Conspicuous phases between P and S arrivals observed in the DPRI seismic network

Abstract

In central Honshu, Japan, numerous phases between P and S arrivals are frequently observed in seismograms of deep earthquake events. We show in this paper some examples of seismograms of these later phases. From the seismograms in the events of the Hokkaido and Ryukyu regions, we can observe triplication resulting from seismic discontinuity at depths of 410km and 660km. Some of later phases observed at events occurring below the Japan Sea region may be related to an anomalous low velocity structure in the upper mantle beneath eastmost Asia. Moreover, we observe some later phases which are generated at the upper portion of the slab or within the slab interface. Using travel time of observed later phases, we estimate that layer structure exists within slab at the depth of 100-150km and deep earthquakes occur 20km apart from upper slab interface at the depth of 300-500km. Moreover, another later phase is observed in the deep events of the Bonin region. Apparent velocity of the later phase is much smaller than that of direct P, which implicates that the later phase is trapped in the crust or uppermost mantle beneath the Nankai trough. We can not, however, explain the nature of all the observed later phases in the present paper.