It's an example of irresistible force meeting an immovable object, but ultimately one cannot hold and dips under the other to release the tsunami, writes Dick Ahlstrom
A "devastating megathrust earthquake" is how the US Geological Survey (USGS) described the cataclysm that struck countries ringing the Indian Ocean on December 26th. The earthquake that resulted in a huge tsunami was the fourth largest to have occurred in the past 100 years.
Quakes are an inescapable part of life on this planet. They arise because of the structure of the Earth's crust, made up as it is of distinct slabs or "tectonic plates" that jostle against each other, says Prof Alan Jones, head of geophysics in the School of Cosmic Physics at the Dublin Institute for Advanced Studies.
The plates - some oceanic, others continental - push, pull or slide against each other and are in relentless but imperceptible motion. The plate boundary that separates us from the US is spreading apart, he says. "We are drifting apart from the US about as fast as your fingernails grow," he says.
The plate boundary along the mid-Atlantic rift is a "passive" boundary, he says. It doesn't usually produce powerful earthquakes, but does deliver volcanoes. Iceland lies along the rift and exhibits significant volcanic activity.
The quake that occurred off the west coast of northern Sumatra, deep under the Indian Ocean, occurred at an "active" boundary. Active plate boundaries either slide along or crash into one another to produce huge numbers of earthquakes both large and small.
Earthquakes in California are caused by plates sliding against one another, says Jones. "You have the Pacific Plate moving to the north relative to the North American Plate which is stationary. You get stress building up and eventually you get enough stress for the rocks to give way."
This results in an earthquake, the release of huge amounts of energy in a wave that moves away from the epicentre, the place where the rocks slipped against one another, says Dr John Walsh of the Department of Geology at UCD.
Similar rock slippage occurs where plates collide, with one plate sliding under or "subducting" another, he says. The African Plate is sliding under Europe where it sits on the huge Eurasian Plate, in turn triggering earthquakes around the Mediterranean Sea.
The Himalayas increase in height every year as a result of the mighty crash of the Indian Plate into the Eurasian Plate, with the Indian Plate moving north by about six centimetres a year.
Ireland sits far from any active plate, hence our relative lack of earthquake activity, he says. "When you are within a plate and far from the boundary, things are usually quiet."
Tectonic plate movement across the globe produces about 500,000 earthquakes a year, perhaps only 100,000 of them strong enough to be felt, according to the USGS. On average no more than 10 of these will cause damage, but when they do they can be devastating.
The quakes most likely to cause tsunamis often occur at plate boundaries undergoing subduction, says Dr Chris Bean of UCD's department of geology. The subducting plate creeps forward, all the time fighting friction between the plates and building up stress.
Eventually friction cannot hold and the advancing plate dips under and slips past the upper plate, Bean says. This jolt triggers the earthquake, but the seabed is also displaced upwards, pushing huge volumes of water out of the way and triggering a tsunami.
"This was a megathrust event, a dip-slip," says Bean. The displaced seabed thrust forward to throw up a tsunami wave.
The Sumatran quake was a rare magnitude nine on the Richter scale, one of the largest recorded over the past 100 years. Early analysis suggests that movement occurred along 1,300km of the plate boundary, with the seabed affected along a 1,000km stretch of this.
"It is not really clear what the displacement was but it was in the order of several metres," says Bean. This means that a pulse of water 1,000km long and two or three metres high was pushed out of the way by the earthquake.
The resulting tsunami rushed across the Indian Ocean, penetrating up to a kilometre ashore in parts of Indian and Sri Lanka.
Such was the power released, that the wave crossed the Pacific Ocean and was recorded in New Zealand and along the west coasts of South and North America.