Deep seismic zone as an upper mantle reflector of body waves

Abstract

THE deep seismic zone associated with the Izu–Bonin arc dips at about 40° towards the south-west beneath the Dodaira Observatory, Japan (Fig. 1). At this observatory an unexplained phase with apparent velocity of 16.5 km s⁻¹ was observed 10–20 s after the first arrival for earthquakes in the Ryukyu Islands. No corresponding phase has been found for earthquakes of different azimuths. This unexpected phase can be explained by post-critical reflection of P waves at the deep seismic zone after strong focusing by the 400-km transition zone: this zone of reflection is apparently sharp, of the order of 10 km in thickness. Typical ray diagrams are shown in the inset of Fig. 1. The ray path involves both the deep seismic zone and the 400 km transition zone.