HR: 0830h
AN: S31D-0786    [PDF]
TI: High frequency local reflections and conversions from upper mantle discontinuities in the Fiji-Tonga subduction zone
AU: * Tibi, R
EM: tibi@wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, One Brookings Drive, St. Louis, MO 63130 United States
AU: Wiens, D A
EM: doug@seismo.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, One Brookings Drive, St. Louis, MO 63130 United States
AB: Recordings of deep Fiji-Tonga earthquakes from an array of 15 broadband seismographs in Fiji are stacked and searched for reflections and conversions from upper mantle discontinuities near the Fiji-Tonga slab. The Fiji array operated as part of the SAFT (Seismic Arrays in Fiji and Tonga) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the local 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 study the phases $s410p$, $P660p$ and $S660p$ where they arrive at least 10 seconds after the direct $P$ wave and prior to the $S$ wave accross the array. To anhance low-amplitude reflections/conversions, deconvolved seismograms from each event are aligned on the maximum amplitude of the direct $P$ wave and slant stacked. Preliminary results indicate that for the northern part of the Fiji-Tonga subduction zone, the 660-km discontinuity varies between 660 and 670 km in depth. In the central part we observe converted phases consistent with a ``410'' depth of 380 km, indicating the effect of the cold subducting plate. The reflections/conversions show only a slight frequency shift relative to the direct $P$ waveforms, suggesting the discontinuities are relatively sharp. The thickness for the 660-km discontinuity is estimated as between 2 and 6 km.
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting