SHORT-PERIOD CONSTRAINTS ON THE PROPOSED TRANSITION ZONE DISCONTINUITY

Short-period P wave data from several events which occurred off the coast of northern Mexico and were recorded on the Southern California Seismic Network are used to investigate the existence and nature of the proposed transition zone discontinuity at 520 km. Synthetically, the 520 discontinuity produces a characteristic signature at 19-degrees to 22-degrees epicentral distance, seen as a small triplication which adds constructively to the first arriving phase to produce a large-amplitude first arrival. Convincing evidence for such an amplitude pattern is not present in record sections. Thus, the possibility of a diffuse discontinuity at 520 km is explored: several models with 1%, 2%, and 2.5% velocity contrasts distributed over zones from 10 to 30 km are tested. Again, evidence for this feature in our data set is negligible and does not justify its inclusion in our model. However, a model with a simple, sharp density contrast at 520 km produces subcritical angle reflections similar to those observed by other authors, and does not violate the short-period triplication data.