HR: 1340h
AN: S43B-0997    [Abstracts]
TI: Detailed Structure and Thickness of Upper Mantle Discontinuities in the Tonga Subduction Zone From Regional Broadband Arrays
AU: * Tibi, R
EM: tibi@seismo.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, St. Louis, MO 63130 United States
AU: Wiens, D A
EM: doug@seismo.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, St. Louis, MO 63130 United States
AB: Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the regional data provide smaller Fresnel zones and high frequency content for precise mapping of discontinuity topography and sharpness. This is particularly important for a subduction zone, where variations in temperature and water content may be expected which should cause changes in the elevation and sharpness of the discontinuities. We studied the phases $s410p$, $P660p$ and $S660p$. To enhance these low-amplitude phases, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct $P$ wave and subsequently slant-stacked. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660 are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660 in terms of local temperatures implies a thermal anomaly at 660 km depth of $-800$ to $-1200^{\circ}$K for the Tonga slab, and $-600$ to $-950^{\circ}$K for the piece of the Vityaz lithosphere. Except for the southern region where it thickens, the Tonga slab seems to penetrate the 660 with little deformation. Waveform modeling susggests that both the 410 and 660 discontinuities are sharp. The 410 thickness is estimated to be at most $\sim$10 km outside as well as within the slab, and the 660 width $\sim$5 km. This suggests that the subduction process has probabily little effect if any on the sharpness of the discontinuities.
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting