Does Japan's megaquake alert mean the 'big one' is coming?

On the face of it, the earthquake that struck southern Japan on Thursday was not a big deal.

The magnitude 7.1 quake did little damage and the tsunami warning was quickly scaled back.

But the earthquake was swiftly followed by a warning - one which had never been given before.

There was, Japan's meteorological agency said, an increased risk of a "major earthquake". Japan's prime minister has cancelled a planned trip to a summit in Central Asia to be in the country for the next week.

For many in Japan, thoughts turned to the "big one" - a once-in-a-century quake that many had grown up being warned about.

Worst-case scenarios predict more than 300,000 dead, with a wall of water potentially 30m (100ft) striking along the East Asian nation's Pacific coast.

Which sounds terrifying. And yet, the overwhelming feeling that Masayo Oshio was left with was confusion.

"I am baffled with the advisory and don’t know what to make of it," she admitted to the BBC from her home in Yokohama, south of the capital, Tokyo.

“We know we cannot predict earthquakes and we have been told the big one is coming one day for so long, so I kept asking myself: is this it? But it does not seem real to me.”

So, what is the "big one", can it be predicted - and is it likely to strike any time soon?

What are Japanese authorities worried about?

The last Nankai Trough earthquake took place almost 80 years ago

Japan is a country used to earthquakes. It sits on the Ring of Fire and, as a result, experiences about 1,500 earthquakes a year.

The vast majority do little damage, but there are some - like the one which struck in 2011 measuring magnitude 9.0, sending a tsunami into the north-east coast and killing more than 18,000 people.

But the one that authorities fear may strike in this more densely populated region to the south could - in the absolute worst-case scenario - be even more deadly.

Earthquakes along the Nankai Trough - an area of seismic activity which stretches along Japan's Pacific coast - have already been responsible for thousands of deaths.

In 1707, a rupture along its entire 600km length caused the second-biggest earthquake ever recorded in Japan and was followed by the eruption of Mount Fuji.

The Nankai Trough sits between Suruga Bay, in central Japan, and the Hyuganada Sea in Kyushu to the south

These so-called "megathrust" earthquakes tend to strike every hundred years or so, often in pairs: the last ones were in 1944 and 1946.

Experts say there is a 70% to 80% chance of a magnitude 8 or 9 quake striking somewhere along the trough in the next 30 years, with worst-case scenarios suggesting it would cause trillions in damage, and potentially kill hundreds of thousands.

And this long-anticipated event is, according to geologists Kyle Bradley and Judith A Hubbard, "the original definition of the 'Big One'”.

"The history of great earthquakes at Nankai is convincingly scary" so as to be concerning, the pair acknowledged in their Earthquake Insights newsletter on Thursday.

But can they actually predict an earthquake?

Not according to Robert Geller, professor emeritus of seismology at the University of Tokyo.

“The issuance of the warning yesterday has almost nothing to do with science," he told the BBC.

This, he argues, is because while earthquakes are known to be a "clustered phenomenon", it is "not possible to tell in advance whether a quake is a foreshock or an aftershock”.

Indeed, only about 5% of earthquakes are "foreshocks", say Bradley and Hubbard.

However, the 2011 earthquake was preceded by a 7.2 magnitude foreshock, they note - one which was largely ignored.

The warning system was drawn up after 2011 in an attempt to prevent a disaster of this scale again, and Thursday was the first time the Japan Meteorological Agency (JMA) used it.

A massive earthquake in 2011 killed more than 18,000 people

But, crucially, while it told people to be prepared, it did not tell anyone to evacuate. Indeed, they were keen to play down any massive imminent risk.

"The likelihood of a new major earthquake is higher than normal, but this is not an indication that a major earthquake will definitely occur," the JMA said.

Even so, Prime Minister Fumio Kishida announced he had cancelled his plans to travel out of Japan to "ensure our preparations and communications are in order".

He added that he feared people would be "feeling anxious", given it was the first time such an advisory had been issued.

Masayo Oshio does not seem to be, however.

“I feel that the government is overplaying it,” she said.

Prof Geller was more scathing, saying the advisory was "not a useful piece of information”.

So why issue the alert?

The system allows for either a warning or a lower-level alert to be sent out. Thursday was an alert, advising people to be prepared to evacuate.

And, anecdotally, it seems to have worked. Even in a country used to receiving alerts on their phones, the "Nankai Trough" effect - and threat of the "Big One" - made people stop and take notice.

“One thing I did when I saw the advisory was to check what we have at home and make sure we are prepared, since I have not done that for a while,” admitted Masayo Oshio.

And this has been replicated along the Pacific coast.

In Nichinan, Miyazaki Prefecture, near the epicentre of Thursday's 7.1, officials were inspecting the conditions of already-opened evacuation shelters. In Kochi Prefecture, western Japan, 10 municipalities opened at least 75 evacuation shelters by Friday morning , according to Kyodo news agency.

The thermal plant operator Jera Co., a joint venture between Tokyo Electric Power Company Holdings Inc. and Chubu Electric Power Co., said it was on emergency alert, reaffirming communication routes with fuel carriers and evacuation protocols for piers.

In the town of Kuroshio, also in Kochi, elderly residents and others were urged to evacuate voluntarily to safer locations. Officials of Wakayama Prefecture, western Japan, confirmed evacuation routes in co-operation with local municipalities.

Prof Geller - for all his scepticism - says it is a good opportunity to "make sure you’re doing all the routine precautions you should be doing anyway”.

"Have a week’s worth of water on hand, some canned food, and then maybe have some batteries for your flashlight," he advises.

Additional reporting by Chika Nakayama and Jake Lapham