A nice visualization of seismic activity occurring under the Tongan Ridge and subduction zone. This animation was done after the magnitude 8 earthquake on May 6th, 2006. There are several layers going on here. First is the surface bathymetry where you can see the Tongan Ridge.
Next is the cross section of the velocity waves (P-waves) that show crustal density. The arc of blue is the seafloor of the Pacific Plate subducting beneath the Australian Plate. The yellow and red color is the less dense rock (due to heating by friction at the plate boundary), which is directly under the Tongan Ridge (and probably portion of the Kermedec Ridge to the south).
The last layer is the points where past seismic activity has occurred. Each cube represents an earthquake, with the large diamond the big one on May 6th, 2006. Note also how deep these earthquakes occur and how they follow the subduction trench.
Given the recent earthquake in the Kingdom of Tonga, I thought it would be neat to revisit this to get a sense of the area and how active it is!