Japan’s first-ever alert for a possible Nankai Trough megaquake, announced Thursday evening following a magnitude 7.1 tremor earlier in the day, left many wondering what to make of the relatively unknown warning.
On Thursday, media outlets sent breaking news notifications about the Meteorological Agency’s decision to issue the alert, urging people to step up preparations in case the "Big One" hits.
The impact of Thursday’s quake so far appears to be minor, with 16 injuries and three buildings damaged reported in the regions affected.
But the alert that followed may have put residents on edge. How serious is the situation, and what should people do in the days to come? Here’s what you need to know.
The alert
At 7:15 p.m. Thursday, over two hours after the quake struck off the coast of Miyazaki Prefecture, the Meteorological Agency issued a megaquake alert.
At a news conference that followed, the agency said the chance of a major earthquake striking along the Nankai Trough is relatively higher than usual, although the alert doesn’t mean a major earthquake will necessarily occur within a specific time-frame.
However, considering that this is the first alert of its kind, the agency is urging people to be prepared for potential disasters while avoiding panic.
The agency issues alerts on the possibility of a megaquake when abnormal phenomena are observed along the Nankai Trough, or when the possibility of an earthquake is believed to have increased.
After initial alerts are issued, an expert committee of the weather agency meets to evaluate the situation and update the alert level based on conditions such as the magnitude of the earthquake that happened at the plate boundary.
Such notices are delivered at two levels based on the committee’s assessment — an alert and a more serious warning. The agency issued the lower level alert on Thursday, which means people should be prepared for potential cases requiring evacuation.
Thursday’s quake was higher than magnitude 7, leading to the lower alert level. A quake of magnitude 8 or higher would trigger the higher level warning.
The alert is expected to last for a week in principle, and can be lifted based on further expert analysis. But that wouldn’t mean the chance of a major quake striking anytime soon is zero.
Nankai Trough earthquakes
The Nankai Trough is a trench-like seafloor structure in the area where the Philippine Sea Plate and the Eurasian Plate meet, extending from Shizuoka Prefecture’s Suruga Bay to offshore Hyuganada, off Miyazaki Prefecture.
Large quakes in the Nankai Trough, which have occurred in the past in roughly 100 to 150-year cycles, have historically caused severe damage. The most recent one, on Dec. 21, 1946, was measured between 8.1 and 8.4 on the moment magnitude scale.
According to the government’s damage assessments, a large quake in the Nankai Trough could cause a maximum of 323,000 deaths and missing person cases in the worst scenario. The government aims to reduce that by 80% with adequate evacuation measures.
As it’s been 78 years since the last major quake in the area, the likelihood that a magnitude 8 to 9 class earthquake will strike in the next 30 years ranges from 70% to 80%, according to government data.
On top of that, there’s a possibility that the regions hit by a magnitude 7.0 earthquake or higher, such as the one that struck Miyazaki on Thursday, could be hit by additional quakes of magnitude 8 or higher.
In the event of a large-scale earthquake along the Nankai Trough, tsunami arrival times could be very short, giving people little time to flee to safety.
Authorities and transportation
Chief Cabinet Secretary Yoshimasa Hayashi said the government is closely monitoring the situation and urged the public to continue their day-to-day activities.
“We urge everyone to take preventive measures such as preventing furniture from falling over, checking evacuation routes and means of communications with their family, and evacuating immediately in case of an earthquake,” Hayashi told a regular news conference Friday.
He added that the government found three cases on social media where misleading information about earthquakes is being used to direct people to harmful websites, stressing the importance of caution online.
The government has two designations for specific areas requiring enhanced prevention measures, based on the projected intensity of a quake and the expected level of tsunami damage. Such cities and towns are required to set up their own procedures specifically for a possible megaquake in the Nankai Trough.
Meanwhile, the Wakayama Prefectural Government, after setting up a dedicated task force Thursday night, said it will have staff on rotation 24 hours a day in each department for the next week to coordinate any response to a potential earthquake. The Tokyo Metropolitan Government also established a similar task force.
The alert has also seen a response from railway operators. On Thursday evening, Central Japan Railway announced the suspension of some of its express trains.
In addition, while the alert is in effect, the Tokaido Shinkansen line, which connects Tokyo and Osaka, will operate at reduced speed between Mishima and Mikawa-Anjo stations, resulting in at least 10-minute delays. East Japan Railway took similar measures on portions of its Tokaido, Ito and Chuo lines.
Being prepared
Getting ready for an earthquake doesn’t require a huge effort. Make sure to have:
- Enough water for you and your family, with 3 liters per person per day for at least three days recommended
- Portable toilets and toilet paper
- A small emergency kit you can carry with you
- A car with a full tank of gas, if you have one
- Some cash readily available