Findings of a new study showed that, contrary to long ago assumptions that destruction of giant volcanoes along subduction zones could add to earthquake risks, the opposite could be true.

Geologists led by Professor Tony Watts of the University of Oxford, based their observations after they scanned the seafloor of the Tonga Trench, a 10.9 kilometers deep, which had been seismically quite. It is an earthquake-prone fault zone running between Tonga and Samoa, similar with the Japan Trench and the even deeper Mariana Trench to the south near Guam.

The images showed dozens of giant, flat-topped old volcanoes, called the Louisville Ridge Seamounts, riding westward atop the Pacific tectonic plate at up to six centimetres per year. Upon reaching the trench, these volcanoes are dragged down into the chasm.

The Tonga Trench is the second-deepest submarine canyon in the world, marking the boundary where a westward-moving chunk of the earth's outer crust, the Pacific plate, is forced downward, beneath the adjacent Indo-Australian plate, along a tectonic subduction zone
Researchers had previously thought that the volcanoes would add friction to the movement of the two plates, leading to a greater build-up of strain and, consequently, to more violent earthquakes in that spot.

However, Watts and his team saw that the doomed seamounts were already highly fractured and could explain why, contrary to expectations, the region is earthquake-free.
The team hypothesized that by breaking up early on, the volcanoes probably provide a kind of buffer that eases the subduction process, and actually reduced the risk of large, tsunami-generating earthquakes.

As to what will ultimately happen to the volcanoes, the researchers said they are still uncertain if these fractured volcanoes will be scraped off onto the Indo-Australian plate or chewed up and carried down deep into the mantle.