Quasi-Love Surface Wave Observations on USArray: Evidence for Upper Mantle Anisotropy Along the North American Plate Boundary

Strong evidence for mantle anisotropy exists along the North American and Pacific plate boundary, likely a result of deformation of upper-mantle rocks from the applied stress of tectonic motion. We have observed anisotropy-sensitive Love-to-Rayleigh scattered waves, also known as Quasi-Love waves, on the USArray component of EarthScope, on propagation paths that cross the North American plate boundary. These observed Quasi-Love waves may indicate the location and orientation of this anisotropy and help understand the tectonic motion along the North American plate boundary on the Pacific coast of the US. We have performed quantitative and qualitative tests to support the interpretation of these Quasi-Love waves against the misnomer of a surface-reflected body wave or Rayleigh overtone. All of our Quasi-Love wave observations are made after applying a 100 second low-pass to the data in effort to mute higher frequency Rayleigh wave overtones. The Quasi-Love waveforms that we have observed are the largest deviation between the USArray recorded wave trains and long-period synthetic seismograms computed using an isotropic Earth model. The observed amplitudes of the Quasi-Love waves fall roughly within 5-10% of the incident Love wave amplitudes, which is consistent with a 3-5% lateral anisotropic gradient along the great circle path [Yu et al., 1995]. Two example observations that we have made that support the proposed hypothesis are from a 3/31/2006 earthquake source near the Kermadec Islands region, where the observed Quasi-Love wave arrives from the west closely after the arrival of the Love wave, suggesting that the anisotropic scattering occurs relatively close to USArray stations in the Pacific Northwest. By contrast, no Quasi-Love waves are observed from a 4/5/2007 earthquake near the Azores Islands, which arrives at the same stations from the east. A particular point of interest is how the anisotropic scattering mechanism changes, if at all, between the Juan de Fuca subduction zone along the coast of Washington and Oregon and the transform plate boundary in California. Though the exact cause of the anisotropy, whether LPO, parallel fracturing, or a combination of both, may not be known, the orientation of the anisotropy will illuminate the stress and strain in the upper mantle and aid in the investigation of tectonic motion along the boundary.