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

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