2-3 Plate tectonics

In the previous section, we described how pressure builds up underground, resulting in an accumulation of strain in the rock mass. An earthquake occurs when this strain reaches its limit. How much pressure is building up beneath the Japanese archipelago and the surrounding areas? This pressure is extremely difficult to measure directly. Therefore, the method used is to observe the extent of the strain and deformation of the earth as a result of the pressure and then to work backwards, estimating the extent of the pressure built up underground over a wide area.

Recent advances in space technology have enabled us to accurately, efficiently, and continuously measure deformation in the earth over a wide area. Typical of these technological advances is a measurement technology using satellites, the Global Positioning System (GPS) for continuous observation. Fig.2-14 shows the extent and direction of the movement of the land throughout the entire the Japanese archipelago for the past year as determined by the use of this technology. This movement is in relation to an observation point in Suwon, Republic of Korea. No large earthquakes occurred in the Japanese archipelago during this period. Therefore, we believe this figure shows the stationary movement of each area of the Japanese archipelago in relation to the Continent. Fig.2-15 shows a calculation of the deformation in the Japanese archipelago during the measurement year based on the results of these measurements. There is compression from a northwest-southeast to a north-northwest south-southeast direction in the area from the Pacific Ocean side of Hokkaido and the southern Kanto area to the Kii Peninsula (Kii Hanto) and Shikoku. In other areas, there are deformations indicating compression in an east-west direction.

Thus, we can assume that strong compression is now occurring in an east-west direction underneath the Japanese archipelago. Depending on the location, this compression sometimes occurs in a northwest-southeast direction. A similar conclusion can be obtained from an analysis of seismic observation data. The results of various topographical and geological surveys are form the basis for the hypothesis that a similar force has been exerted underground for quite some time - at least several hundred thousand years.

Is it possible to present a uniform explanation of earthquake distribution, as described in Section 2-1, and the forces at work in the Japanese archipelago described in this section? In the zone that is part of the belt off the Pacific coast of the Japanese archipelago, the frequent occurrence of large earthquakes indicates that prominent weak planes (fault planes) exist along this zone; in other words, fault movement frequently occurs along the planes. This leads us to believe that the underground structure of the zone in this Pacific Ocean belt is discontinuous and that there must be some sort of boundaries present. Meanwhile, the pressure applied in an east-west direction over a wide area throughout the Japanese archipelago is being applied to the archipelago from the east. It can be conjectured that the Continent is applying pressure from the west. Plate tectonics, a new earth science that emerged at the end of the 1960s, explains this clearly.

Here is the basic idea behind plate tectonic.

The entire surface of the earth is completely covered by dozens of slabs of rock mass of several-tens-km thick. These slabs are called "plates", and drift in different directions at a speed of several centimeters per year. At their boundaries, these plates move in a divergent direction, convergent each other, or passing each other (Fig.2-16).

Next, we will consider the plates in the Japanese archipelago and the surrounding areas. The results of different types of research show that several plates are converging each other in the Japanese archipelago and the surrounding areas. At the boundary of these converging plates, both plates are pushing against each other. As a result, a huge mountain range similar to the Himalayas is being formed, as one plates subducts the other. Along the site of this subduction, large trench-like terrain is being formed, including trenches. This type of submarine topography can actually be seen in areas close to the Japanese archipelago (Fig.2-7).

There are at least three plates in the Japanese archipelago and the surrounding areas: the Pacific Plate, the Philippine Sea Plate, and a plate on land (Fig.2-17). The Pacific Plate approaches the Japanese archipelago from the east-southeast at a speed of 8 cm annually, and subducts under the land plate from the Japan Trench (Nihon Kaiko) and other areas. Observation of earthquake distribution at a deeper location from a profile that cuts across the Japanese archipelago (northeast Japan) from east to west clearly shows that the Pacific Plate is subducting under the archipelago (Fig.2-5). The Philippine Sea Plate is approaching the archipelago from the southeast at a speed of roughly 3-7 cm per year, and is subducting under the land plate at the site of the Nankai Trough. The boundaries between the land plate and the subducting plates are large (weak boundaries). There is a powerful compression force operating between the plates that are approaching each other. It is easy to imagine that many large earthquakes (fault movements) are generated here. One theory postulates that a plate boundary exists off the Sea of Japan (Japan Sea) coast of northeast Japan (the eastern margin of the Sea of Japan) (Fig.2-17, broken line). A series of earthquakes has occurred here in recent years.

Thus, the occurrence of earthquakes along trenches is believed to have a direct relationship with the subducting plates. Moreover, earthquakes in shallow land areas occur a slight distance from the boundary of the subducting plates. These earthquakes are thought to occur due to the pressure exerted on the surrounding area with the subducting plate.

In the next section, we will provide a more detailed classification of the types of earthquakes that occur in the Japanese archipelago, and describe their characteristics.

With the development of new observation and research techniques for studying plates in the Japanese archipelago and the surrounding areas, a more detailed understanding of the positions and movements of the plates can be exported in the future.