Most of the world's earthquakes occur along the boundaries between Earth's constantly moving tectonic plates, like the San Andreas Fault in California. Small quakes along these faults are expected to occur relatively frequently, until they build up to the next big one. However, earthquakes that occur in the middle of continents, such as China's 2008 quake that killed around 70,000 people, seem to occur out of nowhere.
Now, new research from the University of Missouri suggests that inner-continental quakes such as China's may abide by a different set of rules than those that occur along plate boundaries.
Along plate boundaries, small and moderate earthquakes that rupture along a particular fault lead to a build-up of stress along that same fault line, but midcontinent faults are connected to each other in a complex network, and a large earthquake along one fault will instead put pressure on a different fault.
Mian Liu, a geologist from the University of Missouri, and his team studied data from large earthquakes — 6.5 or higher on the Richter scale — that occurred in inner-continental China over the last 2,000 years. Surprisingly, they found that the same fault segment had never ruptured more than once. Instead, the earthquakes migrated throughout the region.
"It's like a game of whack the mole. The mole doesn't come through the same hole twice," Liu told OurAmazingPlanet.
When the team analyzed the locations of the quakes and the energy each released, they found that the data indicated that the earthquakes were not migrating randomly. Instead, the faults seem to be mechanically coupled to each other, so that when one fault segment is ruptured, the energy is passed along to another fault segment, stressing it until it ruptures.
Liu said the new theory is not yet able to predict when or where the next big earthquake will occur along inner-continental fault systems, but he said it is a new way of thinking about midcontinent earthquakes. For instance, a large earthquake occurred in the New Madrid fault system in Missouri in 1811, the largest-known earthquake in the midcontinental United States. Because of that, the fault system has been the subject of much research, but Liu's theory suggests that the next big earthquake in the United States will occur along a separate fault.
The work shows that applying hazard models to continental interior fault systems that have been designed for plate boundaries is likely to be inadequate, said Eric Calais, a geophysicist at Purdue University, who also is a scientific adviser helping revamp a disaster preparedness plan for Haiti.
"When it comes to earthquakes in plate interiors, one truly has to think out of the box and be able — and willing — to abandon plate boundary concepts," he said.