The Characteristics of J-Array Seismograms.

The J-Array is a large-aperture seismic array system in Japan, which consists of 14 regional seismic networks deployed for research on earthquake prediction. During the project period (1991-1992), many seismograms from 61 teleseismic events (M.≥5.8) were collected for studies on structures deep inside the Earth. A report summarizing J-Array seismograms was published in 1994 in order to show the characteristics of seismograms and evaluate the quality of their waveforms. In this paper, the characteristics of J-Array seismograms are introduced and discussed briefly. The seismograms from a large, dense distribution of seismic stations have a high potential to detect weak onsets of unknown phases, which may reveal hidden structures in the Earth's interior. Using the J-Array seismograms, two distinct unknown phases are presented and their tentative interpretations are proposed. One of them is a reflection from the 410 km discontinuity in the upper mantle, and the other is from an unknown discontinuity in the lower mantle with a depth of about 1, 200 km. Both phases might have been missed, if the J-Array did not have a high potential to detect weak and obscure seismic phases, thereby proving the superiority of the J-Array. The limitations of the J-Array also became clear. It is not an ordinary seismic array in a strict sense, but joint observations of regional seismic networks. The use of non-uniform sensors and telemetering systems reduce the resemblance of waveforms among stations, and weaken the ability of the J-Array. In order to enhance the potential of the J-Array, these problems must be solved. The "New J-Array" project, which will start in 1995, is expected to resolve such problems.