Publisher： John Wiley & Sons Inc
E-ISSN： 2156-2202|98|B12|22109-22122
ISSN： 0148-0227
Source： Journal Of Geophysical Research, Vol.98, Iss.B12, 1993-12, pp. : 22109-22122
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Abstract
Travel time tomography was used to obtain the lateral variations in Pn velocities and Pn velocity azimuthal anisotropy beneath the Basin and Range Province. We used 1226 Pn arrivals from 184 earthquakes recorded by 113 stations. The tomographic image shows the lateral variation in Pn velocity from 7.9 km s−1 in the northern Province to 7.6 km s−1 in the southern Province. Azimuthal variations in Pn velocities are less than one percent as determined from the travel time residuals after taking out the contribution of the lateral velocity variations of the Pn tomography. Although the standard errors of the azimuthal variations are ∼ 2%, there is no reason to believe the existence of the Pn azimuthal anisotropy in the uppermost mantle beneath the Basin and Range Province. Beghoul and Barazangi's (1990) suggestion of more than 3% azimuthal anisotropy is the result of the lateral heterogeneity and mantle velocity gradient. Before tomographic inversion, we refined the Pn travel times by considering the lateral variations in the crustal thickness and velocities in the region based on the numerical experiment and theoretical development given in this paper. These refinements are crucial in obtaining an accurate velocity image. The average velocity is 7.72 ± 0.16 km s−1, and an average vertical mantle velocity gradient is 8.0 ′ 10−4 s−1. The resolution for Pn velocity image is generally within 2° to 5°.
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