Figure 1

Event locations and geometry of the slab. (a) Map of the Tonga region showing the relocated epicenters of the events (circles, Table 1), with focal depths shown with a color scale. Thick dotted line is approximately parallel to the subduction direction, along which the vertical cross section of hypocenters and reflection points is shown. Contours of the depths of the Tonga slab are shown every 50 km from 250 to 700 km with thin dotted line (Gudmundsson and Sambridge [1998]). Plotted here and in the cross section (b) are essentially all the events large enough to be relocated accurately by incorporating array measurements of pP. (Inset) PDE epicenters of the deep and intermediate-depth earthquakes from 1970 to 2013 (deeper than 600 km, all magnitudes) are shown with red dots in a map of the larger area of the Tonga subduction zone. The study area is shown in the box in the middle. Gray irregular shapes are the islands of Fiji. (b) Vertical cross section of the relocated hypocenters and conversion (reflection) points along the thick dotted line in (a). The horizontal axis indicates the distance along the dotted line from the origin taken at 20.5°, 181.5°E, WNW to ESE from left to right. Red circles show the hypocenters of the deep events, while blue circles show the locations of the reflection points corresponding to the X phase (interpreted as the Moho). (Inset) Schematic figure of the Tonga slab showing hypocenters (red circles), former Moho (blue line), crust, and ray path of s-to-P waves (dotted line for S, and solid line with arrowhead for P). Dashed line shows the boundary between the former oceanic crust and the surrounding mantle.