Course:EOSC311/2023/The Past and the Future of Megathrust Earthquakes in Japan

Geology and Geography share a lot. Generally speaking, geology has its focuses on the Earth's interior and rarely makes a connection with social aspects. On the other hand, geography covers a lot of things of both physical and social science (things on the Earth's surface), and it is relatively good at making these connections. Since geology tends to focus on the processes or formation that take place within the Earth's interior, however, the impacts on the society that exists on the surface are rarely discussed in geology. In this project, I will explore the geological processes of earthquakes and make a connection between them and human society.

Megathrust Earthquakes

Geological settings: Plate tectonics and Subduction zones

Figure 1: Earth structure

Figure 2: Subduction zone (oceanic and continental plates)

As Figure 1 suggests, the Earth is composed of several different layers: inner core, outer core, mantle, and crust. Crusts, the uppermost layer of the Earth's structure, are the main contributors to the formation of most earthquakes. There are two types of crust, oceanic and continental, and they are moving towards a certain direction that is determined by mantle convection that is occurred beneath the crusts. As the crust moves, they might collide or diverge from one another. It leads to the formation of either convergent or divergent plate boundaries. In another way, a transform plate boundary can be formed, which is a plate boundary at which two plates move one another. Earthquakes are particularly concentrated in the convergent plate boundary, especially, the subduction zone. The subduction zone is one of the convergent plate boundaries where two plates collide with each other. As a result of the collision, one crust began to be subducted beneath another crust due to the difference in density. Generally speaking, as Figure 2 indicates, it is common that oceanic crust is subducted beneath continental crust because oceanic crust is denser than continental crust. At subduction zones, megathrust faults, which are the contact areas between subducting and overriding plates, are formed, and they are the host of Earth's largest earthquakes.

The formation of megathrust earthquakes

The depth of seismic activity can be divided into two categories: shallow and deep. Shallow seismic activity, which is an event that occurs between 0 to 70 km in depth, is usually considered an earthquake that occurs at the subduction zone from the accumulated stresses. As a plate moves, it contact with another plate along the plate boundaries, and stresses and strain are accumulated. Although the plates can hold some degree of stress, there is a point when the plates are no longer able to hold the stress. When the accumulated stresses reach the point, plates move abruptly, which means that the plates release the accumulated stresses or the energy in the form of earthquakes. On the other hand, deep seismic activity takes plate much deeper depth than shallow seismic activity, as the name suggests. It is interpreted that the plate boundary is not the plate of origin of the deep seismic activity. Rather, it originates from the subducting plate itself because of "down-dip compression". Down-dip compression is characteristic of deep seismic activity in which the pressure or stress axis in the focal mechanism solution aligns with the dip of the deep seismic zone. Some areas have many earthquakes along the edge due to the presence of several subduction zones on the edge. One particular example is the Ring of Fire, which is a belt-shaped seismic zones along the edge of the Pacific Ocean.

The 2011 Tōhoku earthquake

Earthquake

Figure 3: Location of plates around Japan

The 2011 Tōhoku earthquake was one of the largest earthquakes that occurred off the northern Pacific coast of Japan on 11 March 2011. Japan, which is situated part of the Ring of Fire, has a number of earthquakes because of its unique geological situation. As Figure 3 indicates, Japan is a country that is situated on four different tectonic plates: the Pacific, the Okhotsk, the Eurasian, and the Philippine Sea plates. A megathrust fault, which was formed in between the overriding Okhotsk plate and the subducting Pacific plate, was the focal point of the earthquake. The focal point is the place inside the Earth where an earthquake originates. The earthquake occurs at 2:46 p.m. local time, and it caused a massive tsunami. In Figure 4, red circles indicate the depth and the distance from the Japan trench, and the magnitude of the earthquake is represented by the relative size of the circles. The largest red circle in Figure 4 is the 2011 Tōhoku earthquake, and it suggests that the focal point of the earthquake is relatively shallow, thus it is categorized as a shallow seismic activity. The magnitude of the earthquake was 9.0, and it was strong enough to generate a devastating tsunami.

Figure 4: Cross-cut of earthquakes along Japan

Tsunamis

Tsunami is a sequence of water waves that have an exceptionally long wavelength and period. It occurs in the ocean due to a sudden disturbance in the Earth's structure, causing a rapid displacement of water. The sudden disturbance in the Earth's structure that happens in the ocean is predominantly a submarine earthquake, which is an earthquake that occurs underwater the beneath water body like the oceans. Since the subduction zones are the place of origin of most earthquakes, they are also the place where most tsunamigenic earthquakes occur. In order to mitigate the impact of tsunamis, seawalls, which are hard-engineered structures that prevent waves from flowing, are often implemented. The affected region had a number of seawalls and some of them were 10 metres high. However, it was not enough to protect the regions because the enormous tsunamis caused the collapse of many seawalls as they surged several meters above their tops. As a result, the tsunamis affected approximately 561 km² and reached to a maximum height of 40 metres.

Effect

The impact of both the earthquake and tsunamis was devastating for physical properties, economy, and humans. Japan has a rich historical record of tsunamis, and the affected region has a long history of being hit by tsunamis that were recorded in paleotsunami, which is a field of study about past tsunamis, and inscribed documents. According to the report by National Police Agency, 15,866 people died and 2946 people are missing by 27 June 2012. In addition to that, the tsunamis and earthquake destroyed 130,411 buildings or houses and caused economic damage of 16 to 25 trillion yen. Some people, who could survive from the disaster, lost their houses and had no options other than moving to shelters. In addition to that, however, the earthquake and tsunamis were not only the disasters that affected the residences but also probably the most dangerous disaster, the Fukushima Nuclear Disaster.

Fukushima Nuclear Disaster

Figure 5: A map of Fukushima nuclear disaster's radiation

The Fukushima nuclear disaster is one of two level 7 nuclear disasters categorized in the International Nuclear Event Scale (INES) along with the Chernobyl disaster. Before it turned into a disaster, Fukushima nuclear plant had six Boiling Water Reactors units that were developed at least older than 40 years ago. While Units 4-6 were out of service and stayed cold when the earthquake occurred, Units 1-3 were in operation. The safety systems automatically triggered the shutdown of Units 1-3, while emergency generators were activated to supply power to the control electronics and coolant systems. However, as tsunamis flowed over seawalls, the water intruded into the building in which emergency generators were placed. It led to overheating of the core because the cooling system could not gain energy or electricity from emergency generators to pump water to cool the core. Eventually, hydrogen explosions occurred and caused a meltdown. As a result, considerable amounts of radioactive materials were released, which forced nearby residences to move out. Residences within the 20-km radius zone around the plant, which later extended to 30-km, were forced or advised to leave the zone. As Figure 5 indicates, the northwestern region of the plant was severely affected by nuclear radiation, which is indicated by the red colour. This map describes the reason why the authorities expanded the evacuation zone from a 20-km radius to 30-km because the 30-km radius zone was the area that experienced high nuclear radiation. Although the number of people who evacuated is the subject of debate, in total, including both mandatory and voluntary evacuation, approximately 164,000 to 380,000 people moved to another place. A study argues that the preparation for tsunamis was not enough. In the designing process, the possibility of tsunamis and earthquakes should be considered more carefully. The design of the room that had an emergency generator indicates that it was the design failure that caused the meltdown. Although the inundation of the room was avoidable, the design of the room was low-lying and unprotected, which resulted in cutting the power supply to the water pump to cool the core.

Oncoming Nankai Megathrust Earthquake

Background

The Nankai megathrust earthquake is an anticipated earthquake that would occur at the Nankai Trough, which extends for 700km from the southwest of Kōchi Prefecture (A in Figure 6) to the east of Shizuoka Prefecture (E in Figure 6). As Figure 3 indicates, the Nankai Trough is a subduction zone where the Philippine Sea plate is subducted beneath the Eurasian plate. The earthquake mechanism is the same as the ones that occur at other subduction zones, at which a plate ruptures or moves abruptly.

Figure 6: The Nankai Trough and its 3 fault segments

Historical occurrence

Historically, the Nankai Trough has hosted a lot of large earthquakes, such as the 1944 Tonankai (M_w 8.1) and the 1946 Nankaido (M_w 8.3). There are three more earthquakes that were larger than the magnitude of 8 since the eighteenth century. All of these earthquakes had an association with tsunamis, which killed over 1,000 people. The historical record is a useful source of information because it can tell a lot of information. For example, the historical record enabled scientists to divide the Nankai Trough into three different fault segments, the Nankai, Tonankai, and Tokai, based on the historical occurrence of earthquakes in these segments (Figure 6). More specifically, the 1944 Tonankai and the 1946 Nankaido did not occur at the same segments because their time interval was too short as the interval of earthquake occurrences is approximately 100-200 years. In the Tokai segment, the most recent earthquake occurred in 1854, which means that it is more likely to experience a megathrust earthquake in the near future.

Earthquake prediction

The term "earthquake prediction" refers to the estimation of a potential seismic event that might happen or not happen within a specific area, timeframe, and magnitude. It involves assigning a probability value, denoted as p (0.0 < p < 1.0), to indicate the likelihood of the earthquake occurring. The purpose of this prediction is to alert the population about the potential risk. This is done by several precursors such as gases, geophysical observations, and statistical patterns. An increase in radon, which is a gas that does not have colour, taste, and odour, was observed before seismic activity at various places, such as Japan and the Philippines. Radon is the most studied gas for earthquake prediction because it is considered that its emanation is associated with increased pressure in the plates. However, it is believed that earthquake prediction is currently impossible because most precursors often occur even though the events were not followed by earthquakes. Since earthquake prediction requires a specific area, timeframe, and magnitude, it is impossible with current knowledge and technology, however, it is possible to know some patterns based on historical and statistical records. Using them, the Japanese government expects that the likelihood of the next megathrust earthquake in the Nankai Trough within the next 30 years is 70-80%. This is not a prediction of an earthquake because it does not specify the exact area where the earthquake occur, the exact time when it occurs, and the exact magnitude.

Potential impacts

In 2012, the Japanese government amended the damage estimate of the Nankai Trough Earthquake because of the 2011 Tōhoku earthquake whose impacts were greater than expected. In addition to that, the damage estimate was relatively outdated since the damage estimates for Nankai and Tonankai were released in 2003 and the one for Tokai was released in 2001. In the updates, the anticipated magnitude increased from 8.0 to 8.4 in the previous version to 9.1 in the 2012 version, and the update was derived from the latest scientific evidence. Based on the damage estimate, it is predicted that the number of fatalities in the worst case would be 323,000, and tsunamis would be responsible for 71 percent of them. For the economy, it is predicted that the earthquake will potentially damage about a quarter of Japan's gross domestic product, which is equivalent to 32.37 trillion yen. The estimation of the damages is significantly influenced by empirical data from the 2011 Tōhoku earthquake. Although it is highly dependent on the type of industry, it is estimated that many industries are still in the process of recovery three months after the disaster.

Figure 7: An example of emergency supplies

Preparation

The preparation or countermeasure for both earthquakes and tsunamis are essential to mitigate the impacts of the disasters. At the national government scale, the disaster management plan and the emergency response are systemized or organized not only in top-down approaches but also in bottom-up approaches. A specific guideline of the countermeasure is generally made by local municipal governments with the respect to following steps of the disaster response levels: preparation, initial response, response, and recovery. The government made basic policies to reduce overall damages, which focus on the protection of people and properties, in the next 10 years from 2021. The policies have seven different measures that are expected to reduce more than 80 percent of fatalities and half of the number of collapses of buildings. The first measure is against earthquakes, in which the reinforcement of buildings and infrastructure against landslides, soil liquidation, fires, and earthquakes is implemented. The second is against tsunamis, and it aims to establish secure evacuation routes and a community that has enough resilience against tsunamis. Other than these, the government is trying to reinforce comprehensive resilience, such as securing food, water, fuels, hygiene, and health of people. In addition to the government's efforts, individuals' efforts are also important since it is ultimately themselves who protect their lives. The local or municipal government often creates a guideline or information booklet for its residents to advise them on what they should do. For example, an information booklet about the Nankai Trough Earthquake, which was published by Kōchi Prefecture, suggests several things that individuals can do. Securing furniture is one of the suggestions in which people can ensure that furniture will not fall over by using metal fittings, tapes, and belts or considering the furniture placement. Another important thing that individuals can do is preparing emergency supplies and stockpiles. Emergency supplies are the goods that can be carried during evacuation time. The items include daily medicine, helmets, flashlights, portable radio, spare batteries, cash, and etc.(Figure 7). On the other hand, emergency stockpiles are things that should be stored for life in a shelter, and they are mostly drinking water and food that should be enough to sustain life for more than 3 days. This is crucial and must be prepared because it takes time to recover infrastructures. There are many other things that individuals can or should do, and it is significantly important to be prepared to protect lives.

Conclusion / Your Evaluation of the Connections

Understanding the mechanism of earthquakes and tsunamis is crucial to protect people's lives because we can know the general patterns or processes so that we can be prepared. A disaster occurs when natural processes impacted human society. In other words, the natural or geological processes that did not have any impacts on human society are not a disaster, they are just natural processes. Making connections between geological processes and human society is crucial since most geological processes affect our lives to some degree. Although it is still impossible to make an earthquake prediction, it is possible to find a general pattern or cycle, like earthquakes are more likely to occur at the subduction zones in the interval of certain years. The general pattern or cycle is often based on historical and geological evidence, and they enable us to be prepared for disasters because we can estimate the likelihood of earthquakes in a certain time interval. Although the effects of earthquakes and tsunamis are dependent on various external factors, such as a country's GDP, budgets, and education, it is ultimately our responsibility to protect our own lives. So, it is important to interpret the geological processes of earthquakes and how they are related to our society.

References

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