Earth's tides may help predict some of the most violent earthquakes on the planet.
In 2004 a magnitude 9.0 earthquake tore through the ocean floor off the coast of Sumatra. In the now infamous Indian Ocean tsunami that followed, nearly 300,000 people died, one of the worst natural disasters in history.
Could the catastrophe have been foreseen?
As tectonic stresses build along a fault, researchers have long suspected that Earth's crust would show some sign that it is about to break. They've examined everything from radon levels in groundwater to changes in the electrical properties of the ionosphere, but earthquake prediction remains tantalizingly out of reach.
But one researcher, Sachiko Tanaka of the National Research Institute for Earth Science and Disaster Prevention in Japan, has quietly built a case that giant quakes may be predictable using Earth's tides.
Strange though it seems, scientists have found that tides — the combined effects of the weight of the ocean and gravitational pull of the sun and moon — do have a small but noticeable influence on earthquake behavior.
In a new study, Tanaka analyzed 1,126 earthquakes up and down the fault zone off Sumatra, where the Eurasian tectonic plate grinds beneath the Australian plate.
In the years before the 2004 megaquake, she found that smaller tremors were increasingly triggered by Earth's tides. That same pattern held for two later quakes to the south of the 2004 event, a magnitude 8.6 temblor in 2005 and a magnitude 8.5 in 2007.
"These results suggest that tidal triggering may appear only when the stress in the focal region is close to a critical condition to release a large rupture," she wrote in a study due to be published in the journal Geophysical Research Letters.
In other words, as a fault zone winds ever tighter with stress and gets ready to pop, tides start causing more and more small earthquakes.
"It's actually pretty encouraging; it's sort of a case where the evidence has been building for a while now," John Vidale of the University of Washington in Seattle said. "The pattern she sees is fairly clear."
Vidale, however, notes that Tanaka's findings, along with a few others in the past few years, are still "controversial" in many ways. For instance, they may only apply to subduction zone faults like the one near Sumatra and others off Japan, Taiwan, and Alaska, leaving many other dangerous faults — like the San Andreas — shrouded in mystery.
Tide-triggered quakes could one day become a potent tool for moving people out of harm's way and saving thousands of lives. However, looking back at earthquakes and analyzing them is one thing; predicting them is another.