DIRECT EVIDENCE FOR THE UNDULATION OF THE 660-KM DISCONTINUITY BENEATH TONGA - COMPARISON OF JAPAN AND CALIFORNIA ARRAY DATA

Short-period seismograms of Tonga deep earthquakes recorded by Japanese and Californian seismic networks are stacked to identify the S-P converted wave associated with the 660-km discontinuity. The travel-time difference between this S-P converted wave and the direct P wave is used to constrain the depth of the 660-km discontinuity. Analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the Tonga subduction zone. Two events which exhibit clear S-P conversions in both Japan and California data are selected to show directly the depth variations of the 660-km discontinuity adjacent to the subducting slab. The S-P conversion points on the ray paths to Japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to California, which represents direct evidence for a slab-induced depression of the 660-km discontinuity.