String of earthquakes in Japan raises fears 'the Big One' is about to hit

A rash of seismic activity across an arc of southern Japan in recent weeks - including the most powerful earthquake to strike Tokyo in a decade - has triggered renewed concern that a major natural disaster may be imminent, potentially the much-feared rupture of the Nankai Trough.

Experts and government agencies have called for calm and insist there are no signs of an impending disaster to rival the March 2011 magnitude-9 quake, a tremor that unleashed a towering tsunami and caused devastation across much of northeast Japan. The Great East Japan Earthquake caused nearly 20,000 deaths and the meltdown of three reactors at the Fukushima Dai-ichi nuclear plant.

Underlining the ongoing instability in the region, a magnitude-5 tremor struck just off the coast of Fukushima at 2.29am on Wednesday, although officials were quick to confirm that it had not triggered another tsunami.

Do you have questions about the biggest topics and trends from around the world? Get the answers with SCMP Knowledge, our new platform of curated content with explainers, FAQs, analyses and infographics brought to you by our award-winning team.

Attempts to play down the public's fears have not been entirely successful, however, with the Asahi newspaper suggesting that "the frequency of tremors indicates that a megaquake could occur in the near future" and warning of "impending doom".

Tokyo was rocked by a magnitude 5.9 quake on October 7 and there have been a series of lower-level aftershocks across the Kanto region since then.

In the small hours of December 3, a level 4.1 tremor shook the flanks of Mount Fuji - one of Japan's most famous landmarks and a dormant, rather than extinct, volcano that last erupted on December 16, 1707.

The initial December 3 quake was followed by two more sizeable jolts in the vicinity over the next four hours, with some locals expressing concern that the tremors could bring the volcano back to life.

On the same day, a magnitude 5.4 tremor caused damage to homes and other buildings in Wakayama prefecture, in central Japan, with workers building a new city hall in Gobo forced to seek shelter outside the partially completed structure.

A few hours later, a level four quake shook Kanagawa prefecture, 500km to the east.

The seismic activity appears to have spread to the south, with more than 200 earthquakes reported in the Tokara Islands, to the south of Kyushu. Most have been relatively small, but there were four significant tremors on the morning of December 8 alone, ranging from a magnitude of 2.4 to a maximum of 3.2.

With much of the nation experiencing subterranean rumblings, many people are giving vent to their fears on social media.

"A level-four incident seems to happen every day somewhere in Japan at the moment; I must be prepared for something serious," said one poster on the Yahoo Japan news site.

Another added, "I heard a report that the 'Big One' will happen this month, so I am being cautious."

Takeshi Sugiya, a professor of crustal dynamics at Nagoya University's seismology research centre, told the South China Morning Post that the situation at present was "normal".

"The quake beneath Tokyo was at a depth of between 50km and 100km, but that is quite a normal event every 10 years or so," he said.

"The Tokara Islands are also a pretty seismically active part of Japan and these 'nests' of tremors occur from time to time," he added.

Despite the experts denying that the risk of a natural disaster has risen, they admit there is no effective way of predicting an earthquake and that any major seismic event is likely to occur with little or no warning.

They also concede that some of the most recent activity coincides with faults linked to the Nankai Trough, a 900km fault beneath the seabed off southern Japan, running from southern Kyushu to near Mount Fuji, which is widely seen as posing the largest seismic threat to Japan.

In late 2017, the Japanese government revised its estimation of the likelihood of a major earthquake in the Nankai Trough from 70 per cent to between 70 per cent and 80 per cent in the following 30 years. Studies suggest that large bouts of seismic activity within the trough occur at intervals of between 100 and 150 years. The most recent was in 1946, when nearly 1,400 people died in a magnitude 8.1 quake. In 1854, more than 3,000 people died in a tremor that measured 8.4 in magnitude.

Since then, however, Osaka has grown into a vast, sprawling metropolis that is home to millions and any quake of a similar scale, particularly one that triggered a tsunami, would cause vastly more damage and loss of life. A study by the Cabinet Office's Disaster Management Bureau estimated that 17,800 people would die as a result of a major earthquake off central Japan, while the economic impact could be as high as 57 trillion yen (US$502 billion).

"A near-future crisis, such as in the Nankai Trough, is predicted before the end of this century, but there are no signs that a big quake is imminent at the moment," Sugiya said. "The Japan Meteorological Agency is monitoring the situation constantly and it will issue warnings if it detects anomalies."

With the destruction caused by the Tohoku quake still relatively fresh in the public's mind - a quake that was not predicted - people will be hoping that the experts are able to give more of a warning before the next natural disaster strikes.

This article originally appeared on the South China Morning Post (SCMP).

Copyright (c) 2021. South China Morning Post Publishers Ltd. All rights reserved.