Japan issued a special advisory today warning of an increased risk of earthquakes at magnitude 8.0 or stronger, after a powerful jolt rattled the country's north and prompted a tsunami warning.
The Japan Meteorological Agency (JMA) said in a statement that "the likelihood of a new, huge earthquake occurring is relatively higher than during normal times".
The warning for the northern region came a few hours after a 7.7-magnitude earthquake struck the area, shaking large buildings in the capital Tokyo - hundreds of kilometres from the epicentre.
The quake, which was initially estimated at magnitude 7.4 before being revised to 7.5 and again to 7.7, hit at 4:53pm (8:53am Irish time) in Pacific waters off northern Iwate prefecture.
There were no immediate reports of serious injuries or significant damage, Chief Cabinet Secretary Minoru Kihara told a news conference.
Two hours after the tremor, tsunami waves as high as 80cm (31 inches) had been detected, though warnings remained for possible bigger waves at the top of Japan's main Honshu island, and the northern Hokkaido region.
Several port towns including Otsuchi and Kamaishi - both hard-hit by a massive earthquake and tsunami in 2011 - earlier issued evacuation orders for thousands of residents, according to public broadcaster NHK. Bullet train services were halted and some motorways were closed due to the tremors.
A Cabinet Office official told a separate televised briefing that "while it is uncertain whether (another) major earthquake will actually occur, we ask that you take disaster preparedness measures based on the principle that you are responsible for your own safety".
The quake measured an 'upper 5' on Japan's seismic intensity scale - strong enough to make it difficult for people to move around and cause un-reinforced concrete-block walls to collapse. The tremor had an epicentre in the Pacific Ocean and was 10km (six miles) deep, JMA said.
A three-metre tsunami could cause damage to low-lying areas by flooding buildings and carrying off anybody exposed in its currents, according to JMA.
Located in the 'Ring of Fire' of volcanoes and oceanic trenches partly encircling the Pacific Basin, Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes.
It accounts for about 20% of the world's earthquakes of magnitude 6.0 or more, such as the 2011 disaster that caused nuclear meltdowns at the Fukushima power plant.
There are no nuclear power plants currently in operation in the affected areas and Hokkaido Electric Power Co and Tohoku Electric Power Co said there were no abnormalities reported at their facilities there.
Japnan haunted by memory of massive 9.0-magnitude quake in 2011
Japan is haunted by the memory of the massive 9.0-magnitude undersea quake in 2011, which triggered a tsunami that killed around 18,500 people and caused the devastating meltdown at the Fukushima nuclear plant.
In 2024, the JMA issued its first special advisory of a possible "megaquake" along the Nankai Trough.
This 800km (497 miles) undersea trench is where the Philippine Sea oceanic tectonic plate is "subducting" - or slowly slipping - underneath the continental plate that Japan sits atop.
The government has said a quake in the Nankai Trough and subsequent tsunami could kill as many as 298,000 people and cause up to $2 trillion in damage.
The JMA lifted the 2024 advisory after a week but it led to panic-buying of staples like rice and prompted holidaymakers to cancel hotel reservations.
It issued a week-long second "megaquake" advisory in December 2025 after a magnitude-7.5 tremor struck off the northern coast.
The 8 December quake triggered tsunami waves of up to 70cms (27 inches) and injured more than 40 people, but no major damage was reported.
