More quakes show power of Pacific 'Ring of Fire'

The quakes shook the South Pacific nations of Papua New Guinea and Tonga within 15 minutes of each other on Sunday morning.

Although the quakes were as powerful as that which hit the central Indonesian island of Java, there were no reports of deaths or subsequent tsunamis.

But experts believe the activity in the Earth's crust over the past two days -- including the awakening of the Mount Merapi volcano near the epicentre of Saturday's quake -- has all been linked to the Ring.

"There's no doubt they are effects of the same cause -- the ring of weakness in the Earth's surface," said Gary Gibson, professor of seismology at the RMIT University in Melbourne, Australia.

The Ring, which stretches along the western coast of the Americas, through the island nations of the South Pacific and on through Southeast Asia, is a series of fault lines -- or weaknesses -- in the hardened upper layers of the Earth's surface, known as the crust.

These lines of weakness are the meeting points of huge continental plates that make up the crust and which literally float on the molten rock of the Earth's core.

These plates are in constant motion, clashing into each other or moving away from each other, creating stresses and pressure build-ups at their margins.