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A set of 142 S and ScS absolute travel times and 62 ScS-S differential travel times from short-period recordings of the January 29, 1971, Sea of Okhotsk deep-focus earthquake has been analyzed to obtain estimates of the source anomaly as a function of position on the residual sphere. In the processing algorithm the station anomalies and travel times are treated as gaussian random variables with known variance matrices. A system of normal equations is found by minimizing a quadratic form that is the sum of three terms: a measure of the misfit to the absolute travel times, a measure of the misfit to the differential travel times, and a measure of the misfit to the estimated station anomalies. An approximate solution to the system of normal equations is derived by requiring that the source anomaly be a smooth function of position on the residual sphere. This fitting procedure yields a saddle-shaped source anomaly pattern. The pattern is compatible with the presence of a planar high-velocity zone beneath the source with a nearly vertical dip and a strike parallel to the Kuril-Kamchatka Arc. Ray tracing calculations have been used to model the anomaly. A high-velocity slab extending to a depth of 1000 km along the extrapolation of the Benioff Zone, with a velocity contrast of 5%, is consistent with the data. The anomaly is interpreted to be the expression of lithospheric material that has penetrated the lower mantle. This study and data from other subduction zones suggest that lithospheric slab penetration below the 650 km discontinuity is a general feature of mantle structure in regions of rapid plate convergence. It is concluded that the lower mantle participates in the thermal convection responsible for plate motions.
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