The Nankai Earthquake

What is the Nankai?

It’s a huge earthquake that occurs every 100-150 years and affects all of Kochi Prefecture.

The Nitty Gritty:

The Nankai is a subduction fault earthquake. What does this mean? Well, earthquakes are caused by the movement of tectonic plates, which creates breaks/ruptures in the earth’s crust called faults. There are 3 main kinds of faults: The first kind is a normal/divergent fault, which is when the plates move away from each other and create volcanic ridges, or rift valleys. The second kind is a reverse/convergent faults, which is when the plates push against each other to create folded mountains. The third kind is a transform fault, which is when the plates slip/slide along each other to create something like the San Andreas Fault in California. In regards to the Nankai, is the second kind of fault, a reverse/convergent fault. Basically, the plates push against each other and create a lot of tension. This builds over time until eventually, one plate slips below the other in one big movement. This results in a mega earthquake which can be highly destructive.

Actually, the Nankai is part of a chain, part of a larger system of faults that affect all of southern Japan. This chain of faults is part of the Pacific Ring of Fire. Essentially, the Ring of Fire is the area where the Pacific plate interacts with other plates, often resulting in subduction faults (where one plate goes over the other). In other words, it’s where many earthquakes and volcanic eruptions occur because the tectonic plates in this area just can’t sit still. In fact, about 90% of the world’s earthquakes (and most of the biggest ones) and volcanic eruptions occur. To put it simply, because Japan is on the Ring of Fire, it gets a lot of earthquakes including major ones like the Nankai.

What you need to know:

  • It occurs every 100-150 years.
  • It’s typically a magnitude 8+ on the Richter scale or a 5-7 on the Shindo scale in Kochi. The next Earthquake is est. to be M 8.5+. For more on the Shindo scale, click here. To understand the difference between the Richter/magnitude scale and the Shindo scale, click here.
  • The last Nankai was in 1946 but as it turns out, this earthquake was quite small, which means there is still a great deal of stress left to be released. As a result, there is a strong likelihood the fault-line will not wait the usual 100 years, and the next earthquake can be expected in the first half of this century. Estimates say there is a 70-80% chance that the Nankai will occur within the next 30 years. After that the chances will further increase. To clarify, this doesn’t mean that there’s a 70% chance of it happening tomorrow. This means that as we get closer to 30 years, the more the likelihood of it occurring will incrementally increase.

The Showa Nankai Earthquake of 1946

The earthquake occurred on December 21st, 1946. It was a magnitude 8 earthquake that struck 50km off of Shiomizaki, Wakayama. That’s where the epicenter was. While this earthquake was smaller than expected, there was still a lot of damage: 679 killed, 1,836 unaccounted for and 4,846 homes destroyed. The earthquake wasn’t the only cause of the damage; a 4-6 meter tsunami resulted from the earthquake and also caused a lot of damage.

  • The worst-case scenario prediction for the tsunami that will be caused by the Nankai is 34 meters in Kuroshio. This is about as tall as the 11th floor of a building. Keep in mind that this is how high the tsunami will be AT SEA LEVEL. So how high the tsunami will be for you will depend on where you are and how high about sea level you are. But even then, it will still be really high and very destructive.
  • This is a prediction of how long it is expected for the Nankai tsunami to hit Kochi prefecture.
Nankai evac pic
Those in the Red have the least amount of time, about 5-30 minutes. Assume you have less time than is predicted. Those in the Yellow have about 15-35 minutes. Keep in mind that while the earthquake is happening, you won’t be able to move. But the second the earth stops shaking, you need to get going.

Preparing for the Nankai

You should have all received this booklet by the KIA about how to prepare for the Nankai. Be sure to check it out! Just in case, here is some more information on preparing for the Nankai.

What to do before it comes:

Prepare an evacuation kit.

Quakeproof your home – This includes securing large furniture to walls, fastening down smaller objects, keeping heavy objects close to the ground, ets. Make sure to ask your landlord before doing any major changes! In regards to your bedroom, keep it as clear of non-bed stuff as possible, and make sure escape route is not blocked! Try to keep a flashlight, whistle and a pair of comfortable, practical shoes nearby.

Plan your escape. Make sure you know where your evacuation points are and how to get to them. Even when you are out and about, keep an eye out for potential evacuation spots. Also, familiarize yourself with vocabulary related to emergencies so you are prepared to understand what is happening should an emergency occur.

Find your closest evacuation point with this site: (Japanese only)

Stay informed.  You should get lots of warnings before the quake happens. Pay attention to evacuation/emergency alerts (via NHK or apps like Safety Tips or Yurekuru Call).

What to do when it comes

What to do in an earthquake? Drop, cover hold. If you’re in bed, roll over, assume the fetal position, and guard the back of your neck. It is better to stay put than trying to find a distant shelter in an earthquake. Familiarize yourself with safety procedures during an emergency.

Next, when the earthquake is over, you’ll need to grab your evacuation kit and evacuate immediately. Get to the closest evacuation point or to the closest higher ground – whether this is a tsunami tower or anything else that is higher.

While you’re evacuating, be careful to avoid the following:

  • Falling debris
  • Any and all buildings if possible (Except tsunami towers/designated escape buildings)
  • Fallen or falling walls or signs
  • Fallen power lines/utility poles
    • Puddles or bodies of water these may be touching

Upon reaching your evacuation point, you must wait until you have been given the all the clear by an official. Because tsunamis come in a series of waves that can last up to six hours, you must wait at your evacuation point until it has passed.

Next, do your best to get in contact. Phones will likely be down or jammed. In past major earthquakes such as the Sendai Quake in 2011, internet was the most reliable means of communication. Take a look at the Block System to understand how to get in contact and respond using the Block system.

Again, when it comes:

  1. Stop, take cover, wait until the earthquake is over.

  2. Grab your evacuation kit and go. Get to the nearest evacuation point.

  3. Wait until it is safe

  4. Get in contact