An attempt to detect reflected waves from the lower boundary of a descending slab

Abstract

We attempted to detect reflected waves from deep-focus earthquakes at the lower boundary of a descending slab. For this purpose, we calculated theoretical waveforms in the realistic heterogeneous velocity and Q structures beneath the Japanese islands, and compared them with seismic waveforms observed in the Kanto-Tokai districts of central Japan. Assuming that earthquakes occur within the deeper portion of the slab, we investigated the effects of variation in focal mechanisms, focal depths, velocity contrasts near the lower plate boundary, and the Q structures on theoretical seismograms on the earth's surface. The results show that the amplitudes of the P to P and S to S reflected waves gradually increase toward the subduction zone, and take maximum values near the trench axis. However, travel time difference between the direct wave and the following reflected wave decreases there, making it difficult to detect the reflected wave. Based on these theoretical calculations, we investigated digital three-component seismograms observed in the Kanto-Tokai districts by the National Research Institute for Earth Science and Disaster Prevention, Science and Technology Agency (NIED). The seismograms observed were obtained from deep-focus earthquakes, from 1990 through 1996, with magnitude of 5.0 and greater. After carefully checking seismograms from 15 deep-focus events, we could not detect the reflected waves from the lower plate boundary systematically at expected arrival times from the theoretical calculation. This indicates that the reflected waves are either masked by coda waves followed by the direct P and S waves or below the noise level that is detectable by seismometers. There is also a possibility that the seismic velocity contrast between the slab and the underlying upper mantle may not be so sharp. However, the possibility to detect the S to S reflected waves increases if we check observed waveforms in the horizontal range between 200 to 300 km landward from trench axis for a moderate size deep-focus earthquake with down-dip compression, which occurs near the upper plate boundary at depths of 300 to 400 km.