Topography on the 410-km seismic velocity discontinuity near subduction zones from stacking of sS, sP, and pP precursors

We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near-source, underside reflections off the 410-km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high-quality depth phase arrivals recorded by, several long-period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel-time curves reveals dear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross-correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25-s period corrected to an oceanic crustal thickness. The lateral resolution of our long-period stacks for 410 topography is limited compared to that sometimes achieved in short-period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410-km discontinuity in the local areas sampled by the precursor bounce points compared td broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410-km discontinuity neat subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long-period data. Coverage for the Tonga and Peru-Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the Slab orientation as large as 13 km. The cross-slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.