Stress-induced upper crustal anisotropy in southern California

Yang, ZH, Sheehan A, Shearer P.  2011.  Stress-induced upper crustal anisotropy in southern California. Journal of Geophysical Research-Solid Earth. 116

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anza seismic network, earthquake prediction, measurements, polarizations, premonitory variations, quantitative, san-andreas fault, shear-wave anisotropy, temporal variations, united-states, velocity anisotropy, western


We use an automated method to analyze shear wave splitting from local earthquakes recorded by the Southern California Seismic Network between 2000 and 2005. The observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression sigma(Hmax), suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern California is regional stress. However, at other stations, fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock. Splitting delay times range widely within 0.2 s. These upper crustal anisotropy observations, together with previous studies of SKS shear wave splitting, surface waves, and receiver functions, suggest different mechanisms of anisotropy at different depths under southern California. Anisotropy in the upper crust appears to be in response to the current horizontal maximum compression sigma(Hmax), which differs from the cause of anisotropy in the lower crust and mantle. We also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays.






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