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.

This stress is released through volcanic eruptions, when the molten rock is ejected as magma through fissures in the crust, or via earthquakes, when the pressure causes the crust to buckle and shift.

Most of these seismic events are small and occur under the sea, where the majority of the continental plate margins are found.

But occasionally they generate volcanic explosions, earthquakes or landslides.

According to the US Geological Survey (USGS) there has been an average of 19.4 quakes of 7.0-plus strength on the Ring each year. Indonesia has suffered from three catastrophic earthquakes in the past 18 months.

The 9.3-magnitude quake on December 26, 2004 unleashed tsunamis that crashed into Indian Ocean shorelines and killed 168,000 in Indonesia's Aceh province alone.

An 8.7-quake just 160 kilometres (100 miles) to the south on March 28, 2005, killed more than 600.

Gibson said that the flurry of seismic activity of the past days was comparatively small.

"On any given day you would find seismic activity greater than this throughout the Pacific region," he said.

"Last week there was a sequence of earthquakes between New Zealand and Tonga that were far greater -- we got up to magnitude of 7.8 on one day," he said. "But nobody knows anything about it because it didn't affect anybody."

Although this activity is the result of the weak points in the Earth's surface, experts are not convinced that they create a ripple, or domino effect, of one quake setting off another.

"Earthquakes do tend to happen in clusters but they aren't triggered by one another," he said.

"The seismic wave created by an earthquake can travel for hundreds of kilometres and then dissipate. The Earth's crust is actually very good at absorbing that energy.

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