What is the significance of the 2011 Japan Tohoku Earthquake and how has it shaped our thinking?

 

SANTOSH SHAH

On 11th March 2011, just over 10 years ago, Japan experienced the strongest earthquake in its recorded history. The earthquake took place at a destructive plate boundary, a unique junction of 3 plates in a part of the world referred to as the Pacific Ring of Fire. Here the convergence of tectonic plates leads to the "subduction" of the denser plate, meaning it is being pulled under another plate. In Japan the complex situation of the Pacific Plate being subducted under the Phillipine Plate, itself being subducted under the Eurasian Plate was a fundamental reason behind the devastating catastrophe in 2011. 

[Figure 1 - the Pacific Ring of Fire]

As well as having a very shallow focus point (where the earthquake starts) of only 24km and an epicentre (the point on the surface above the focus) 72km off the coast of Sendai, the real severity of this 'quake was not caused by the actual ground shaking : usually buildings are said to kill humans in an earthquake rather than the earthquake itself. Fortunately, however, after earthquakes such as the 1995 Kobe earthquake, Japan has tightened its building codes and restrictions. Multiple hazard resistant structures such as base absorbers (absorb the shock of seismic waves) and pendulums on rooftops (to absorb the vibrations in buildings) have started to be installed. By protecting the buildings, such techniques have saved both the economy and thousands of lives. Despite all these mitigation measures, however, more than 300 000 buildings were destroyed but in terms of casualties caused by building collapses the figure was relatively low. 

So what really caused all the havoc?

Usually when a plate is being subducted, friction between the plates results in very slow movement. This leads to sudden jerking movements as a plate gets stuck on the other, then freed, hence resulting in the shaking of land: earthquakes. But when this occurs in the vicinity of water, these abrupt movements displace the surface of the water and generate long wavelength waves. As they approach land they grow in height as the water becomes shallower, resulting in the uneven movement of the whole wave. Friction at the bottom of the wave (from the sea bed) slows down the motion whereas near the top the speed remains constant. Hence this difference makes the waves more elliptical and taller in height, and this is where the Japanese term for harbour wave, tsunami, is used. However what was unique about the 2011 earthquake was that the Pacific Plate had clay on top of it which acted as a lubricant. The clay had trapped water in it which made it slippery and this reduced the friction between the plates resulting, in more movement. As a result the Pacific Plate amazingly slipped by as much as 50m, an unusually high value, thereby amplifying the tsunami created. Within 30 minutes up to 40m high waves flooded into Japan and travelled as far as 10km inland. All in all an estimated area of \(550km^2\) was flooded. To put this into perspective, this is 1½ times bigger than Isle of Wight. The majority [90%] of deaths were caused by the inundation of water drowning the inhabitants.

In addition to the devastating secondary hazards (e.g. a tsunami), the final part of the disaster was provided by the tertiary effect of the Fukushima Nuclear Power Plant, which was what made this one of the worst natural hazards in our history. The tsunami overtopped the 14 metre high sea wall encircling it and this flooding knocked out the emergency generators. This led to major power cuts for residents, since nuclear powered energy is one of the main sources, but worse effects were faced by the environment. The nuclear fuel in three of the reactors overheated, which partly melted the cores. This released chemicals into the atmosphere as well as radioactive material which further contaminated the Pacific Ocean. Although no deaths occured due to this nuclear meltdown, it caused many criticisms of nuclear power and its consequences on the environment, especially in disasters like the tsunami. Due to the hazard posed by radiation contamination, 80000 people in a 12-mile radius had to be evacuated. Ten years on from the disaster, the radioactive waste is kept in storage tanks on the site. But eventually this will be released into the Pacific Ocean as space is limited. Questions are being posed about how long this storage method will last. Moreover many towns are still abandoned after the incident as it is simply unsafe for residents to return. The empty houses, communities and towns would need to be fully rebuilt as they remain in tatters after ten years of abandonment. While this could all be built back the main worry is with the radioactive contamination which is much harder to avoid and remove. 

Most importantly what did society learn from this? 

Despite the breakwaters and seawalls put in place to prevent potential tsunami waves, the shear size of this event meant the waves still "overtopped" such barriers and inundated Japanese coastal towns. At the time many people had put faith into the walls for protection and hence had a false sense of security. The tsunami easily surpassed the top of both the inner and outer wall (34 feet in height) in the town of Taro as well as countless other seawalls and breakwaters. People had trusted these and hence, had been put into a fake sense of reassurance. But furthermore, they have had to endure a feeling of containment and a lack of freedom as a result of the massive barrier despite it not being able to prevent the tsunami reaching them. In Kamaishi, a 1.5 billion dollars project took place to complete the world’s deepest breakwater after around 30 years of construction, but it was damaged in a matter of minutes by the tsunami. Hence, this earthquake has taught the Japanese people to reduce their use of expensive "hard engineering" strategies and rather invest more in education and evacuation drills. Nonetheless 245 miles of seawall structures have since been installed at a cost of around $12 billion. This action was taken because they can still be effective in smaller tsunami events in the future, and they do minimise the force when overpowering tsunamis occur. 

Conclusion

This all leads to the conclusion that the 2011 Japan Tohoku Earthquake was a major point in history, because of the damage it caused socially, economically and environmentally. The poor preparation and safety precautions by the Fukushima Daiichi nuclear power plant, as well as the false reassurance provided by the trust in the seawalls and breakwaters to prevent the tsunami reaching land, all highlighted the dominance of nature over humans. Regardless of all current efforts it boils down to the fact that humans can’t win this battle. Natural disasters can never be prevented and this was a major lesson learnt from the 2011 disaster. The key takeaway is illustrated by these words “Seawalls should be designed with the assumption of overtopping and destruction, and communities should not rely on coastal infrastructures alone for protection. Tsunami warning information can inform people that they are in danger, but it cannot guarantee people's safety. The most important lesson is that one should not wait for official information to act.” 

Credits:

• Quote: https://royalsocietypublishing.org/doi/10.1098/rsta.2014.0373
• Ring of fire graphic: Wikimedia Commons (https://en.wikipedia.org/wiki/File:EQs_1900-2013_worldseis.png)
• Other images from "Chulmin Park" and "Walkerssk" on Pixabay.