Do double ‘SS precursors’ mean double discontinuities?
Article first published online: 26 OCT 2012
© 2012 The Authors Geophysical Journal International © 2012 RAS
Geophysical Journal International
Volume 191, Issue 3, pages 1361–1373, December 2012
How to Cite
Zheng, Z. and Romanowicz, B. (2012), Do double ‘SS precursors’ mean double discontinuities?. Geophysical Journal International, 191: 1361–1373. doi: 10.1111/j.1365-246X.2012.05683.x
- Issue published online: 12 NOV 2012
- Article first published online: 26 OCT 2012
- Accepted 2012 September 18. Received 2012 September 9; in original form 2012 May 10
- Body waves;
- Wave scattering and diffraction
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.