What is Megaquake and why Japan has issued advisory on it

On August 8, 2024, Southern Japan was struck by a magnitude 7.1 earthquake. In response, the meteorological agency issued a warning for another potential earthquake, with a high risk expected over the coming week. This is the first time Japan has ever issued a 'megaquake' alert. The concern centers on the Nankai Trough, a subduction zone where there is a significant danger of strong shaking and tsunamis, with the potential to produce earthquakes of magnitude 8 or 9.

The Nankai Trough is an underwater subduction zone nearly 900 kilometers long, where the Eurasian Plate collides with the Philippine Sea Plate. This collision forces the Philippine Sea Plate under the Eurasian Plate and into the Earth's mantle. Stress builds up in the faults within the subduction zone, and when the locked fault slips, a megathrust earthquake, often referred to as a "megaquake," occurs. This region has a history of generating massive earthquakes.
A 2023 research study titled “High Probability of Successive Occurrence of Nankai Megathrust Earthquakes” explains that these earthquakes occur every 100-150 years, typically coming in pairs, with the second occurring within two years of the first. The most recent of these 'twin' earthquakes on the Nankai Trough occurred in 1944 and 1946. In 2022, Japan’s Earthquake Research Committee stated that there is a 70-80% chance of a megaquake occurring within the next 30 years. Such an event could send tremors to areas ranging from central Shizuoka, about 150 km south of Tokyo, to southwestern Miyazaki, according to reports from Reuters.
The United States Geological Survey noted that the August 8 earthquake occurred on or near this subduction zone. While the Nankai Trough has several segments, if the entire margin of the fault were to slip, it could trigger an earthquake with a magnitude of 9.1.

Japan, being a nation highly prone to earthquakes and tsunamis, cannot predict earthquakes but continues to develop techniques to recognize periods of heightened risk in disaster-prone areas.
In a news briefing, Japanese Prime Minister Fumio Kishida stated, "We ask all citizens to carefully check every new piece of information issued by the government,