TOHOKU, Japan, Friday, Mar. 11 -Yesterday, a 9.0 magnitude Earthquake struck the east of Tohoku, Japan. The epicentre of the earthquake was located approximately 72 km east of Tohoku (38.3 degrees North latitude and 142.4 degrees East longitude), or 130 km east of Sendai as shown by the X in figure 3 below. The focus of the earthquake was 24.4 km under the epicentre. The earthquake therefore caused a tsunami due to the displacing of water above the Pacific Plate. The earthquake and tsunami together have caused approximately 20,000 deaths, 6152 injuries and 2500 people missing. Approximately 90% of these deaths were by drowning. Approximately 140,000 people have been displaced and over 500,000 buildings and structures have been destroyed or damaged. The early warning system of Japan stopped many bullet trains, and many residents received texted warnings of the earthquake and tsunami on their mobile phones giving them time to evacuate. Figures 1 and 2 show exactly how destructive the tsunami caused by the Tohoku Earthquake was.
The earthquake negatively affected Japan in multiple ways. It disrupted electricity, water, food and gas supplies in Japan followed by transportation and telecommunication failure.
The earthquake also caused fires due to destruction of gas lines or electrical power, dam failure due to the tsunami causing overflow of water, landslides due to the earth movement and soil liquefaction. Floods and washing away of homes was caused by the rupture of the Fujinuma irrigation dam in Sukagawa. The flooding of the dam caused the drowning of many people and was a factor in the 90% deaths by drowning. Soil liquefaction is caused by applied strength from an earthquake and is where soil behaves as if it is liquid, and at a water purification centre in Tohoku, the soil sank 1 metre. Many coastal towns and cities were destroyed as a result of the tsunami that followed the initial ground shaking. The tsunami even spread to locations across the world such as Hawaii, Ecuador, Chile, Peru, Antarctica, California, Oregon and the Galapagos Islands. However, there were no deaths due to these waves.
How did this tragedy occur?
Researchers say the Tohoku earthquake, measured as a magnitude 9.0 on the Richter Scale, was caused by converging plate boundaries that cause subduction. This subduction zone is where the Pacific and Eurasian tectonic plates meet, as shown in figure 6. The Okhotsk microplate is the more specific area of the Eurasian plate that meets the Pacific plate. In a subduction zone, one plate slides under the other into the mantle, a hotter layer underneath the crust. East of Japan, the Pacific oceanic plate descends under the Okhotsk microplate of the Eurasian continental plate above it, forming the Japan Trench which is caused by the pressing down of the Pacific plate under the continental plate, with the Japanese islands above. The Japan Trench is part of the Pacific Ring of Fire, and is shown in Figure 4 below. The plates are rough and stick together and build up energy that is then released as earthquakes, and today, this tension along this boundary that has built up over thousands of years, was released as the Tohoku Earthquake. According to studies, at the subduction fault, the oceanic and continental plates slid past each other around 80 metres.
The subduction zone as shown in figure 4 (left), is where the depth, focal mechanism and location of the Tohoku earthquake are consistent. The earthquake’s rupture indicates the fault was pushed vertically upwards about 10m.
The point located directly above the focus of an earthquake on the Earth’s surface, is the epicentre, which is 130 km east of Sendai. The focus which is 2.4 km below the epicentre is the earthquake’s beginning location. This spot is where the ground ruptures. Seismic waves then radiate outwards in every direction. The fact that the focus was only 2.4 km deep explains why the magnitude was so high, as the closer the focus to the surface, the more impact there is. There were a number of high magnitude foreshocks two days before March 11. A magnitude 7.2 quake that was situated approximately 40 km off the main earthquake’s epicentre began a series of foreshocks. The first day of these foreshocks continued with three other earthquakes of greater magnitude than 6 on the Richter scale.
The rupture zone that caused the sudden 10m upthrust of the Pacific plate, displaced the water above it, causing the tsunami. Tsunamis start with a disturbance of a water column by a force. In this case, the force was the earthquake under the water. There is then displaced water that moves across the deep ocean and spreads out in all directions from the earthquake epicentre. The further the tsunami travels, the less impact there is on the coastline as the wavefront spreads. The Tohoku earthquake was only approximately 72 km off the shore of Tohoku and therefore there was substantial destruction to the coast. The diagram below (figure 5) is from a satellite altimeter that shows tsunami wave movement, and that only 24.5 minutes was needed for the tsunami to impact the east shore of Japan. There were 40.5m wave heights recorded to surge up to 10km inland, causing 90% of the 20,000 person death-count to be by drowning. Honshu was even moved 2.4 metres to the east due to the earthquake. The next stage of a tsunami is inundation, and this is where the fast moving tidal waves overflow onto the shore. Figure 2, at the beginning of this article, shows how Japan’s seawalls offered minimal protection against the rapid waves of the tsunami. The seawalls on the shores of Japan indicate that the coastlines of Japan are tsunami prone areas. The 24.5 minutes taken for the tsunami to hit the Eastern shore of Japan after the earthquake infers that the government had very limited time to warn and evacuate everyone and therefore the tsunami hit many off guard, and this relates to the mass deaths by drowning.
Scientists drilled into the subduction zone after the earthquake and found a thin layer of slippery clay that lined the fault. This layer of clay is hypothesised to be the reason the two plates were allowed to slide the 50 metre horizontal distance and 10m vertical upthrust. This is the main scientific theory of how the earthquake and tsunami occurred.
However, there were other extreme circumstances to be considered. The Fukushima Power Plants in Fukushima were severely damaged by the tsunami after the earthquake. Due to the 9.0 magnitude earthquake, a 15 metre tidal wave disabled the cooling system of three nuclear reactors and this caused a nuclear disaster which received an International Nuclear Events Scale rating of 7. As shown in figure 7, the power plant burst into flames due to overheating as a result of the destruction of the emergency generators that cooled the power plant. The controversial matter to question is, why were the power plants built so low-lying and close to the sea? The power plants were built only 10m above sea level, and the disaster may have been preventable if built on higher land.
Nearby cities were endangered as the incident caused high radioactive releases that will last over 4 to 6 days. Approximately 100,000 people have been evacuated from their homes in order to protect them from the hazards of the radiation from the power plant. The power plants are being cooled by water to stabilise them as shown in Figure 8. Apart from cooling, the aim is to prevent the release of radioactive materials, especially the radiation-contaminated water that leaks from the three power plants. Government caution delayed the return of people home in order for residents to avoid radiation sickness. Attempting to maintain the evacuation itself however has caused 1000 deaths due to the tsunami, in comparison to the risk of radiation if people were allowed back to their homes. Long term damage is also an issue in this matter. Many buildings have been destroyed and as a result these buildings have released large amounts of greenhouse gases into the atmosphere. The 500,000 buildings destroyed are responsible for a large number of the 20,000 casualties.
How Has This Earthquake Compared To Others In Recent Years?
The 2011 Tohoku Earthquake (magnitude 9.0) was of a much higher magnitude than Haiti 2010 which was a magnitude 7.0, as displayed by the relative scales in Figure 9. Tohoku 2011 had 500,000 buildings and structures damaged, more than Haiti.
However the Haiti earthquake caused many more significant casualties. According to estimates by official researchers, there were approximately 316,000 people killed, 300,000 injured, 97,000 destroyed houses, 1,300,000 displaced people and 188,383 houses damaged in Haiti. This is significantly higher than the 20,000 deaths, 6,152 injured, 562 people missing and 140,000 displaced in Tohoku.
This is most likely due to the Haiti earthquake striking more populated areas than the Tohoku earthquake. An earthquake warning was given 1 minute before the earthquake for the Japanese people to have a chance to escape, however the warnings for the Haiti 2010 earthquake were ignored by officials, claims ‘The Australian’. Skyscrapers in cities of Japan are designed with shock absorbers and withstand earthquakes by swaying side to side when an earthquake occurs. The ratio of buildings destroyed to people killed infers that houses that were destroyed did not kill as many people, mainly due to the one minute warning that all Japanese citizens received for the aware people to evacuate the buildings, but also because of the way Japanese houses are constructed; with regard to threat of earthquakes.
On a tectonic level, the Tohoku Earthquake was caused by subduction due to converging plates. On the other hand, Haiti 2010 was caused by the North American and Caribbean plates that move against each other in a left lateral strike slip motion. This indicates a transform plate boundary, rather than a convergent boundary, so large magnitude high damage earthquakes are common in this region. The 2011 Tohoku Earthquake was the most powerful in Japan’s history as no other Japanese earthquake has reached the 9.0 magnitude on the Richter scale. The second highest magnitude ever recorded in Japan was an 8.9 on the Richter scale; however that was the Jogan Sanriku earthquake that took place in the year 869.
Jinius Kangius, an eyewitness at the scene, told The Australian Times about his experience of the tsunami. “The tidal wave of the tsunami rushed in like it was eating up the city. I looked on from my 14th floor apartment and saw the water destroy every building in its path. The tsunami was relentless, just like the earthquake.” he said. Overall, this disaster can be summarised by the utter destruction shown in figure 9. The Tohoku earthquake and tsunami has wreaked havoc across Japan.