Seismic forces that shape the world

The earth's crust is composed of giant, slowly moving tectonic plates.

On a fault line, these plates gradually slide over one another, but they can get stuck at their edges due to friction, which causes a build-up of stress.

This pent-up energy can be suddenly released when the plates begin to move past one another again, causing the ground to shake violently and generating energy waves that can travel round the world.

In short, an earthquake is a sudden slip of the land lying on a fault line.

The Indonesian island of Java is dominated by the movements of the Australian tectonic plate, which is sliding underneath the Sunda plate at a rate of about 6 centimetres per year.

The Australian plate dips north-north eastwards under the Sunda plate from the Java trench, a deep valley under the sea off the island's western coast.

Java is sitting on the Sunda plate, which is being lifted by the Australian plate sliding beneath it at depths of between 100 and 200 kilometres.

Further north, the Australian plate reaches depths of 600 kilometres.

The earthquake on Saturday occurred at a relatively shallow depth within the Sunda plate and well above the deeper Australian plate.

It measured 6.3 on the Richter scale.

Was the earthquake linked with the eruption of the Merapi volcano further north?

In general terms the seismic forces that cause earthquakes are separate from the magmatic forces that generate volcanic eruptions.

"Earthquakes may occur in an area before, during and after a volcanic eruption but they are the result of the active forces connected with the eruption, and not the cause of volcanic activity," according to the US Geological Survey (USGS).

In this case, the jury is still out on whether the earthquake on Saturday had anything to do with the on-going volcanic activity on Merapi, which is tens of kilometres further north.

"We do not know if there is a direct link," said the USGS over the weekend.

"The occurrence of shallow-focus earthquakes near volcanoes is not unusual worldwide. Sometimes the association of earthquakes and volcanic eruptions is so close in space and time that it is clear that the earthquakes are triggered by the magmatic processes that are causing the eruption. In the cases of many earthquakes that occur in the general vicinity of volcanoes, however, there are not obvious links to volcanic eruptions."

One possibility is that there is an underlying tectonic process behind both the seismic activity that caused the earthquake and the magmatic forces that generated the volcanic eruption.

Is it true that we are having more of these big earthquakes now than in the past?

There is no evidence that earthquakes are becoming more common.

In fact the USGS said it has recorded fewer of the really big earthquakes - magnitude 7.0 or greater - over the past 20 years.

The number has gone down very slightly from about 20 in 1970 to about 18 in recent years.

What has changed is the ability of scientists to detect earthquakes and the ability of the media to report from some of the most remote regions of the world.

Both could make it appear as if the Earth is becoming more seismically active.

In 1931, for instance, there were about 350 seismic recording stations in the world and they were limited in how fast they could transmit information.

Now there are more than 4,000 stations in a global network that can, with the help of satellites and modern telecommunications, transmit data across the world within seconds of an earthquake occurring.

- INDEPENDENT