29 Small-scale Variation of SS Precursors Observed by US Transportable Array

SS precursors are the underside reflections from the upper mantle discontinuities (400 and 670 primarily) at half distance. They have almost identical ray paths as SS except in the vicinity of the bounce point; therefore their differential travel times (with reference to SS) and amplitudes provide ideal constraints on the discontinuity topography and the impedance contrast across it. They have been extensively used to investigate the discontinuities, mainly on the global scale (for a review, see Deuss, 2009) but also in regional studies if there is enough coverage (e.g. Schmerr and Garnero, 2006; Cao et al., 2011). SS precursors are usually at or below noise level. Therefore, they are difficult to identify on individual seismograms. In most previous studies, stacking (including non-linear) techniques are employed to bring out a clear phase, which results in robust travel time measurements. The amplitude information, however, is usually distorted or lost after stacking. In addition, constrained by data Dep= 650 km