Exploring Success: Inside JTRACK’s Historic Expedition to a Major Japanese Seismic Fault 13 Years After the Catastrophic Earthquake

More than 13 years after the Tohoku earthquake, as part of the JTRACK mission (September 6, 2024 – December 20, 2024) within the International Ocean Discovery Program (IODP), scientists are studying the fault zone present in the area of subduction, particular zones which are often the site of mega earthquakes.

It is in this sense that Science & Life spoke with Marianne Conin, University Professor at Mines Nancy and researcher in the GeoRessources laboratory attached to the University of Lorraine. She was, just a few days ago, in the open sea on the Chikyua scientific drilling boat designed to study rocks under the ocean.

Subduction zone, mega earthquake and tsunami

Before getting to the heart of the matter, it is important to understand some vocabulary. Let's start by defining what a subduction zone is, the subject of the study of the JTRACK mission in which Marianne Conin is participating.

In Japan, the Pacific plate is sliding at a speed of 8.5 cm per year under Japan. But this movement is not continuous over time. It blocks for periods of a few dozen to a thousand years, called “interseismic periods”. During these periods the rock deforms around the blocked areas and accumulates elastic energy (a bit like a rubber band being stretched). When the deformation becomes too great, the fault which limits the two plates suddenly ruptures and begins to slide: this is an earthquake. If the slide propagates near the surface, the ocean floor also moves, and displaces the ocean water: this is the tsunami. In subduction zones the faults have large surfaces; and can cause mega earthquakes and mega tsunamis.

These mega earthquakes, if they occur at sea, lead to devastating tsunamis, as in Sumatra, Chile, and Alaska, in the Aluetians.

“Since 1900, there have been 16 mega earthquakes of magnitude greater than 8.5 recorded. Among them, five had a magnitude of 9 or more. We of course have the one from Tohoku which is the most recent. Before that, we had that of Sumatra which was catastrophic with more than 230,000 deaths. There was one in Chile in 1960 and in Alaska in 1964.” recalls the researcher.

“At sea, in a subduction zone when a magnitude 9 earthquake is triggered, the ocean floor moves, therefore also causing the water above to move. And that's what we call a tsunami. It is not a wave but a shift in sea level. The water will then respread everywhere, including on land. And when the tsunami arrives, we have a height of water which invades the earth in a very energetic way.” specifies Marianne Conin.

Science & Vie: What is the JTRACK mission?

Marianne Conin : We are an international cooperation. Around me, there are 200 people on board the boat representing 30 nationalities. The mission takes place in the International Ocean Discovery Program (IODP) and the latter was built like international space programs to pool efforts and resources in order to be able to go to places that are extremely difficult to access. And JTRACK is in fact the last mission of the IODP3 which will now begin with Japan and Europe, but without the United States whose drill ship has just retired.

JTRACK consists of drilling the front of the subduction zone, in particular the fault zone which slipped during the earthquake and its immediate environment. To do this, you have to cross 7,000 meters of water, then core around 900 meters of rock. During this operation, 10 meter long cores are gradually brought back onto the boat, cut into 1m sections, then scrupulously studied by around fifty scientists.”

SV : When did this mission begin?

MC : In fact, it all started in 2009. At the time I was just a post-doctoral student and colleagues who are currently participating in the mission said to themselves “the day there is a big earthquake, it doesn't matter where it is. or, that we be there in less than a year to install temperature sensors to observe the quantity of heat released during the earthquake”. And in 2011, the Tohoku mega earthquake occurred. The following year, the JFAST mission was born, which consisted of drilling the fault zone to successfully obtain samples. A success which did not arrive without some technical difficulties.

But once the mission was over, we were a little left wanting because we would have liked to know more and learn more about this subduction zone. And so the idea was that, 10 years after this first mission, we would return to the site to see if this fault zone was starting to repair itself, to heal and therefore to store energy again.

Today, [ndr, le 13 décembre 2024 lors de l’interview], I can say, after four months of expedition, that the mission is a real success, we have achieved all the ambitious objectives that we had set for ourselves. This became possible because we managed to bring together technical skills and scientific research to achieve a level of excellence.

SV: What do you mean by observing the healing of the fault?

MC : The healing of a fault is its ability to regain its properties before the earthquake.

A subduction zone, as long as it is active, will not stop seeing one tectonic plate diving under another. Thus, at more or less regular intervals over relatively long periods of time, perhaps 1000 years for example, an earthquake will occur, releasing energy.

Then it will block, seeing one of the plates storing energy which, when it reaches too high a quantity, releases, then restarting a new seismic cycle. By focusing on the healing of the fault, we seek to know if a tectonic plate regains its characteristics after an earthquake.

SV: Has this latest expedition already provided answers to what you were looking for?

MC : With the JTRACK mission, we seek to understand if anything has changed in 13 years.

Without saying too much, we have new information that allows us to develop models on how this fault zone is repaired and that's what we were originally looking for. From a geological point of view we have also been able to broaden our knowledge of certain types of subduction zones.

All the information collected on the boat directly, but also with the help of the underwater observatory which will remain in place for another year, will be used to deliver scientific publications, for overall knowledge of the functioning of these areas. All this knowledge is important to better understand seismic hazards and protect populations.

SV: Based on your expertise, when could Japan experience another earthquake similar to that of Tohoku in 2011?

MC : We cannot give precise dates, but we have historical traces of several mega earthquakes. Because Japan was able and knew how to preserve its archives enormously, particularly in temples which made it possible to save writings.

And in Japan, there were written traces of a mega earthquake in the 9th century, in 861, which corresponded, according to the accounts preserved from the time, to a magnitude similar to that experienced by Japan in 2011. Then , based on this story, we will try to find the seismic and geological evidence of this event.

Thus, based on the evidence found, but also thanks to the stories, it is estimated that the recurrence of such an earthquake would be a little over 1000 years. But just because it happened a thousand years ago the last time doesn't mean the probability of a mega earthquake occurring before then is zero. It could happen, and these expeditions to subduction zones allow us to better understand the processes at play.

SV: Can we expect a similar sort of JTRACK mission for the so-called Big One earthquake, for example?

MC : Maybe. But what we must understand is that one country does not have the capacity to do it alone. But it is an international cooperation which therefore requires mobilizing a lot of people for very long periods and which also requires very significant financial resources. Because drilling into an earthquake fault can cost tens or even hundreds of millions of dollars.

It is therefore a human budget, but also a financial one. So, if all the conditions are met to make such a project possible. And it would be necessary that, in less than a year, as soon as the next magnitude 9 earthquake appears, we could set up a team and achieve what we did for JFAST and JTRACK, while having the experience and expertise acquired during these two expeditions.

It would be an excellent idea, but it comes from the good will of scientists who will be able to help each other and come together to convince their respective governments to be able to finance such projects which, in the end, will serve humanity to better understand and better understand each other. prepare for mega earthquakes.