Small-scale lateral variations in azimuthally anisotropic D″ structure beneath the Cocos Plate

Shear waves from intermediate and deep focus South American events recorded by California broadband seismometers reveal complex anisotropy in the lowermost mantle. ScS waves densely sample the D″ region beneath the Cocos Plate, just west of Central America (1–16°N; 267–277°E). Our data show evidence for significant near-source anisotropy, which we constrain for each event by performing polarization analyses on an expanded dataset of S waveforms. ScS waveforms are corrected for upper mantle and near-source anisotropy, and estimates of D″ anisotropy are made using the covariance method. We find splits between fast and slow components of ScS ranging from 0.4 to 2.5 s, with an average of 1.6 s. Fast polarization directions in the southern portion of our study area are approximately orthogonal to the raypaths. North of 7° latitude there is a rapid change in fast polarization direction to nearly raypath parallel. Beyond 10° latitude fast directions are more highly scattered than in southern regions. Past work in the region has inferred strong lateral variations in shear wave velocity, but fairly uniform transverse isotropy. In contrast, our results indicate that the lowermost mantle beneath the Cocos plate has azimuthal anisotropy that varies laterally over scales of 100–200 km. In addition, our data suggest connections between previously imaged topography on the D″ discontinuity and the character of anisotropy.