Published in The Hindu on March 4, 2010
The 8.8 magnitude earthquake had its epicentre in the Pacific Ocean about 115 km from Concepcion, Chile; it occurred at a depth of 35 km.
This is not the first time that Chile has been struck by high magnitude earthquakes.
The coast of Chile has witnessed 13 events of magnitude greater than 7 since 1973. In fact, the most powerful one (the largest instrumentally recorded quake in the world) occurred off the coast of Chile in 1960; the magnitude was 9.5.
Seismologists are interested in Chile not because of the number of high-magnitude earthquakes that have struck Chile but for its tectonic setting.
While most places have trenches where two large tectonic plates meet, Chile is one of the few places where three major plates meet. In other words, it is a triple junction.
The Nazca Plate, borders the South American Plate for most part of the continent. But towards the tip of the continent and bordering Chile is the Antarctic Plate.
The Nazca Plate is subducting beneath the South American Plate at a rate of 80 mm a year; the Antarctic Plate is subducting beneath the South American Plate at a rate of 20 mm per year.
What makes Chile suffer from very powerful quakes is the fact that the angle of subduction (diving) by the Nazca Plate below the South American Plate is very shallow — 5 to 15 degrees.
The place where one plate subducts beneath another is called a trench. The place where new ocean crust is formed is called a ridge. It can be seen as the recycling of the ocean crust — new ocean crust is formed at the mid-oceanic ridges and is consumed (destroyed) at the subduction zones (trenches).
Sea floor spreading
This process of new ocean crust forming at the ridges and moving towards the trenches where it is consumed is the essence of sea floor spreading. It is because of this reason that the ocean crust is always younger than the continental crust.
In general, the rate at which ocean plates move is determined by the rate at which new ocean crust is formed at the ridges.
Mid-oceanic ridges are generally located far away from the trenches. This allows the ocean crust to age a bit before it is consumed at the trenches. However, in the case of Chile, the Chile ridge is located close to the Trench. That is not the only complex tectonic feature.
The Nazca-Antarctic Plate boundary is comprised of the Chile Ridge where new ocean crust is formed. And this Chile Ridge is subducted beneath the South American Plate at 46 degree south latitude. The southern Chile triple junction is a magnificent showpiece of a ridge-trench collision.
The Chile triple junction region is one of only two presently active examples of a ridge-trench collision. The other being the west coast of North America; the ridge-trench collision is very prominent along the California continental margin.
The Chile Ridge collides with the Chile Trench at a highly oblique angle. As the newly produced ocean crust moves parallel to the South American coast, the age of crust becomes progressively older from south to north along the eastern edge of Nazca Plate.
There have been 3 instances when the Ridge had collided with the Chile Trench. The first time was about 14 million years ago; the second was between 10 and 14 million years ago. The last time was 3 million years ago.
Studies have also shown that the rate at which the Nazca Plate is subducted beneath the South American Plate has not been constant. According to the USGS, the rate at which the Nazca Plate subducted was rapid (about 130 mm per year during Late Miocene; it is now subducting at a rate of 80 mm per year). The Chile triple junction is a magnificent place to study the ridge-trench subduction. It provides a window to understand what happened along the west coast of North America over the last 20 million years.