Depression of the 660 km discontinuity beneath the Tonga slab determined from near-vertical ScS reverberations

The occurrence of the March 9, 1994 (M-W 7.6) deep Tonga earthquake during a temporary deployment of broadband seismographs provides an excellent opportunity to study the seismic discontinuity structure in the vicinity of a subducting slab, Near vertical ScS reverberations from the 410, 520, and 660 discontinuities are visible at 10 stations in the South Pacific at frequencies between 0.003-0.033 Hz without stacking. We use a regional velocity model and ray theory to model the interval from ScS to ScS3. A grid search is then performed on the depth and impedance contrast at each of the seismic discontinuities to optimize the fit to both the underside and topside reflections. Stacked records indicate prominent discontinuities at 400 km and 665 km depth beneath and to the west of the slab, and 410 lan, 500 km, and 640 km to the east of the slab. Analysis of individual records suggests a local depression of 30 km in the 660 and a broadening of the transition zone directly beneath and to the west of the Tonga slab, in a region of possible near horizontal slab. These results are consistent with a large cool region just to the west of the intersection of the Tonga slab with the 660.