Do double ‘SS precursors’ mean double discontinuities?



Stacks of SS precursors have been widely used in the past two decades to investigate the existence and characteristics of upper mantle discontinuities on a global scale as well as in several regional cases. Here, we present observations of SS precursors from an Mb 6.7 earthquake recorded at the US Transportable Array in 2010. In this particular case, the S660S precursors on the transverse component are strong enough to be identified on individual seismograms across the array without any stacking procedures. Two S660S precursors are observed, seeming to suggest double discontinuities around 660 km depth in the bounce point region. Through careful analysis of 1-D and 3-D synthetic seismograms, we however discover that, although they have arrival times and slownesses that are very close to the theoretical values of SS precursors, the apparent ‘double precursors’ are artefacts because of mantle heterogeneity in the upper mantle near the receivers away from the bounce point region. This suggests that caution must be taken about appropriate azimuthal coverage at the SS bounce point, before interpreting double SS or PP precursors in terms of complex mineralogical transitions.