Two months after a massive earthquake triggered the catastrophe still plaguing Japan, the island nation continues to experience powerful aftershocks. But, contrary to the popular belief that quakes release the geological forces that caused them, a new study concludes the threat of major aftershocks looms larger than ever.
The common perception is that earthquakes relieve stress in the Earth's shifting tectonic plates. Applied to Japan, that would suggest the massive 'Tohoku-Oki' quake will translate into another three or four decades before the next mainshock. After all, that's been the pattern in there recent years, as magnitude 7 or higher quakes have struck the same area with a degree of regularity.
But, after poring over the reams of data recorded by "the extraordinary quality and accessibility of the Japanese seismic and geodetic monitoring networks" active during the magnitude 9.0 quake that struck on March 11, scientists say the quake might have actually added stress elsewhere along faults in the region.
"A major shock does relieve stress -- and thus the likelihood of a second major tremor -- but only in some areas. The probability of a succeeding earthquake adjacent to the section of the fault that ruptured or on a nearby but different fault can jump" significantly, the researchers wrote in a summary of their study published in the Tohoku Earthquake Special Issue of the journal Earth, Planets and Space.
"While the frequency of aftershocks decays rapidly with time after the mainshock, their magnitude does not decay at all, and so large late aftershocks are an unexpected consequence of a great earthquake," the authors said.
Basing their research on a theoretical model known as Coulomb stress triggering, the U.S. and Japan-based researchers found measureable increases in stress to the northeast at Sanriku-Hokobu, south at Off Boso and to the east at the Outer Trench Slope. Stress increases are also evident along on the base of the Kanto plate fragment they believe underlies Tokyo.
"Based on our other studies, these stress increases are large enough to increase the likelihood of triggering significant aftershocks or subsequent mainshocks," the researchers wrote.
As a result, they are warning that residents of central Japan -- including the 30-million strong capital region, Mount Fuji and central Honshu -- should be wary the threat of large, late aftershocks will continue to loom for years.
"So, in our judgment, Central Japan, and Tokyo in particular, is headed for a long vigil that will not end anytime soon," Ross Stein of the U.S. Geological Survey wrote.
While the quake that triggered a massive tsunami off the northeastern coast of Japan's main island was the fourth-largest in history, it stands as the best-recorded event of its kind ever.
