HR: 0800h
AN: S51B-0169    [Abstracts]
TI: Precise Measurements of Depth Phase Relative Arrival Times to Refine Estimates of Double Benioff Zone Geometry
AU: * Brudzinski, M R
EM: brudzins@geology.wisc.edu
AF: University of Wisconsin, Department of Geology and Geophysics, Madison, WI 53706
AU: * Brudzinski, M R
EM: brudzins@geology.wisc.edu
AF: Miami University, Department of Geology, Oxford, OH 45056
AU: Thurber, C H
EM: clifft@geology.wisc.edu
AF: University of Wisconsin, Department of Geology and Geophysics, Madison, WI 53706
AU: Engdahl, E R
EM: engdahl@iaspei.org
AF: University of Colorado, Department of Physics, Boulder, CO 80309
AB: In subduction zones, inclined Wadati-Benioff zones of seismicity occur within the subducting lithosphere, and thus can be used to examine the structure of downgoing slabs. A particularly detailed set of information about the slab can be obtained from curious Double Benioff Zones (DBZs), where two sub-parallel planes of seismicity are separated in depth by as much as 30 km. Based on a handful of observations of DBZs, the planes have been proposed to mark the location of petrologic dehydration reactions, which could in turn trigger earthquakes via dehydration embrittlement. However, identification of DBZs has typically been limited to areas where dense local networks cover the subduction zone. We demonstrate a new technique that strictly uses globally recorded teleseismic waveforms to more precisely determine the location of subduction zone earthquakes, which in turn facilitates identification and characterization of DBZs. We obtain better precision in the relative depths of earthquake hypocenters by cross-correlating depth phase waveforms to determine their relative arrival times ($pP-P, sP-P,$ and $sS-S$) for a group of nearby events recorded at a common station. Relative depths are a critical measurement for determining the separation between planes of seismicity in DBZs. This technique is first applied to the Alaska subduction zone to facilitate comparison with hypocenters determined from local network data, and then we apply the technique to the Sunda subduction zone to illustrate a previously unreported DBZ. In the latter case, subsequent analysis of corresponding fault plane solutions indicates the typical pattern of down-dip extension in the lower layer, but the upper layer shows along-strike compression whereas other prominent DBZs show down-dip compression in the upper layer (i.e., Japan, Kamchatka, Tonga). The ability to identify DBZs away from local networks will be a key step in determining the overall prevalence and regularity of this feature on a global basis, which is necessary to understand the conditions, both seismic and petrologic, in the evolution of subducting slabs in general.
DE: 7230 Seismicity and seismotectonics
DE: 7294 Instruments and techniques
DE: 7203 Body wave propagation
DE: 7215 Earthquake parameters
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting