HR: 0830h
AN: S31D-0785    [PDF]
TI: Upper Mantle and Transition Zone Discontinuities by Comparison of PP and SS Precursors
AU: * Chambers, K
EM: Kit.Chambers@earth.ox.ac.uk
AF: Dept. of Earth Sciences, University of Oxford, Parkes rd, Oxford, OX1 3PR United Kingdom
AU: Woodhouse, J H
EM: John.Woodhouse@earth.ox.ac.uk
AF: Dept. of Earth Sciences, University of Oxford, Parkes rd, Oxford, OX1 3PR United Kingdom
AU: Deuss, A
EM: Arwen.Deuss@earth.ox.ac.uk
AF: Dept. of Earth Sciences, University of Oxford, Parkes rd, Oxford, OX1 3PR United Kingdom
AB: A number of previous studies have confirmed the global existence of the major mantle discontinuities by aligning seismograms on a surface reflection phase. Stacking large numbers of traces aligned this way yields signals from precursor phases, which have amplitudes too low to be seen in individual traces. Previous comparisons of PP and SS precursor data sets have considered global discontinuities (Flanagan \& Shearer [1999]). Here we also study the lateral variations in mantle discontinuities by considering PP and SS precursors from certain -well sampled- regions. The stacking procedure utillises a slant stack in which traces with varying offsets are aligned to a reference distance using a range of slownesses. The slant stacks are then converted to a trace in which the stacking slowness is time dependent, the time dependence being chosen to maximise the stacked amplitude corresponding to reflections from a continuous range of mantle depths. The weak precursor signals are enhanced using deconvolution techniques. We present stacks for PP and SS precursors which contain bounce points within a specified region. We observe and compare the observations of PP and SS precursors consistent with reflections from upper mantle and transition zone discontinuities. By comparison with WKBJ synthetics we attempt to constrain the variations in seismic parmeters which can explain the precursor arrival times and ampilitudes. The comparison of S-wave and P-wave underside reflections from mantle discontinuities allows better constraints to be placed on the discontinuity characteristics such as impedance contrast and discontinuity thickenss. Such information is a prerequisite for the integration of results from seismology and mineral physics to establish the causes of mantle discontinuities.
DE: 7207 Core and mantle
SC: Seismology [S]
MN: 2003 Fall Meeting