Propagation of high frequency P and S waves under oceanic structures
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The high frequency Po and So seismic waves observed at long distances under deep oceanic basins are investigated by means of numerical simulation. These waves are propagated particularly along the ray paths in relatively homogeneous and tectonicly stable regions of oceanic crust and upper mantle, and in continental shield areas. Propagation is terminated by structures such as midocean ridges, deep ocean trenches, and continental mountain chains. When expected variability in crust and upper mantle velocity depth structure is disrupted by such structural discontinuities, Po and So waves seize to travel. The seismic wave guide of crust and upper mantle may be continuous for hundreds of kilometers, as is the case in oceanic basins and continental shields. For these seismic ray paths, the wave guide is so efficient that waves recorded at large distances (as large as 3000 km or more) could include frequencies as high as 35 — 40 Hz. The long coda duration of 1 — 2 minutes is typical in these phases. The apparent velocity varies around 8.3 km/sec. for Po and 4.7 km/sec. for So. A special name, Po and So, has been given to these high frequency waves because of their predominance along oceanic paths.