Pervasive Complex Seismic Azimuthal Anisotropy beneath the Western United States Orogenic Zone


Whether the western US Orogenic Zone is underlain by a single layer or double layer anisotropy is still a debated issue. A careful examination of NA-SWS-1.1, a coherent shear-wave splitting database for North America [Liu, 2009] finds that the splitting parameters at the majority of the stations in the western US with adequate azimuthal coverage demonstrate a clear azimuthal variation with a 90 degree periodicity. Specifically, events from the southwest show a splitting time that is about 60% of that for events from the west and northwest. In terms of fast directions, events from the southwest have a fast direction that is greater than 90 degree, while those from the west and northwest result in mostly E-W fast directions. This particular pattern of azimuthal dependence breaks down locally in "tectonically anomalous" areas such as the Yellowstone and the Great Basin where a high velocity lithospheric drip has recently been proposed [West et al., 2009]. One of the simplest explanations of this apparent two-layer phenomenon is the existence of a stratified mantle flow system consisting of two layers of plastic flow, as recently suggested in the Afar depression in Ethiopia. Alternatively, one layer of anisotropy could reside in the lithosphere and the other in the asthenosphere. However, this is an unlikely model because the western US has a thin lithosphere with spatially varying dominant tectonic characteristics, yet the azimuthal variation of the splitting parameters seems similar across a large area. We fitted the apparent splitting parameters under a two-layer model using the procedure of Silver and Savage [1994], and found a two-layer model in which the lower layer has a nearly E-W fast direction which is parallel to the absolute motion direction of the NA plate, and the upper layer has a fast direction that is approximately parallel to the San Andreas fault (SAF). These observations support a pervasive two-layer anisotropy beneath the western US orogenic zone, and suggest the existence of SAF-parallel lithospheric or asthenospheric fabric extending at least 800 km from the Pacific-North American plate boundary.

Meeting Name

AGU Joint Assembly (2010: Aug. 8-12, Foz do Iguassu, Brazil)


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Seismology; Body Waves; Mantle; Lithosphere

Document Type

Article - Conference proceedings

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