abstract

Theoretical ray paths through velocity models constructed from numerically calculated thermal models of slabs were computed. The results were in good agreement with observed travel times. First motion amplitudes of P waves at teleseismic distances were measured from long- and short-period WWSSN records of intermediate focus earthquakes in the Tonga, Kermadec, and Kurile regions and of nuclear explosions and shallow earthquakes in the Aleutian region. These amplitudes were corrected for source mechanism. The Aleutian data were sufficient to show that intermediate focus earthquakes in that region occur in the colder regions of the slab. At short periods, for regions other than the Aleutians, shadowing effects which could be associated with the slab were not very marked, less than a factor of 2 reduction for epicentral distances between 30° and 50°. No systematic effects due to plates were found in the long-period data. Some stations in the predicted shadow zone of a Tonga earthquake recorded low amplitude precursors which probably were greatly defocused waves which ran the full length of the slab. Simple diffraction is incapable of explaining the short-period results.

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