The 520 km discontinuity

摘要: Although some of the earliest suggestions a seismic discontinuity near 520 km depth in upper mantle came from studies structure to north Australia, results more recent deployments portable arrays short-period instruments are equivocal. Of several hundred earthquakes recorded during these experiments, only small fraction provide data that could help constrain properties discontinuity. few earthquake recordings can address question whether such exists, most indicate it does not exist as sharp feature (i.e., less than 50 thick) at depths 540 km. The do not, however, existence greater Furthermore, velocity increase be distributed over significant range; even 50-km transition would appear observations and still sufficient reflectivity vertical incidence compatible with long-period for waves traveling steeply mantle.