slab and high attenuation regions beneath the arc and the backarc spreading center and in the core of the mantle wedge. A quasi-columnar-shaped high QP-1 region persists beneath the backarc spreading center to &sim;100-125 km depth and is distinctly separate at shallow depths (< &sim;50 km) from the high attenuation island arc anomaly. In addition, we observe high Q P-1 and QS-1 beneath forearc seamounts, likely indicative of serpentinization. (3) Results from &sim;300 shear wave splitting measurements show dominantly arc-parallel fast directions near the island arc and between the arc and the trough. Only from deep (>250 km) earthquakes and at stations located in the far backarc are fast splitting directions concurrent with a corner flow regime. These results suggest that the typical interpretation of mantle wedge flow strongly coupled to the downgoing slab is valid only at depths greater than &sim;250 km and large distances from the trench. We suggest that shallower than &sim;250 km depth, the arc-parallel fast splitting orientations are likely the result of physical arc-parallel mantle flow, which precludes the typical subduction zone corner flow regime.
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773 0 $t Dissertation Abstracts International $g 69-02B.
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791 $a Ph.D.
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