Strong quakes cause delayed worldwide tremors – study

The person looks at the buildings destroyed by earthquake.(Reuters / Tarmizy Harva)
A massive earthquake that struck beneath the Indian Ocean in April went on to trigger a global wave of tremors. The discovery has revealed that one great tremor can reverberate throughout the world for days.

­In the six days following the 8.7 magnitude quake near the Indonesian island of Sumatra, the number of tremors registering at 5.5 or higher on the Richter scale jumped fivefold around the globe, a US Geological Survey-led (USGS) study published in the journal Nature revealed on Thursday.

The quake was caused by four undersea fault ruptures, all of which occurred in less than three minutes. If each rupture were to be considered a separate earthquake, their magnitudes would have been 8.5, 7.9, 8.3 and 7.8, the researchers found.

It was the largest of its kind ever recorded, and a fifth quake with a magnitude of 8.2 followed within two hours.
Hitting the seabed near Sumatra, the earthquake sent seismic shockwaves pulsing through the earth’s surface in every direction, sparking tremors up to 20,000 kilometers away.

Surface waves from the quake, described as Love-wave radiation, raced across the entire globe in a snake-like motion over the course of three hours. The vibrations triggered many more small tremblors, the report found, with faults as far away as Mexico and Alaska rupturing six days later.

Scientists believe the 2012 quakes were strike slip tremors, where one side of the Earth's crust moved horizontally past the other side.
As the horizontal movement did not induce a massive upheaval from the sea floor, it was notable for not causing tsunamis like the devastating 9.2 megathrust earthquake that occurred off the west coast of Sumatra in December 2004.

A megathrust quake occurs when two plates collide at their boundaries and one is forced underneath the other.

Seismologists have known for more than 20 years that major earthquakes can trigger small seismic slips thousands of miles away. The study, however, has redefined the threat level that large-scale quakes present on a global scale.

“Aftershocks are usually restricted to the immediate vicinity of a main shock,” lead author Fred Pollitz, a geophysicist at the USGS told Nature. He says that the April 11 earthquake should redefine conventional wisdom about how soon and how close aftershocks can occur to large earthquakes.

"The Pollitz paper shows that you can get significant, potentially destructive-sized earthquakes thousands of kilometers away from a mid-8 earthquake," Kerry Sieh, director of the Earth Observatory of Singapore and author of a number of studies on earthquake hazards in the Sumatra region, told Reuters.

Great quakes in this part of the world are likely to continue for some time, as a separate report stemming from the study confirmed the belief of many scientists that the Indo-Australian Plate is breaking into separate subplates.

"This is part of the messy business of breaking up a plate. … This is a geologic process. It will take millions of years to form a new plate boundary and, most likely, it will take thousands of similar large quakes for that to happen," says study co-author Keith Koper, the director of the University of Utah Seismograph Stations.