Direct evidence of the undulation of the 660-km discontinuity beneath Tonga:
comparison of Japan and US regional array data
Fenglin Niu, Hitoshi Kawakatsu
Geophys. Res. Lett., 22, 531-534, 1995.
Abstract
Short-period seismograms of Tonga deep earthquakes recorded by Japanese and US
seismic networks are stacked to identify the
S-P conversion wave associated with the 660-km discontinuity.
The travel-time difference between this S-P conversion wave and the
direct P wave is employed 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.
Moreover, two events which exhibit clear S-P conversions in both Japan and US
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 US,
which represents direct evidence for a slab-induced depression of
the 660-km discontinuity.