News

Study: Break-up of the Indo-Australian tectonic plate

Media Release, Thursday 27 January 2005

Australian and American researchers investigating forces exerted on the Indo-Australian tectonic plate have discovered that the considerable stresses being exerted on the plate could be leading to its breaking up.

ARC Professorial Fellow, Mike Sandiford, from the University of Melbourne’s School of Earth Sciences was recently awarded ARC funding for research aimed at understanding the forces that drive the motion of the Earth’s tectonic plates and the distribution of stresses in the Earth’s crust that give rise to great earthquakes such as the magnitude 9 Boxing Day Sumatran quake.

Professor Sandiford says the research shows that as much as 10 per cent of the huge amounts of energy being created at plate connection points at Sumatra and Java are being transferred back into our plate and causing major stresses.

“This is enough stress to contribute to mild earthquake activity in the central regions of the plate, such as in the Australian continent or central Indian Ocean, and provides us with clues as to why our plate has been slowly breaking up,” he says.

“The Indian Ocean quakes are, in effect, leading to the active rupture of the Indo-Australian plate into separate Indian and Australian plates. This new research provides us with important information about the stresses that are driving this drawn out tectonic plate divorce.”

The research, which was conducted in collaboration with Wouter Pieter Schellart of the Australian National University and David Coblentz of the Los Alamos National Laboratory in the US, will be published in the journal Geology (27 January).

The research is also important for understanding why smaller intra-plate earthquakes such as the 1989 Newcastle quake, which occurred nowhere near the edge of the plate, take place. Up to now, why earthquakes occur in apparently safe zones in the centre of plates has not been very well understood.

Professor Sandiford says, “Earthquakes such as the 1989 Newcastle quake that killed 13 people and caused more than $1 billion in damages are just one manifestation of mild tectonic activity that has been affecting the Australian continent for the last 5-10 million years.”

The new research shows that stresses originating at points of collision between two plates are being dissipated back into our plate and generating enormous internal stresses.

The ARC funded project will map the spatial and temporal pattern of this tectonic activity and relate it to the factors that drive the motion of the Indo-Australian plate.

“This research will contribute to our understanding of the factors that drive plate motion, to earthquake risk assessment in Australia and other comparatively stable continental regions, and to the factors that have shaped our distinctive Australian landscapes,” he says.

BACKGROUND INFORMATION

What causes tectonic plates to move?
The Earth is made up of a series of layers. The outermost layer is called the crust and the innermost is the core. In between lies an area called the mantle (in turn made up of upper and lower mantle), which is arguably the most important when it comes to understanding where big earthquakes occur.

The outer part of the Earth, comprising the crust and top 100 or so kilometres of the mantle (together called the lithosphere), consists of a collection of eight major plates which almost fit perfectly together. The continents and oceans all lie on these plates, the major ones referred to as the African, Eurasian, Indo-Australian, Antarctic, Philippine, Pacific, Nazca, North American and South American, depending on their geographic location.

These plates (also referred to as tectonic plates) float on the softer mantle underneath. Convection currents within the upper mantle layer are a major driving force driving plate motion and therefore behind earthquake activity.

The lower sections of the mantle closest to the molten core of the earth are also the hottest parts of this layer. These hottest parts of the mantle begin to rise through the layer (just like hot air in the atmosphere) until it reaches the lithosphere. Here, it cannot go any further and spreads out along the surface. As it rises it also cools and eventually reaches a point where it turns and starts to flow downwards again, carrying with it old parts of the oceanic lithosphere which descends back into the mantle along the deep ocean trenches at subduction zones, such as lies along the Indonesian archipelago. Thus convection causes very slow overturn of the mantle.

The movement is very slow, comparable to the length of time that it takes fingernails to grow, but it can lead to large motions between neighbouring plates. This motion generates huge amounts of energy and stresses, capable of breaking of the Earth’s crust along discrete fault planes – and thereby generating large earthquakes.

The forces that Professor Sandiford and his colleagues looked at:
When two adjacent plates collide, the one with ocean crust tends to lodge under the other. Portions of individual plates that extend down into the mantle and under an adjacent plate are called subducting slabs.

Because these subducting slabs are colder and denser than the mantle surrounding them, they tend to sink into it as slabs, pulling on the plate to which they are attached.

Professor Sandiford and his colleagues looked at the stresses generated along two segments of the subducting slab in the subduction zone between the Indo-Australian and Eurasian plate borders. These zones are at Sumatra and Java.

They then modelled how the energy generated by these subduction zones are distributed and found that about 90 per cent of the energy released by subduction beneath Indonesia is dissipated deep within the Earth’s mantle and the area where the Indo-Australian plate bends before sliding beneath Indonesia.

The remaining 10 per cent of the energy is transmitted back into the Indo-Australian plate.