The Tokyo Inland Earthquake and the Great Kanto Earthquake | Satoko Oki, Assistant to the Dean, Faculty of Environment and Information Studies

September 1 of this year marked the 100th anniversary of the Great Kanto Earthquake. As it is a milestone year, more media outlets than usual are covering the Great Kanto Earthquake and the Tokyo Inland Earthquake. In fact, I (and probably many other seismologists) view the number '100 years' not as an anniversary, but from a slightly different perspective. To put it bluntly, the conclusion is that 'the Tokyo Inland Earthquake is finally becoming imminent.'

Here, I will briefly provide a seismological explanation of the major earthquakes that occur in the Kanto region. The 'Great Kanto Earthquake' is the name of the disaster caused by the earthquake; the earthquake itself is called the '(Taisho) Kanto Earthquake.' This 'Kanto Earthquake' is an earthquake with a magnitude (hereafter, M) of around 8 that occurs at the plate boundary between the Kanto Plain and the Philippine Sea Plate (the Sagami Trough) (Figure 1). Earthquakes at plate boundaries are known to occur repeatedly in cycles of several decades to several hundred years, and the Kanto Earthquake is no exception. The one before the 1923 Taisho Kanto Earthquake was the 1703 Genroku Kanto Earthquake, a difference of about 200 years. This means that for an M8-class Kanto earthquake, we are now about halfway through the cycle. This is the '100 years' as the halfway point of an approximately 200-year cycle.

Source: Headquarters for Earthquake Research Promotion, MEXT

So, what is the difference between the commonly heard 'Tokyo Inland Earthquake' and the 'Kanto Earthquake'? The Tokyo Inland Earthquake is a general term for 'earthquakes of around M7 that occur directly beneath the capital and its surrounding areas, as well as the aforementioned Kanto Earthquake.' While the Kanto Earthquake has a clearly defined location and magnitude (around M8 in the Sagami Trough), these are ambiguous for the Tokyo Inland Earthquake. In other words, whether it occurs in Tokyo, Kanagawa, or off the eastern coast of Chiba Prefecture, if it has a magnitude of M7 or greater, it is considered a Tokyo Inland Earthquake. In fact, the Cabinet Office has created and published multiple damage estimation scenarios by assuming various source fault models at different locations.

Now, let's consider the frequency of the 'Kanto Earthquake (around M8)' and the 'Tokyo Inland Earthquake (around M7).' Generally, the smaller the M, the higher the frequency, so a Tokyo Inland Earthquake of around M7 is far more likely to occur than a Kanto Earthquake of around M8. Furthermore, looking back at historical data on the Kanto Earthquake reveals important information that cannot be ignored. Earthquakes of around M7 in the Tokyo metropolitan area have occurred frequently in the latter half of the cycle of M8-class earthquakes. As mentioned earlier, we are currently at the 'halfway point of the approximately 200-year cycle,' in other words, at the 'beginning of the latter half.' You have probably guessed it by now. This is the breakdown of what I stated at the beginning: 'the Tokyo Inland Earthquake is finally becoming imminent.'

Of course, there are issues of accuracy, such as whether the actual cycle is truly 200 years, and limitations in the data, such as whether they truly occur more frequently in the latter half, so we cannot say for certain that we have entered the latter half starting this year. However, if we are to make use of scientific and historical knowledge within the scope of what we know, we should assume that a Tokyo Inland Earthquake will occur and prepare for it immediately.

As I wrote in the Dean's Diary about a year ago, my research lab conducts 'hands-on drills' for public facilities such as schools and airports. In these drills, we simulate the conditions immediately after a disaster, and have the staff respond. Specifically, students act out and recreate various incidents and injuries that have occurred in past earthquake disasters, and the staff must deal with them. Sometimes first aid is necessary, other times they need to calm people down by talking to them, or gather colleagues to transport the injured. They must accomplish all this in a simulated situation where there is a power outage and communication cannot be done in the same way as usual.

Since the drills are conducted without prior notice of where and what kind of incidents will occur, in most organizations where we have conducted them, information becomes confused, people run around in disarray, and a situation arises where no one has a grasp of what is happening throughout the facility. When this happens during a disaster, the responders on the scene will either keep calling for help from their colleagues ('Someone, help!'), try to be the first to get a stretcher, or escape from reality by spending time on unimportant things. Meanwhile, the headquarters staff will either repeatedly shout 'Don't move without permission! Report!', leave the headquarters to see for themselves, or panic with their minds going blank. The idea that 'if a disaster happens, we'll figure it out on the spot' simply does not work.

On the other hand, organizations that have discussed in advance how to communicate during a power outage and the priority for transport, and have practiced repeatedly, move splendidly even in the face of a new situation we create. For example, at one elementary school, a teacher who found a seriously injured person administered first aid while loudly calling for help from other teachers, and immediately reported to the headquarters, 'There is a level-red injured person on the third-floor landing of the east staircase.' Although information was coming into the headquarters from various other parts of the school, this school had a rule that minor injuries were not to be reported immediately but were to wait in a safe place, so the headquarters did not suffer from information overload. The headquarters was able to make decisions with composure about where to send stretchers and where to dispatch additional support staff, while having a bird's-eye view of the entire school.

What gives the headquarters this composure is nothing other than the fact that the teachers on the scene are prepared to take on the injured with resolve. This readiness to handle the injured is backed by their enhanced skills, such as first aid, and a sense of trust in their colleagues that help will surely come. And this sense of trust as a team is born from the experience of helping each other in past drills. In other words, through the regular drills conducted in normal times, they are achieving ongoing team building.

Turning this back to ourselves, if a Tokyo Inland Earthquake were to occur now, would SFC be able to function as such an organization? Or would each campus and the Mita Headquarters (or a temporary headquarters set up elsewhere) be able to share information appropriately according to the severity of the disaster and the time elapsed? At SFC, where there are many students with excellent autonomy and initiative, I want to create a good model case for how a university should respond in a disaster. I can only hope we will be ready in time for that inevitable day...