Sumatra: Caught between two plates

The violent earthquake that shattered Sumatra is the result of the island's precarious position, perched on the edge of a tectonic plate.

Just off Sumatra's south-western coast, two plates meet. The oceanic plate is being thrust under the island and its grinding motion sends periodic seismic tremors through the region.

Sumatra forms part of the Pacific Ocean's "Ring of Fire", a circle of violent earthquakes and volcanoes around the ocean's fringes.

In 1883, this region saw one of the most catastrophic volcanic eruptions in human history, when the island of Krakatoa all but vanished in a colossal explosion.

Trouble ahead?

The Sumatran earthquake is the first major tremor in the region for many years and will raise fears of further major quakes in the near future.

This happened after last year's Turkish earthquake. But whether an earthquake increases or decreases the chances of further quakes is difficult to predict and depends on the exact local tectonics.

On the one hand, a quake can release pent-up stress and therefore decreases the likelihood of further movement.

But on the other hand, a rupture can simply pass on the stress to the next segment of the fault, making a break more likely there.

Plate patchwork

The surface of the Earth is covered in a patchwork of about a dozen massive rigid plates, each about 80km (50 miles) thick. These lie over the Earth's hot, soft interior.

The hot rock deep under the surface slowly rises and falls, bringing heat to the surface and moving the plates. The two Sumatran plates are moving towards one another at six centimetres per year. The heat and the friction of the motion cause the earthquakes and volcanic eruptions.

The process by which denser ocean plates are thrust beneath lighter plates carrying land is called subduction. This process of destroying crust leads to the largest and deepest earthquakes and the most explosive volcanoes.

Two other main boundaries between plates exist. At mid-ocean ridges, two ocean plates move apart and new sea floor is created. The geological activity at these sites is less violent and usually far from populated areas.

At strike-slip boundaries, two plates slide past each other. Here, earthquakes can be violent, shallow and near cities, e.g. the San Andreas fault in California.

Shallow focus

Sunday's earthquake in Sumatra is believed to have occurred at a relatively shallow location, just 50km below the surface. Earthquakes in subduction zones can occur hundreds of kilometres below the Earth's surface.

A strike-slip fault exists in Sumatra, called the Great Sumatran fault. It has the potential to cause severe seismic damage, but the most recent quake is believed to have been caused by the subduction zone.

Due to the existence of four colliding plates, the whole Indonesian region is geologically active. In 1998, an earthquake struck Sulawesi island, in the east of the archipelago.

Ring of fire

These events are just a fraction of the activity on the Pacific Ring of Fire. The Pacific Ocean plate is growing in its centre, but being destroyed by subduction at its edges.

Consequent activity stretches from the frequent earthquakes in Japan and the Philippines, the volcanoes and tremors in the Andes, Alaska and Siberia.

Recent events include the severe quake that devastated Taiwan last year and the tsunami wave, which destroyed several coastal villages in Papua New Guinea in 1998. Tsunamis are caused by earthquakes under the sea.