In brief
Following a magnitude 7.5 earthquake in northeastern Japan, officials have issued warnings about the possibility of a future megaquake occurring in the Nankai Trough, raising concerns about potential catastrophic impacts. With predictions citing a 60-90% chance of a megaquake in the next 30 years, the government has advised residents to prepare for emergencies while emphasizing that no evacuation order has been issued and major earthquakes, despite being within statistical likelihood, cannot be predicted.
After a magnitude 7.5 earthquake struck north-eastern Japan on Monday, authorities again warned of the possibility of a future megaquake.
It means that thoughts in Japan are turning to the 'big one' - a once-in-a-century quake.
In September, Japan's earthquake investigation panel said there was a 60-90% chance that a megaquake would occur in the Nankai Trough - an area of seismic activity which stretches along Japan's Pacific coast - within the next 30 years.
In April, authorities had warned that a megaquake had the potential to trigger a tsunami of more than 20m (66ft) which could hit parts of Tokyo and other prefectures. They predicted that there would be around 300,000 deaths and trillions of dollars in economic damage.
So, what is the 'big one', can it be predicted - and is it likely to strike any time soon?
What did the latest warning say?
Officials urged residents in seven prefectures from Hokkaido in the north to Chiba in central Japan to stay on high alert for a potential megaquake.
This is a vast area with millions of people.
A government official said there was a possibility that 'a large-scale earthquake with a magnitude of 8 or higher could occur as a follow-up earthquake' in the region.
Authorities also told people to check evacuation routes, secure furniture, and prepare emergency kits, including food, water, and portable toilets.
However, an evacuation order was not issued.
Japan's director for disaster management said at a news conference that global earthquake data suggests there's a possibility, not a prediction, of a larger tremor to come.
Officials said the possibility of a larger quake occurring is about one in 100.
What is a megaquake?
Japan is a country used to earthquakes. It sits on the Ring of Fire and experiences about 1,500 earthquakes a year.
The vast majority do little damage, but significant quakes - like the one which struck in 2011 measuring magnitude 9.0 - sent a tsunami into the northeastern coast, killing more than 18,000 people.
But the one that authorities fear may strike in this more densely populated region to the south could be even deadlier.
Earthquakes along the Nankai Trough have already been responsible for thousands of deaths.
These so-called 'megathrust' earthquakes tend to strike every hundred years or so, often in pairs: the last ones were in 1944 and 1946.
But can earthquakes actually be predicted?
Not according to Robert Geller, professor emeritus of seismology at the University of Tokyo.
While the 2011 earthquake was preceded by a 7.2 magnitude foreshock, experts argue that predicting the timing or even the likelihood of a major earthquake is fraught with uncertainty.
