Japan raises probability of megaquake to over 80% in next 30 years

The probability of a “megaquake” happening in Japan within the next 30 years has risen to over 80 per cent, the government’s earthquake investigation panel found.

A megaquake is defined as an earthquake with a magnitude of 8 or greater, bringing the potential for exceptional destructive power and a strong likelihood of generating a tsunami.

The most likely location for a megaquake in Japan is along the Nankai Trough, an 800km-long undersea trench near Japan’s Pacific coast, where the government panel said there was now more than 80 per cent probability of such an earthquake – up from a previous figure of 70-80 per cent.

Each year, the Headquarters for Earthquake Research Promotion updates the probability of earthquakes occurring along active faults and seabed areas around Japan, using data from 1 January.

“This probability is a number indicating that it would be no surprise if an earthquake were to happen at any time,” Naoshi Hirata, head of the expert panel and professor emeritus at the University of Tokyo, said during a press conference.

“We’d like to ask people to continue to be prepared.”

The panel found that the likelihood of a megaquake in the Japan Trench and Chishima Trench has also increased, with a 20 per cent chance of an 8.6-magnitude quake off the coast of Tokachi.

The cause of the potential earthquake is the movement of two tectonic plates: the Philippine Sea plate is slowly slipping beneath the continental plate that Japan sits on. As these plates move, they get stuck against each other. Over time, this causes energy to build up. When the plates finally break free from their stuck position, this energy is suddenly released, which could result in a megaquake.

Megaquakes in the Nankai Trough have occurred every 100 to 200 years over the past 1,400 years, with the most recent one recorded in 1946, according to Japan’s Headquarters for Earthquake Research Promotion. The earthquake, which was felt throughout Japan, destroyed 36,000 houses in the southern part of Honshu alone.

In August last year, the Japan Meteorological Association issued its first megaquake advisory since the 2011 Tohoku earthquake and tsunami, warning of an increased likelihood of a major earthquake along the Nankai Trough.

One year has passed since the 1 January earthquake on the Noto Peninsula in Ishikawa Prefecture last year. The panel also released an assessment report saying that the repeated occurrence of large earthquakes in the region over several years was “something Japan has never seen before”.

Japan is highly earthquake-prone due to its location on the “Ring of Fire”, where intense seismic and volcanic activity occurs.

“Japan sits on the boundaries of four tectonic plates, which makes it one of the most earthquake-prone areas in the world,” Shoichi Yoshioka, a professor at Japan’s Kobe University told CNN last year.

“About 10 per cent of the world’s earthquakes of magnitude 6 or higher occur in or around Japan, so the risk is much higher than in places like Europe or the eastern United States, where earthquakes are rare.”

The panel’s report, released on Wednesday, also mentions that regions with a high probability of experiencing a megaquake in the next 30 years include the coast off Nemuro in Hokkaido, along the Chishima Trench, and the coast of Miyagi Prefecture, along the Japan Trench.

The likelihoods for these areas are projected to be around 80 per cent and between 80 per cent and 90 per cent, respectively. The probability for Miyagi’s coast has risen from a range of 70 per cent to 90 per cent last year.

On 26 December 1707, all segments of the Nankai Trough ruptured simultaneously, causing an earthquake that remains Japan’s second-most powerful on record – referred to as the Hoei Earthquake – triggering the last eruption of Mount Fuji.

After World War II, Japan faced two megaquakes along the Nankai Trough in 1944 and 1946.

Not all experts are convinced by predictions of an imminent Nankai Trough megaquake, however. Robert Geller, a seismologist and professor emeritus at the University of Tokyo, criticises the idea of issues regular warnings about a Nankai Trough quake, calling it a “made-up construct” and a “purely hypothetical scenario” in a CNN report last year.

He argued that earthquakes don’t follow predictable cycles and can occur unpredictably, making it pointless to forecast future quakes based on past ones. This view contrasts with the traditional scientific belief in the “stick-slip” process, where stress builds up along faults and is eventually released in earthquakes.