Yogyakarta, Indonesia's ancient capital and now a densely populated university city, lies almost exactly halfway between the rumbling volcano and the epicentre of Saturday's violent earthquake.

While the "Mountain of Fire", whose belching of searing clouds of gas and volcanic dust has forced the evacuation of tens of thousands of villagers, lies 35 kilometres (20 miles) to the north, the quake's epicentre is 37 kilometres to the city's south.

Both are created by a single force: the meeting of giant, shifting plates of the Earth's crust.

"It wasn't Merapi that set off the earthquake today," said Giuseppe Arduino, an Indonesia-based UNESCO geologist, but "the increase in tectonic activity which is causing the Merapi eruption also started the earthquake."

The Indo-Australian and Eurasian plates, as they are known by scientists, grind towards each other at rate of about five centimetres (two inches) a year, the first passing under the second.

The movement, called subduction, creates two results: a build-up of tension resulting in earth tremors, and the release of magma as the melting lower plate shoots back up to the surface.

The Indonesian archipelago, made up of thousands of islands and islets, is the world's most active zone for volcanoes with 130 still dangerous and has been plagued by earthquakes and eruptions for centuries.

Indonesia has seen the two biggest volcanic disasters of modern times: the Tambora eruption of 1815, the most deadly in history, and the cataclysmic Krakatoa explosion of 1883 which unleashed waves around the world and altered climates across five continents.

More recently, a major earthquake prompted the Asian tsunami of December 26, 2002, which left 220,000 dead mostly in Indonesia's Aceh province and was followed by volcanic activity on Sumatra island.

"In the framework of plate tectonics, it's true that there are links between tectonic activity in the Indian Ocean and volcanic activity in Sumatra," said Fauzan, an Indonesian geophysicist.

Similar activity was seen at the volcanic lake Toba after an earthquake at nearby Nias on March 28, 2005.

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