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Geology is the Way

Living in the tsunami hazard zone

Japan is a country that is blessed by the sea. The island nation, surrounded by ocean on every side, is famous worldwide for its picturesque markets full of fish street food and unique seafood delicacies. It is impossible not to imagine postcard panoramas of old fishing towns, small rocky coves, shrines and temples with red torii gates, standing out over the blue waters, and mountains in the background, when thinking about Japan. However, as soon as you reach the country, you immediately start to realize that the relationship of the Japanese with the sea looks more like a cold war.

Anti-tsunami seawall
The coast of the Kochi plain is dammed by an extensive megastructure, consisting of seawalls, dams, and breakwater barriers. Shimojima coast, South of Kochi Ryoma Airport. Photo credits: Google Earth.

I have been in Japan for more than a year now, after being granted a position at the Kochi Core Center on Shikoku, the smallest of Japan’s four home islands. On arrival, as my flight was approaching Kochi Ryoma airport, it was impossible not to notice the peculiar coastline arising in front of me, stretching for kilometers on every side: tall and large walls of reinforced concrete and steel, walled fishing harbors, dams regulating inflow and outflow of water from canals to the sea, and uncountable breakwater barriers made of artificial rocks and concrete blocks battered by the sea waves. Japan from above looks like a city from ‘Attack on Titan‘, but there are no titans outside of the walls, only the infinite ocean.

Japan coastal defenses
Walking along the coast of Nankoku-shi (Kochi) feels like standing next to a fortress or a prison. A massive 5 to 10 meter high seawall and its associated dams, breakwater barriers, and artificial cliffs separates the Pacific Ocean from the mainland.
Tsunami seawall
The other side of the wall. Many coastal communities in Japan are used to overwhelming walls obstructing any sense of the ocean behind them. Nagahama district, southern Kochi.

These barriers are just the expression of the dual relationship of the Japanese with the sea, source of  wealth and life, but also of threats and death. It is the response of this country to all the natural hazards coming from the ocean it is exposed to – not only tsunamis but also powerful storms. The barriers and sea defenses are expensive megastructures that surround entire parts of the country and that cost trillions of Japanese yen (tens of billions of US dollars). The public opinion is split on their very existence: many other barriers are planned or under construction and they cost billions to the stagnating Japanese economy. They are massive and overwhelming. They totally obstruct the view of the ocean, change the daily life of coastal communities, and damage entire ecosystems irreversibly. Nevertheless, their utility as a disaster mitigation measure cannot be denied.

During the rain season in the summer, typhoons regularly batter the coastline of SE Japan, with winds up to 250 km/h sustaining waves and storm surges several meters high. The barriers regularly face and withstand these tropical storms. Indeed, the destructive coastal flooding that occurs during typhoons is relatively rare and limited in extent in Japan, compared to other tropical cyclone-prone countries that lack extensive barrier systems. In 2022, Nanmadol, one of the strongest typhoons to ever hit Japan with winds up to 250 km/h, made landfall in Kyushu. It caused power outages, extensive rainfall, and damage due to the strong winds, claiming the lives of 4 people and costing 1.2 billion US $ in damage. However, there was no coastal flooding due to the storm surge thanks to the extensive barriers that protect the coast. A similarly strong hurricane, Ian, which swept through the Southeastern United States on the same year, was the third-costliest ever hurricane on record, causing 113 billion US $ in damage and killing 149 people just in Florida, due to the extensive coastal flooding it caused.

Japan’s extensive and impressive coastal barrier system can withstand waves and storm surges up to 8 meters, resisting very well to typhoons. However, there are monster waves out there, tsunami, that can easily overcome even these impressive defenses. Japan sits in front of a subduction zone and the Pacific Plate is constantly moving to the northeast, sliding underneath Japan at a relatively high convergence rate of 9 to 13 cm/year. The friction between the Pacific plate and the Japanese crust above it, causes the two plates to become locked and slowly accumulate elastic energy on the timescale of years as the Pacific Plate is pushed downward. Every several tens or hundreds of years, all this energy is released all at once in a destructive megathrust earthquake. We all still have in our mind the disastrous Mw 9.1 Tohoku earthquake that hit northeastern Japan (Sendai) in 2011. In that case, the Pacific plate is estimated to have moved suddenly 20 meters to the west, causing an upthrust of the seafloor of 6 – 8 along the 180 km rupture. This caused tsunami waves up to 38 meters, which were able to overcome the barriers and flood the plane of Sendai. More than 20,000 died or went missing.

Japan earthquake and tsunami 2011
The moment the 2011 tsunami overcame the barriers at Miyako city, Iwate prefecture. Photo by: Miyako city Officers/Rex Features.

Is Kochi facing a similar threat? What’s the tsunami hazard in Kochi?
Kochi lies on the edge of the Pacific Ocean and also sits on a subduction zone. However, in contrast to other parts of Japan, where small and relatively strong earthquakes are felt from time to time, the Kochi area is completely silent, giving the misplaced illusion of safety. On the bottom of the ocean, right offshore Kochi city, the Philippine Sea Plate is moving to the northeast, colliding with the Eurasian plate and being pushed in subduction under the Japanese archipelago. The subduction of the oceanic lithosphere, sliding at depths of hundreds of kilometers in the mantle, is accompanied by the formation of a large oceanic depression, the Nankai trough. The trough sits right where the Philippine Sea Plate is dragged under an underwater collisional belt offshore Shikoku, the Nankai accretionary prism, made up of sediments and slices of oceanic crust detached from the subducting plate. This subduction happens, apparently, in complete silence, but not for those that have the right tools to listen.

Earthquake and slow earthquake Nankai
The Nankai Trough is the source area of thousands of geodetically and seismologically detectable tremors (red dots) and long-term slow-slip events (collectively known as ‘slow earthquakes’) occurring between 35 and 50 km depth. These events are slowly charging the silent and locked megathrust zone (red circles), capable of releasing destructive ‘fast earthquakes’. After Obara & Kato (2016), Science: connecting slow earthquakes to huge earthquakes.

It is here, indeed, that a new geological phenomenon was first detected about 20 years ago: thanks to a network of high-resolution seismometers Japanese scientists discovered a silent seismic chatter, almost indistinguishable from background noise. Low-frequency and very low-frequency tremors with barely detectable signals are periodically occurring along the subduction interface at a depth between 35 and 50 km. These tremors are associated with the slow movement of parts of the seafloor for several millimeters over days to months, silently accompanying the downward sliding of the subducting plate. Today we call these events ‘slow earthquakes’, ‘slow’, because they move the ground only a few millimeters over days to months, ‘earthquakes’, because they release the same energy of a conventional earthquake, although the longer timescale makes them not destructive at all, like receiving a soft touch instead of a fast punch.

But if these slow earthquakes are dissipating all the energy, why does Kochi need tsunami barriers? Because, unfortunately, while some parts of the subduction are slipping, others are completely silent, said to be ‘locked’. Slow earthquakes occurring at depth are, indeed, slowly and silently invading a part of the subduction zone, called the megathrust zone, at a depth between 10 and 30 km. This zone is completely quiet: no earthquakes, no microearthquakes, no movements of any sort at all. It is slowly accumulating energy, slow earthquake after slow earthquake, and has been doing so for years. One day all this energy will be released in a large megathrust earthquake.

Nankai subduction sketch
Sketch of the Nankai subduction. Parts of the plate interface slip periodically in slow earthquake events (episodic tremor, long-term slow slip events, shallow very lof-frequency earthquakes), while others (megathrust asperity) are locked and slowly building up energy. After Obara & Kato (2016), Science: connecting slow earthquakes to huge earthquakes.

The last Nankai earthquake happened in 1946 and killed at least 1362 people with a magnitude estimated between 8.1 and 8.4 and tsunami waves up to 6 meters. Only 2 years before, the M 8.1 Tonankai earthquake hit the Kii peninsula, further to the North on the Island of Honshu, with tsunami waves of at least 10 meters, killing other 1223 people. Similar events have been occurring in this area every 100-150 years on average, which means that the next one has a high probability to occur between the year 2020 and 2060. However, when the next megathrust earthquake happens it might be far more costly in terms of damage and human lives lost.

1946 Nankai earthquake and tsunami
The town of Mugi (Tokushima prefecture) after the 1946 Nankai earthquake and tsunami. Source: Tokushima Local Meteorological Observatory.

In the 1940s, Kochi was very sparsely populated. Today, Kochi prefecture has a population of 757,914 people, and most of them live close to the coastline. Furthermore, studies have revealed that the Nankai trough is able to release earthquakes with magnitude exceeding 8.5, way stronger than those that occurred in the 40s, and tsunami waves up to 35 meters are expected to hit the coast. What would happen today if such an earthquake occurs and tsunami defenses are overcome?

The sad truth is that, despite the incredible effort that building these coastal defenses was, a tsunami of 10 meters or more can simply pass over them. Just look at the tsunami hazard map for Kochi, below. Despite the presence of the barriers, tsunamis are expected to flood large areas of the countryside and Kochi downtown, reaching areas several kilometers inland as they overflow rivers and canals.

Kochi tsunami hazard map
Map of tsunami risk for the Kochi area. The areas in orange and purple are forecast to be inundated by tsunami waves above 5 and above 10 meters respectively. Green to yellow: between 1 and 5 meters. Blue: less than 1 meter. Source and full-resolution pdf: Kochi kia.

In Kochi city center there are tall buildings where people can find shelter in case of a tsunami, but what about the countryside? Kochi has many rural communities – areas of flat land with rice fields and small villages with no tall buildings at all. These areas are the most vulnerable to a tsunami, because there are no natural elevated areas where to escape. For this reason, the Japanese government has built many tsunami evacuation towers every few hundreds meters along the coast. In the case of a tsunami, there will be a few minutes available for dwellers to find shelter in one of those towers (less than 5 minutes in some areas, see map above!). For this reason, the local government runs several earthquake and tsunami drills – I also did one last year at my workplace – to raise awareness and preparedness in the population, because every minute is crucial to save lives in the event of a Nankai megathrust earthquake. Furthermore, Japan has an excellent emergency broadcast service called J-Alert that quickly sends information about natural disasters to local medias and directly to citizens via loudspeakers, emails, and cell broadcast. This system allows to alert people automatically seconds before the arrival of seismic waves, and minutes before the eventual tsunami generated by an underwater earthquake reaches the coast.

Tsunami evacuation tower
A massive tsunami evacuation tower built next to the seawall and houses in the coastal community of Shimojima, Kochi. Link to location.
Tsunami evacuation tower
Another massive tsunami evacuation tower, surrounded by rice fields, in Yoshikawa-cho, south of Noichi, Kochi. Link to location.

Now, let this situation sink in for a moment. The sole fact that this area is expected to be devastated by an earthquake and a tsunami in the next 30 years is frightening to say the least. The sole fact of living behind walls, under the shade of tsunami evacuation towers, knowing that one day your house, the memories within it, and even your loved ones may disappear sounds frightening. I feel this oppressive feeling myself: after having lived here for more than a year, having eaten in izakayas and restaurants, shopped in malls, been to shrines and temples, and met many wonderful people. I cannot help but think that one day all of this may be erased. But the Kochi people, they are probably stronger and show a proud ‘ganbatte’ spirit in face of this reality, not willing to give up their lives no matter how hard the challenges might be. I know the next Nankai earthquake cannot be avoided, and I simply and genuinely hope that these people will remain strong and rise again from the waves.

Katsurahama Kochi
People praying at Kaitsumi shrine at Katsurahama, Kochi.

Introducing the ‘hazards’ series
Tsunami hazard markerWe live on a hazardous planet. This is the first of a series of posts in which I will talk about geological hazards and how people and nature coexist with them in different parts of the world.
← look out for this marker on the map! (the ‘hazard’ in the marker may change… well, depending on the hazard I am talking about!)

I am glad that as I continue my blog posts, many of you keep supporting me! It means a lot for me! Thank you very much Michele Lustrino, Martha Carr, Sandra McLaren, Marta Codeço, Jerry Nelson, Helfe, Deemery, Finn, Maria, and to an anonymous supporter! If you like my geological posts and you wish to support me, you can do it by offering me a coffee at ko-fi!

Special thanks to a very special reviewer that helped me a lot with this blog post: thank you very much, Sandra McLaren!

Araki, E., Saffer, D. M., Kopf, A. J., Wallace, L. M., Kimura, T., Machida, Y., … & IODP Expedition 365 Shipboard Scientists. (2017). Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust. Science, 356(6343), 1157-1160.
Hirose, H., Asano, Y., Obara, K., Kimura, T., Matsuzawa, T., Tanaka, S., & Maeda, T. (2010). Slow earthquakes linked along dip in the Nankai subduction zone. Science, 330(6010), 1502-1502.
Obara, K. (2002). Nonvolcanic deep tremor associated with subduction in southwest Japan. Science, 296(5573), 1679-1681.
Obara, K., & Kato, A. (2016). Connecting slow earthquakes to huge earthquakes. Science, 353(6296), 253-257.
Rikitake, T., & Aida, I. (1988). Tsunami hazard probability in Japan. Bulletin of the Seismological Society of America, 78(3), 1268-1278.
Rogers, G., & Dragert, H. (2003). Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip. Science, 300(5627), 1942-1943.

News articles and others
Residents lose coastal vistas to fortress-like tsunami walls. The Asahi Shimbun. March 15, 2021.
Tsunami-Stricken Town Resists “Great Wall” Mentality: Kesennuma’s Quest for a Middle Way. March 8, 2021.
Kochi tsunami hazard map and Information booklet on ‘Preparing to the next Nankai trough earthquake‘. Kochi kia.

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