Japan Lifts Tsunami Alert After 7.7 Quake, Issues Rare Megaquake Advisory

A magnitude 7.7 earthquake struck off the Pacific coast of Iwate Prefecture in northeastern Japan on Monday, triggering a tsunami warning that was later downgraded and prompting the Japan Meteorological Agency to issue a rare advisory on the heightened probability of a "megaquake" along the Japan Trench.

Shaking felt as far as Tokyo

The quake hit at 4:53 pm local time (07:53 GMT), with JMA placing its magnitude at 7.7. The US Geological Survey initially listed the event at 7.4. The tremor was felt across much of northern Honshu and shook buildings in Tokyo, hundreds of kilometers to the south, Al Jazeera reported.

Authorities initially warned of tsunami waves up to 3 meters (10 feet) along the Iwate coastline. Observed waves peaked at roughly 80 centimeters (2 feet 7 inches), and the warning was downgraded to an advisory before being lifted entirely, according to CNN and CNBC. No deaths or significant damage were reported.

Megaquake probability raised

Several hours after the quake, the JMA issued a rare special advisory for residents of northeastern Japan. The agency said the probability of a magnitude-8-or-greater event along the Japan Trench in the following week had risen to around 1 percent, up from a normal baseline of about 0.1 percent. Scientific American and the Japan Times reported the figures.

The JMA's 7-day elevated-probability metric is a standardized estimate and does not mean a megaquake is imminent. Scientists note that the 1 percent figure is roughly 10 times the ordinary baseline, a statistically meaningful jump that warrants public preparedness.

Tohoku echoes

Monday's quake occurred along the same Japan Trench subduction zone that produced the magnitude 9.0 Tohoku earthquake and tsunami on March 11, 2011, a disaster that caused widespread destruction across northeastern Japan. The JMA stressed that this week's advisory is a probabilistic precaution, not a forecast of an imminent event, and urged coastal communities to review evacuation plans.