A STUDY OF P-WAVE VELOCITY DISCONTINUITY IN D'' LAYER WITH J-ARRAY RECORDS - PRELIMINARY-RESULTS

We analyzed digital waveform data recorded at J-Array, a large-apertur e short-period seismic array in Japan, in order to search for P-waves reflected and/or refracted at a velocity discontinuity possibly existi ng at the top of D'' layer. During the period of April 1991-June 1992, Ne selected waveform data with good quality for 8 events; 3 events in California-Nevada, and 5 events in Fiji-Tonga-New Zealand. They are l ocated in the distances of 70 similar to 85 degrees from the J-Array, where the phases originating at D'' layer are expected. However, the P -waves from the events display no clear later arrivals with correspond ing slowness in the raw paste-up traces. In slant-stacked records from two events of Fiji Islands, there is a small peak with slowness and d elay corresponding to waves originating at D'' layer although these pe aks are not so prominent. For the other events, we could not observed the phases even in the stacked records. Comparing synthetic data with observed ones, there can exist a P-wave velocity discontinuity with a jump of 1.1 similar to 1.6% at the top of the D'' layer beneath the pa ths from Fiji Islands to Japan. However, the data set is only for the period of a year and we might need more good quality data for complete analyses.