Being able to predict an earthquake has long been a dream for the geosciences, but an Australian research team may have brought the world a small step closer to achieving this capability.
Key to this recent development is a greater understanding of the immense forces that created the Andes mountain chain of South America, and the emergence of a potential explanation for a mysterious time gap in that mountain-building process.
Until now, geologists have had a reasonable grasp of the role played by movement in the tectonic plates into which the earth’s upper layer, or crust, is segmented.
The Andes are a textbook example of an oceanic plate (called the Nazca Plate) driving under the South American continent; a ‘subduction’ movement invisible to the naked eye, but so powerful that it created the 7000-kilometre mountain chain – complete with its unusual bulge in the middle known as the Bolivian orocline (a distinct bend or deformation in a mountain range).
But, if plate tectonics are so easy to understand then why did the subduction process of the Nazca Plate start some 125 million years ago, but the Andes themselves only start to form around 45 million years ago?
“That time lapse of up to 80 million years has never been satisfactorily explained,” says Dr Fabio Capitanio, research fellow in the school of Geosciences at Monash University.
“A smaller time gap of, say, 10 million years would be easier to explain, but the very large lag required a fresh way of thinking, which is what we have been doing by using physics rather than geology to provide an explanation.”
That thinking led Dr Capitanio to the creation of a three-dimensional numerical subduction model that ‘explored’ what happened to the slow-moving Nazca Plate at the trench where it started to thrust under South America. The model, which incorporated variations in the thickness of the plate, arising from its age at the trench, produced a cordilleran morphology (mountain range shape) consistent with what we see in South America.
“The model predicted the behaviour of tectonic plates, and when applied to actual data tracing the Andes back 60 million years, there was a match,” says Dr Capitanio.
It means that for the first time researchers now appear to have a reliable computer model for predicting mountain-building forces, and these are the primary cause of earthquakes.
Dr Capitanio, in collaboration with researchers at the University Roma Tre, UPC Barcelona Tech and the University of California-San Diego, is now applying the model to other tectonic plate subduction zones, starting with India and its role in creating the highest mountains of all, the Himalayas.
“It is going too far to say that we can accurately forecast the time of a particular event, though in geological time which is measured in millions of years we are able to say which areas of the earth’s crust are susceptible to earthquakes, and broadly when they might occur,” Dr Capitanio says.
“But what we do have is a greater understanding of the forces driving tectonic plates which will help predict shifts in those plates and the consequences – the formation of mountain ranges, the opening and closing of oceans, and earthquakes.”
The research by Dr Capitanio and his collaborators was published as a letter in the December 2011 issue of Nature.
Technical as such a scientific report has to be, there is a simple message in the work: the forces driving the subduction of the Nazca Plate built up over tens of millions of years until a critical point was reached and the force was released in a comparatively rapid series of events that pushed the Andes higher in stages, with the Bolivian orocline an imperfection that proved the model.
In effect, Dr Capitanio's research team has applied the laws of physics to create a model of how the Andes might have been created, complete with the time lag between the forces that started the mountains rising. The researchers were then able to show that the model matches the geology.
Though he studied geology, Dr Capitanio says for this three-dimensional model work he ignored the geology.
“The model and the real geology are completely independent and yet they match. This is the novelty of the approach. It is the approach of the physicist to start with the balance of forces and then see how the model evolves.”
In terms of practical application Dr Capitanio says the model takes the understanding of major earth movements to a higher level. “The conventional approach describes what has happened so far … how many earthquakes occurred, what was the force released, and how much was the motion of the plates. But, we cannot tell where it is going.
“Now, with this model, we have shown that everything matches because our knowledge of the deformation is matched by the model. We can’t measure forces, but we can measure motions and deformations, so, we can make inferences about the trend of forces over time.”
And it is this that brings the potential for earthquake prediction that little bit closer to a future reality.
