Whether on land or undersea, the volatile edges of the north Pacific, bounded by the east Asian rim and the west coast of the Americas, are alive with near-constant seismic activity.

Some of the most dramatic natural disasters of recent history have happened within the Ring's arc, which stretches from Chile, north to Alaska and then west to encompass Japan, Southeast Asia and the Pacific islands.

From the nuclear-like explosion of Krakatoa volcano off the coast of Indonesia in 1883 to the Indian Ocean tsunami disaster that killed 220,000 in late 2004 the Ring's awesome power is legend.

The eruption of Mount St Helens in the United States in 1980, the freak quake that felled San Francisco in 1906 and the one that devastated Kobe, Japan, in 1995, were all part of the Ring's devastating toll.

Since the start of the year there has been an increase in seismic activity with dozens of earthquakes and a volcanic eruption within the Ring.

From January 3, when a huge quake occurred between Fiji and Tonga, to May 23, when a temblor shook Kamchatka, in Russia's Far East, there were 33 earthquakes on the Ring that have measured 6.0 or more on the Richter scale.

Adding to the tableau of upheaval is the angry awakening of the volcano Mount Merapi on the Indonesian island of Java, just north of the Saturday quake that rocked the island's southern coastline.

Twenty-nine of the quakes have taken place on the western side of the Rim, in an arc running from Kamchatka to New Zealand.

Six of the total involved quakes bigger than 7.0, topped by a 7.9 behemoth off Tonga on May 3. Six took place in Kamchatka and three near Tonga.

The reason for such geological volatility is the fragile fault lines that skirt the zone.

The Earth's crust is made up of a series of rocky plates that literally float on the molten rock of the Earth's mantle and core, interlocked over the entire planet like the pieces of a puzzle.

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.

The edges, or fault lines, are weak points in the planet's surface where the crust drops to just a few miles in thickness; at its thickest it is about 20 miles deep.

Many, mostly small eruptions occur, but occasionally huge volcanic explosions, earthquakes or landslides are generated, as pent up energy is released through the weak fissures.

According to the US Geological Survey (USGS), since 1900 there have been on average 19.4 quakes of 7.0-plus strength on the Ring each year, although there were only 11 in 2005. So this year's burst may not prove to be so exceptional.

Indonesia, which sits on the ring of fire, has suffered three catastrophic earthquakes within 18 months.

The 9.3-magnitude December 26 2004 quake that occurred west of Sumatra, unleashing tsunamis that crashed into Indian Ocean shorelines and killed 168,000 in Aceh alone, was followed by an 8.7 quake just 160 kilometres (100 miles) to the south on March 28 2005, killing more than 600.

Tremors continued to shake the region and scientists had warned a third big seismic event was possible in the region.

"The probability of a third quake in the coming months and years, cannot be excluded," Mustapha Meghraoui, of the Institute for Planetary Physics in Strasbourg, eastern France, told AFP last year.

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