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USArray Observations of Quasi-Love Surface-Wave Scattering:
Orienting Anisotropy in the Cascadia Plate Boundary
Duayne M. Rieger (Yale University), Jeffrey Park (Yale University)
Love surface waves scatter to elliptically-polarized Rayleigh waves when they encounter regional concentrations of anisotropy.
These scattered waves, called Quasi-Love, or QL waves, are useful for detecting shear near plate boundaries, where the strong lithosphere
deforms mantle asthenosphere surrounding the plates as lithosphere sinks at oceanic trenches, spreads at rift zones, or crumples
where continents collide. QL waves can be observed on individual seismograms, and complement other analyses of seismic data.
We found QL scattering in and around the Cascadia subduction zone in seismic data from the land-based USArray, a component
of Earthscope. The dense sampling and broadband response of the USArray transportable array allows us to detect and
correlate the scattering of 100-sec surface waves at closely-spaced locations. We demonstrate that a pattern of Love-to-Rayleigh
scattering seen across the western USA (Figure 1) can be related to an anisotropic gradient that scatters energy from a location
offshore the Juan de Fuca trench. The azimuthal variation of scattering (Figure 2) indicates an alignment of the anisotropic symmetry
axis with present-day plate motion. We reject slab rollback as the causative process and suggest the entrainment of asthenosphere
with the overriding Juan de Fuca and Gorda plates.
The strong coherence of scattered waveforms between neighboring stations in USArray suggests that such long-period surface
wave motion can detect lateral gradients of anisotropy beneath Cascadia and, potentially, the rest of the western USA.
Detailed reconstruction of anisotropy could be tried by grouping great-circle paths that intersect at a distribution of possible
scattering points throughout the subduction system in an effort to further constrain its mantle dynamics.
References
Rieger, D. M., and J. Park, USArray observations of quasi‐Love surface wave scattering: Orienting anisotropy in the Cascadia plate boundary,
J. Geophys. Res., 115, B05306, doi:10.1029/2009JB006754, 2010.
Acknowledgements: This work was supported by NSF Grant EAR-0208652.
Figure 1. USArray record sweeps along the great-circle path for the Loyalty
Islands 4/9/2008 (Ms=7.3) earthquake. The Love wave on the transverse components
and the QL wave on the vertical-components are highlighted. The station
each record was recorded at is along the left Y-axis and the distance from the
great-circle path intersection at 42.5°N 127.5°W is along the right Y-axis. The
variation in amplitude of the QL wave with distance appears to be accompanied
by the gradual modulation of the waveform. This can be characteristic of an
interference pattern that would suggest a more complex anisotropic structure
than a single point scatterer with a horizontal axis of symmetry.
Figure 2. Spatial plot of the QL/Love amplitude ratio at 100s-lowpass, normalized
by the Love-wave amplitude along each great-circle path. The black circles
are station locations. The red line is the zero-scattering azimuth, parallel to our
proposed anisotropic axis of symmetry. This orientation correlates well with the
hotspot referenced Juan de Fuca plate motion, illustrated by the black arrows,
and regional shear-wave splitting measurements. The orange line represents
an oblique orientation to the anisotropic axis of symmetry suggested by the
data. The largest QL/Love amplitude ratios are measured close to this orientation
which is encouraging. The green line is the azimuth normal to the zeroscattering
azimuth.
II-196 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics