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An array study of upper mantle velocity in Washington State

GEOPHYSICS PROGRAM UNIVERSITY OF WASHINGTON, SEATTLE, WASHINGTON 98195

Abstract

Short-period seismograph networks on both the western and eastern sides of the Cascade Range of Washington State were utilized to measure upper mantle P-wave (P_n) velocity from earthquakes and explosions at intermediate distances. Both modified time-term analyses and apparent velocity measurements from individual events were used to yield upper mantle velocities and sets of relative station delays. Absolute depths to the refracting interfaces are not determined from this analysis although the relative station delays may be interpreted as due to dip of the refracting interface (M-discontinuity) and shallow crustal structure variations. West of the Cascades a P_n velocity of 7.72±0.06 km/sec was determined, whereas east of the Cascades the corresponding velocity is 8.10±0.05 km/sec. No significant component of the anisotropy was reflected in the velocity measurements although the data suggest a shallow slope of the M-discontinuity, thickening toward the Cascades on both sides of 3° to 5°, or a slight decrease in average crustal velocity toward the core of the mountain range.

Footnotes

^* Now at Union Oil Company of California, Research Center, P.O. Box 76, Brea, California 92621

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