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?
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 been 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.
