Radial Anisotropy in the Deep Crust beneath the western US Caused by Extension

Radial Anisotropy in the Deep Crust beneath the western US Caused by Extension Amplitude of crustal radial anisotropy in the middle and lower
crust. Strong anisotropy is mainly restricted to the predominant extensional provinces (Basin and Range, Rocky Mountain Basin and Range, and Omineca Extended Belt) of the western US.
Although laboratory experiments have established that crustal rocks can be strongly anisotropic, the evidence for crustal anisotropy across large regions of the western US has not previously been documented. Surface waves can be used to identify crustal radial anisotropy (VSH != VSV), but the short period (< 20 s) surface wave dispersion measurements that are predominantly sensitive to crustal velocity structure are largely missing from distant earthquake signals because of scattering and attenuation. The development of noise interferometry methods now allows the measurement of surface wave dispersion at these shorter periods. Because Love and Rayleigh waves are predominantly sensitive to VSH and VSV, respectively, the simultaneous inversion of these measurements allows us to investigate the effects of crustal radial anisotropy. We invert Rayleigh and Love wave dispersion measurements from ambient noise and earthquake tomography for a radially anisotropic shear-velocity model of the crust and uppermost mantle beneath the western US (Moschetti et al., 2010a, 2010b). Where the Earth exhibits radially anisotropic properties, the effect of inverting for an isotropic model results in a characteristic data misfit termed the "Rayleigh-Love discrepancy" for which the predicted Rayleigh and Love wave speeds are faster and slower, respectively, than the observed surface wave speeds. An isotropic model results in large Rayleigh-Love discrepancies across most of the western US. We find that a model with an anisotropic uppermost mantle also results in a Rayleigh-Love discrepancy at periods that are mainly sensitive to crustal depths, but that the discrepancy is generally restricted to the Basin and Range (BR) and the Rocky Mountain BR (RMBR) provinces. The introduction of radial anisotropy in the deep crust resolves this discrepancy at all but a few, small regions in the western US. Within those geologic provinces that have experienced significant extension during the Cenozoic Era(~ 65 Ma), crustal anisotropy is often required to resolve the Rayleigh-Love discrepancy. Radial and azimuthal anisotropy in the upper mantle are generally ascribed to the alignment of olivine, and we similarly propose that the deep crustal anisotropy is caused by the alignment of anisotropic crustal minerals during crustal extension. This observation also supports the hypothesis that the response of the deep crust to crustal thinning is widespread within the extensional provinces.
</p><p>Moschetti, M.P., M.H. Ritzwoller, F. Lin, Y. Yang (2010a) Seismic evidence for widespread western-US deep-crustal deformation caused by extension, Nature, 464, 885-889.
</p><p>Moschetti, M.P., M.H. Ritzwoller, F. Lin, Y. Yang (2010b) Crustal shear-wave velocity structure of the western US inferred from ambient seismic noise and earthquake data, J. Geophys. Res. (In press)
</p><p>Acknowledgements: Research support from the National Science Foundation (NSF) (EAR-0450082 and EAR-0711526) and an NDSEG Fellowship from the American Society for Engineering Education to M.P.M. are acknowledged. The facilities of the IRIS Data Management System, and specifically the IRIS Data Management Center, were used to access the waveform and metadata required in this study.</p>


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