The Meteorological Agency on Thursday issued its first-ever alert warning about a possible megaquake around the Nankai Trough, following a magnitude 7.1 quake that struck earlier in the day off the coast of Kyushu.
It is believed that the chance of a major earthquake occurring in the Nankai Trough is relatively higher than usual, according to the agency, but it added that this does not mean an earthquake will necessarily occur within a specific timeframe. The warning is expected to be in place for a week, but officials said residents should not let their guard down even after it is lifted.
The JMA issues such warnings when abnormal phenomena are observed along the Nankai Trough, or when the possibility of an earthquake is believed to have increased, according to the Cabinet Office.
Information from the agency on Nankai Trough megaquakes is delivered at two levels — an alert and a higher warning. The weather agency issued the lower level alert on Thursday, urging people to be prepared for cases requiring evacuation. No evacuations are required for the alert level.
A warning means people would need to be on higher alert, and officials would urge those who wouldn’t be able to evacuate quickly to do so before any large quake occurs.
After initial alerts are issued, an expert committee of the weather agency meets to evaluate the situation and update the alert level based conditions such as the magnitude of the earthquake that happened at the plate boundary.
Thursday’s quake met the condition of being magnitude 7 or higher, leading to the lower alert level. A quake of magnitude 8 or higher would trigger the higher level warning.