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.
Even before Thursday, it wouldn’t have been surprising for a large quake to occur at any time in the Nankai Trough, an official from the Meteorological Agency said, but the likelihood has now increased even further.
From a seismological perspective, the probability of a large quake in the Nankai Trough has become several times higher, the agency said. Such quakes happen roughly once every 100 years, but the exact timing cannot be predicted.
The most recent one took place on Dec. 21, 1946, which was measured between 8.1 and 8.4 on the moment magnitude scale.
Large earthquakes can occur without much warning, and people need to incorporate preparedness into their daily lives, the agency said, adding that steps like suspending train services, closing schools, and shutting down businesses in anticipation of an imminent major earthquake are often seen as difficult to implement effectively.
In the event of a major earthquake in the Nankai Trough, intense shaking is expected over a wide area from the Kanto region to Kyushu, and high tsunami waves are anticipated along the Pacific coast from the Kanto region to Okinawa, the agency said in a statement.
If an earthquake with a magnitude of 9.0 struck in the Nankai Trough, the shaking in the Kanto region could be around a lower or upper 5 on the Japanese seismic intensity scale, similar to the tremors the region experienced during the Great East Japan Earthquake in 2011.
Large quakes in the Nankai Trough have historically caused severe damage, originating from the plate boundary region stretching from Suruga Bay to offshore Hyuganada, according to the Cabinet Office.
According to the government’s damage assessments published in 2012, a tsunami caused by a Nankai Trough megaquake could result in up to 224,000 deaths, which can be reduced by roughly 80% with sufficient evacuation efforts immediately after the earthquake.
Thursday’s alert has already prompted some precautionary measures. Central Japan Railways, which operates the bullet train line that connects Tokyo and Osaka, announced Thursday evening that it would operate trains at reduced speed between Mishima and Mikawa-Anjo stations, resulting in at least 10-minute delays until the quake.