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Empirical Models for Anomalous High‐Frequency Arrivals from Deep‐Focus Earthquakes in South America

Summary 
 
Anomalous high-frequency arrivals from deep-focus earthquakes have been observed for epicentral distances of 13–21 degrees in Japan, Tonga and South America. Their arrival times are up to a minute later than those of S, their durations are longer than those of the direct phases and they have an apparent phase velocity of 4.7 km s-1. Isacks and Barazangi have proposed a model which fits the data for arrivals of this type observed at NNA in South America. Their model assumes the existence of structure heretofore not predicted by independent observation. Assuming the uniqueness of their model in its ability to fit the data, they have used it as evidence for the existence of these structures. We have found, however, alternative models which fit the data for arrivals of this type observed at ARE, CUS and TRU in addition to those at NNA. According to these models, a P phase travels from the focus to some discontinuity where it is converted to an S phase which travels on to the station. The conversion points predicted by these models lie near a boundary of the inclined seismic zone which, we speculate, defines a dipping lithospheric slab. We conclude that these anomalous arrivals can be interpreted using seismic-ray calculations based on laterally homogeneous velocity-depth models and a dipping interface.