East of Fiji, between Tonga and Samoa, is a feisty, earthquake-prone fault zone called the Tonga Trench that is the second-deepest submarine canyon in the world. Researchers have found that dozens of giant, flat-topped old undersea volcanoes quickly march toward the trench, ultimately taking the final plunge into the abyss.
Earthquakes and resulting tsunamis are a concern at the Tonga Trench, just as they are along the Japan Trench and the even deeper Mariana Trench to the south, near Guam.
A whopping 10.9 kilometers deep in some areas, the Tonga Trench marks the boundary where a westward-moving chunk of the earth's outer crust, the Pacific plate, is forced downward beneath the Indo-Australian plate next door.
Geologists long assumed that the destruction of giant volcanoes along these so-called subduction zones might add to the risk of earthquakes there. But Tony Watts of the University of Oxford and his colleagues noticed recently that the opposite might be true: Right at the spot where giant volcanoes are sliding to their doom, the Tonga Trench is surprisingly, seismically quiet. This past summer the researchers scanned the seafloor with sonar to find out why.