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On January 17th 1995, the Great Hanshin Earthquake, commonly referred to as the Kobe Earthquake, killed over 6000 people in Japan. This natural disaster has made people wonder how large earthquakes could affect nuclear power plants. However, none of teaching materials refer to the risk of nuclear disasters caused by earthquakes. Does that sound right to you?
In 1922 the Great Kanto Earthquake struck Kanto and shattered downtown Tokyo. In Kansai (west side of Japan), despite the former large earthquake that occurred in that area after the WWII, there was little or no warning there would be another one. People did not expect a large earthquake in that area until the 1995 Great Hanshin Earthquake devastated them. The occurrence of the Great Hanshin Earthquake has changed the general view of earthquakes.
The world seismic zoning map shows large earthquakes of over M (magnitude) 4 as shown on the right. Above all, whole Japan is located in the area where earthquakes frequently occur. You cannot see the shape of Japan here because dots of earthquakes cover all over Japan. Japan is one of the places where earthquakes occur most frequently on the earth.
Why do earthquakes occur in specific areas? By comparing the world seismic zoning map (Fig.1) with the distribution of the world’s major plates and plate boundaries (mobile belts) (Fig.2), you can see that earthquakes frequently occur on the plate boundaries (mobile belts) where continents and the ocean lie. As you see, earthquakes do not occur throughout the earth but only on specific belts.
Geological processes deep underground that have continued for millions of years are responsible for the geological features we see on the surface of the earth today; land has become the ocean; the ocean floor has upheaved and become mountains. The active tectogenesis concentrates on the narrow area called the “mobile belt”, and geological research shows that the mobile belts have been transiting with time. The area where earthquakes frequently occur lie on the mobile belts formed within last several hundred thousand years or several million years.
The reason why major tectogenesis and seismic volcanic activities occur only in narrow mobile belts is because whole lithosphere that covers the earth are divided into several blocks, and each of them are moving slowly towards different directions on the surface of the earth with little internal change, and then excessive transformation converges on the boundaries. This situation is clearly explained through global space scale and geological time scale; ‘plate tectonics’, a system of earth science, explains this movement.
There are three types of movements of adjoined plates: moving away from each other, colliding to each other, and passing each other. The movement that is related to the occurrence of earthquakes in Japan is the ‘colliding’ action.
As shown in Fig.3 a, the Pacific Ocean plate subducts under the land plate of the Japanese archipelago. Accompanied with this movement, excessive transformation and its resistance accumulate around the plate boundary and the weak plane inside of the plate. As shown in Fig.3 b, when the transformation reaches its limit, the land plate tries to repel and it causes an earthquake. The majority of large earthquakes, such as the 1960 Chile Earthquake (M9.5) and the 1964 Alaskan Earthquake (M9.2), have occurred on this subduction boundary.
As shown in Fig.1, Japan is one of the countries that earthquakes occur most frequently in the world. Why is that?
As shown in Fig.4, the Japanese archipelago is located on the middle of the Pacific Ocean Plate that has a long subduction zone, and offshore there is a range of Kuril Trench, Japan Trench, and Izu-Ogasarawa Trench. In these trenches the subductions have been continuing for approximately 40 million years. On the west of Izu-Ogasarawa Trench there is the Philippine Sea Plate, on the north there is Amur Plate and the Sea of Okhotsk Plate. Around the Japanese archipelago, four plates are competing with each other. That’s why Japan has one of the highest seismic activities in the world.
As shown in Fig.4, there are working nuclear power plants on the ground of the seism activities, and most of them are located in the observation area for earthquake prediction, or adjoining this area.
Earthquakes occur periodically. Let’s take a look at great earthquakes in Suruga-Nankai Trough in southeast offshore Japan as an example (Fig.5). Many great earthquakes occur in Japan because four plates are competing with each other. It is considered that the Philippine Sea Plate is involved in the occurrence of most Japanese earthquakes. This plate has been subducting rapidly towards the Kanto and Tokai area, where it accumulates colossal stresses. In Tokai area, there has not been any large earthquake since 1854; it is said that Tokai Great Earthquake will occur in the near future when these stresses are released.
Other than earthquakes mentioned above, there is a type of earthquake that occurs inside of a plate, such as the South Hyogo Earthquake (1995, M7.2), the off Kushiro Earthquake (1993, M7.8). This is a type of earthquake directly above its epicentre and it causes enormous damage, but it is not very clear what condition causes intraplate earthquakes.
Fig.4 shows both regions where earthquakes frequently occur and locations of nuclear power plants. Considering the mechanism of earthquakes, it is understandable why nuclear power plants in Japan are called “nuclear power plants on tofu” or “nuclear power plants on a catfish”. Why have so many nuclear power plants been constructed even though Japan is well-known as an earthquake kingdom? Let’s listen to what power companies say.
Power companies insist that they are taking the surest measures for earthquakes when building nuclear power plants. Nuclear safety committee that is supposed to protect citizen’s safety has no complaint about it. The followings are examples of earthquake policies shown in Chubu Power Company’s website (http://www.chuden.co.jp/torikumi/fr_atom.html)and problems based on their earthquake policies.
(Translated by Noriko Matsubara)