On April 11, 2012, an 8.6 magnitude earthquake in the Indian Ocean shook the Sumatran coast. Only a day later—3,900 miles (6,230 km) away—seismologists detected a set of smaller temblors rattling the eastern coast of Japan.
But this was no aftershock, those smaller rumblings that usually occur in the aftermath of an intense seismic event. Yet the two quakes may still have been related, according to a team of researchers from Los Alamos National Laboratories.
Earthquakes happen when pieces of the Earth's crust slip by each other, are stretched, or compressed. The points of contact are called faults (essentially, cracks). The stress builds and is eventually released, resulting in a sudden movement. After an earthquake, the affected region may, of course, experience aftershocks. For example, the Tohoku earthquake of 2011 moved parts of Honshu Island a full 13 feet closer to the U.S.
According to the research published today in the journal Science Advances, large quakes can also set set off smaller ones on a distant part of the globe by altering the way the rock responds to stress.
"In any kind of fault, you have everything from fractured rock to granular material," says Andrew A. Delorey, a geophysicist at Los Alamos National Laboratories who led the recent study. "When you shake that up, the way force is transmitted through it will change."
Whether a distant, large earthquake will trigger another fault the way the Indian Ocean quake did in Japan depends on a number of factors: The amount of activity that has already occurred, stress the fault has already endured and the kind of material in the fault itself.
Earthquakes and faults come in several varieties. At the boundaries between plates, faults generate quakes because the plates don't always smoothly slip by each other. In California and in the Indian Ocean off Sumatra, the plates slide against each other laterally; this is known as a strike-slip fault. In Japan, the Pacific plate is being driven underneath the one that carries the main islands, and that boundary is a convergent-type fault.
The area Delorey studied consists of so-called "normal" faults, which are areas the crust is stretching out and breaking, and the two sides of he fault are moving up and down relative to each other.
An earthquake sends seismic waves through the surrounding rock, and those waves can, and do, travel great distances. (This is one reason seismic detectors can pick up both earthquakes and nuclear weapons tests even when they are very far away). The Los Alamos study posits that those waves jostle the rocks in the areas immediately around faults, as well as the faults themselves, changes the way the material in the fault responds to stress.
A good analogy is a pile of gravel: Depending on its initial shape, the form it takes after you shake it will differ and with it, the way it would transmit force, Delorey says.
If there has been a lot of recent seismic activity in an area with faults, those faults can be put under more stress very quickly—this is what happened in Japan. An additional seismic wave can push them over the top so that they slip, causing a secondary earthquake.
In this case, the seismic wave from the Indian Ocean Earthquake hit the already stressed rock of Japan, which had experienced the 9.0 magnitude Tohoku quake only a year before.
In the study, Delorey's team looked at two small earthquakes that occurred just off the eastern coast of Japan 30 and 50 hours after the Indian Ocean quake. The temblors themselves were relatively mild, magnitude 5.5 and 5.7, respectively—people on shore wouldn't have noticed them.
The quakes occurred in a line, one after the other, describing a path that led right back to the Indian Ocean quake's epicenter. But the odds were against that pattern, with a chance of only 1 in 358 that they would happen coincidentally, according to the study.
The team also found that seismic activity in that area generally showed a sharp increase just after the Indian Ocean quake, which tailed off after several days. Delorey notes that he happened to study the area near Japan because the seismic monitoring there is exceptionally good, but if his hypothesis is correct, the same thing would show up elsewhere in the world.
Delorey's study isn't the first time that anyone has theorized large quakes causing smaller cascading ones, but it has never been directly measured.
This does not mean that a quake in Sumatra—or anywhere else—would necessarily cause problems for residents of California, for example, nor does it mean that a distant quake will always cause smaller ones somewhere else. Changes to the faults are also not permanent. The faults can recover their strength and resistance to slippage after weeks or months. It doesn’t even make an area more prone to shaking, explains Delorey. "It depends on the properties of the material."
The real benefit of knowing this happens is learning about the structure of faults. Large seismic waves can act like radar—by studying what happens to them before and after they trigger earthquakes elsewhere, it's possible to see the structure of a fault system more clearly. "If we do see triggered quakes we can learn something about stresses on that fault," Delorey says. "We really don't have a good handle on temporal changes in response to seismic hazards. These [studies] can get us a bit closer."