The dimensions of scatterers in the lower mantle using USArray recordings of S-wave to P-wave conversions

Abstract

Seismic recordings from the USArray of the October 1, 2013 Sea of Okhotsk earthquake and the May 24, 2010 Western Brazil earthquake include high-amplitude SxP signals due to S-wave to P-wave scattering about 150 to 200 km below the 660-km discontinuity below these events. The scattering structures have been mapped previously and interpreted as fragments of folded crust that subducted into the uppermost lower mantle. The USArray recordings are unique because SxP is visible at all stations of the USArray over a distance of 2000 km. The arrival times of SxP with respect to the P arrival times vary by 2–3 s across the USArray.The 2–3 s arrival time variations of SxP suggest the finite dimensions of the scattering structures. We estimate the dimensions by beamforming waveforms for subsets of USArray stations and the uncertainty ranges by bootstrapping. In our analysis we assume that the high-frequency wave traveltimes can be modeled using ray theory and that wave-speed heterogeneity between the scattering point and the array does not affect SxP-P differential traveltimes. The results indicate that the scattering structures beneath the Sea of Okhotsk and western Brazil have dimensions of 20–160 km but the uncertainties are large. The variation of the SxP amplitude by a factor of two to three is likely due to the complex geometry of the folded crust. We attribute the down-and-up SxP waveform to the interference of S-wave to P-wave conversions at the top and bottom of a 10–20 km thick portion of the folded crust. Our results suggest that the subducted crust can retain thicknesses of 10–20 km even after being folded by subduction into the viscous lower mantle.