Anisotropy in the oceanic lithosphere from the study of local intraplate earthquakes on the west flank of the southern East Pacific Rise: Shear wave splitting and waveform modeling

Shear wave splitting is observed on ocean bottom seismometer records from l ocal, intraplate microearthquakes on the west flank of the East Pacific Ris e at 18 degrees S. Split times reach a maximum of about 0.2 s. For most of the incoming waves with long mantle paths (similar to 20 km), the polarizat ion direction of the fast arrival is subparallel to the spreading direction , which we attribute to anisotropy caused by the strain-induced preferentia l orientation of olivine. In contrast, for some records with short paths or shallower sources or propagation along the spreading direction, the fast d irection is nearly parallel to the ridge axis. These polarizations are prob ably caused by seismic anisotropy from aligned cracks in the uppermost crus t. In addition, for some events, the apparent splitting is frequency depend ent. To explore the pattern of shear wave splitting that would be expected for nonvertical paths in an oceanic lithosphere with two distinct anisotrop ic layers, we generate synthetic seismograms for a variety of source depths and mechanisms. We employ a multidomain, pseudospectral method to simulate the elastic wave fields from point sources in an inhomogeneous, anisotropi c medium. Splitting parameters measured from synthetic S waves demonstrate that the apparent fast direction is not always parallel to the symmetry axe s and that, in some cases, fast directions at higher frequencies will be mo re characteristic of the shallower crustal anisotropy while fast directions at lower frequencies are dominated by the mantle portion of the path. Most of the observed characteristics of splitting can be reproduced if there is approximately 8% S wave anisotropy in the mantle and an average of about 6 % S wave anisotropy in the upper crustal, seismic layer 2.