Rim of Fire: Repeat Event Still Threatens Japan, As Well
March 24, 2011 • 11:41AM

Strong new earthquakes struck near Japan on March 23: a 6.6-magnitude earthquake struck at a depth of nine miles (15 kilometers), about 353 miles (568 kilometers) northeast of the capital Tokyo, at 9:44am GMT (about 3:00 PM in Japan, 4:00 AM on the U.S. East Coast), according to the US Geological Survey. One-half hour earlier, a 6.4-magnitude earthquake had struck at a depth of 16.7 miles (27 kilometers), about 81 miles (131 kilometers) from the city of Fukushima in northeastern Japan. And before that, about noon local time, a 6.6-magnitude earthquake struck at a depth of 16.5 miles (26.5 kilometers), some 194 miles (312 kilometers) east from Fukushima.

None of these quakes was sufficiently large or slow in developing to generate a tsunami; none of them apparently interrupted the work of rehabilitating electricity and cooling at the Fukushima nuclear reactors, or resulted in additional deaths or injuries. However, a University of Colorado geologist writing an expert column for Bloomberg News today, described the aftershocks and follow-on earthquakes moving southward from the epicenter of the March 11 superquake, toward another region which is "overdue" for a 9 quake. Roger Bilham writes, "The nightmare scenario now unfolding in Japan could get worse. Seismologists have documented numerous times when an earthquake in one place has stressed a neighboring area, triggering another major tremor. Could this month's earthquake trigger what Japanese call the Tokai earthquake, which last ruptured the region south of Tokyo in 1854? With a recurrence interval of 110 years, Tokyo's residents know this event is overdue. Ominous aftershocks have been approaching Tokyo daily since March 11. A few sizeable aftershocks have even migrated to its south."

Bilham also says that the characteristics of the March 11 earthquake were not routine for geologists, who did not expect, in particular, that such a quake would produce dramatic vertical motion of the coast of Japan's main island, Honshu. "Had Japan's eastern shore not sunk as the sea surged toward it, the tsunami that drove onshore would have been less damaging, and its aftermath less tragic. Port facilities, beaches and most important, the 16-foot-high tsunami barriers for the Fukushima nuclear power plant, were lowered 3 feet by the earthquake. The barriers needed to be twice as high."