AN ATTEMPT TO DETECT REFLECTED WAVES FROM THE LOWER BOUNDARY OF A DESCENDING SLAB

We attempted to detect reflected waves from deep-focus earthquakes at the lower boundary of a descending slab. For this purpose, we calculat ed theoretical waveforms in the realistic heterogeneous velocity and Q structures beneath the Japanese islands, and compared them with seism ic waveforms observed in the Kanto-Tokai districts of central Japan. A ssuming that earthquakes occur within the deeper portion of the slab, we investigated the effects of variation in focal mechanisms, focal de pths, velocity contrasts near the lower plate boundary, and the Q stru ctures 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 grad ually increase toward the subduction zone, and take maximum values nea r the trench axis. However, travel time difference between the direct wave and the following reflected wave decreases there, making it diffi cult to detect the reflected wave. Based on these theoretical calculat ions, 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 caref ully checking seismograms from 15 deep-focus events, we could not dete ct the reflected waves from the lower plate boundary systematically at expected arrival times from the theoretical calculation. This indicat es that the reflected waves are either masked by coda waves followed b y the direct P and S waves or below the noise level that is detectable by seismometers. There is also a possibility that the seismic velocit y contrast between the slab and the underlying upper mantle may not be so sharp. However, the possibility to detect the S to S reflected wav es increases if we check observed waveforms in the horizontal range be tween 200 to 300 km landward from trench axis for a moderate size deep -focus earthquake with down-dip compression, which occurs near the upp er plate boundary at depths of 300 to 400 km. (C) 1997 Elsevier Scienc e B.V.