Japan On Alert for ‘Megaquake’ After Magnitude 7.6 Shakes the Nation

A magnitude 7.6 earthquake struck the eastern coast of Aomori, the northernmost prefecture of Japan’s main island, on Monday. As residents assess the damage, officials warn that another major quake could be close behind.

On Tuesday, the Japan Meteorological Agency (JMA) issued the “Off the Coast of Hokkaido and Sanriku Subsequent Earthquake Advisory†to 182 municipalities, from Hokkaido to Chiba prefecture.

This type of warning follows earthquakes of magnitude 7.0 or greater in or around what are known as “megathrust earthquake zones†along the Japan and Chishima undersea trenches. Seismic activity of that nature is associated with an increasing likelihood of a magnitude 8 or larger quake in the coming days.

The probability of such an event is still only about 1%, but if an offshore megaquake does strike the Hokkaido-Sanriku area, the Japanese government estimates the worst-case scenario could include a tsunami wave as high as 98 feet (30 meters) along Japan’s northeastern coast from Chiba to Hokkaido.

“Please understand that it remains uncertain whether a large earthquake will actually occur,†Prime Minister Sanae Takaichi said in a Tuesday statement. “On that basis, I would like to ask everyone to take appropriate disaster-prevention actions in line with the principle that one must protect one’s own life.â€

History no one wants to repeat

Together, the Japan Trench and the Chishima Trench form the subduction zone where the Pacific plate slides beneath Japan. This region has been known to produce clusters of large earthquakes, according to the JMA.

Monday’s magnitude 7.6 quake struck the subduction zone around 11:15 p.m. JST, with an epicenter located roughly 50 miles (80 kilometers) off the coast of Aomori. According to Prime Minister Takaichi’s statement, the shaking caused one residential fire and injured at least 30 people. Officials are still assessing the damage.

In 2011, a similar quake preceded the magnitude 9.0 TÅhoku earthquake and massive tsunami that devastated much of Japan’s northern coast. More than 18,000 people died, including several thousand victims whose remains were never recovered.

In the worst-case scenario, the Japanese government estimates that another offshore megaquake within this subduction zone could cause as many as 199,000 deaths. Due to the mass destruction of homes and buildings, up to 42,000 could be at risk of hypothermia.

Prepare, don’t panic

These figures paint a terrifying picture, but officials do not want this advisory to spark panic. Rather, they hope it will urge residents to exercise an abundance of caution, even if the risk of a megaquake is only slightly higher than usual.

“In addition to routinely confirming earthquake preparedness—such as identifying safe evacuation sites and routes and securing furniture—please stay prepared to evacuate immediately if you feel any shaking, while continuing your socioeconomic activities,†Prime Minister Takaichi said.

Monday’s earthquake serves as a stark reminder of Japan’s seismic volatility and the need for disaster preparedness. Even in the absence of another major quake, smaller aftershocks continue to rock the region.