Japan’s Earthshaking Reality

Land of the Shifting Plates

Japan is one of the most earthquake-prone places on Earth, accounting for no less than 10% of all tremors worldwide despite making up just 0.3% of the world’s total land mass. What is the reason for this abundance of earthquakes?

The answer lies in Japan’s geographical location, close to the tectonic plates whose shifting movement causes earthquakes. The surface of the Earth is covered by numerous plates that vary in size from a few dozen kilometers to more than 200 kilometers in thickness. The plates under the seabed, called oceanic plates, expand by several centimeters each year. This forces the denser oceanic plate underneath the continental plate—a process called “subduction.” As a result, tension builds up within the adjacent crust over the years. When this tension reaches a certain point, the continental plate buckles, fracturing the rock in the plates and causing an earthquake.

The Japanese archipelago is located close to the Japan Trench, where the Pacific Plate pushes under the North American Plate; and to the Nankai Trough, where the Philippine Sea Plate meets the Eurasian Plate. This is an area of violent seismic activity. Earthquakes in this region, called trench-type earthquakes, occur cyclically in roughly the same location at intervals of decades or centuries.

Centered on the ocean floor, trench-type earthquakes carry a strong risk of tsunamis. The buckling of the continental plate that triggers the earthquake can also displace seawater vertically, causing a tsunami to occur. The tsunami gains in height as waves run into each other as they approach the shoreline. When the tsunami enters a bay, the shallower water and the shape of the bay itself interact to cause the tsunami to increase even further in height. The Tōhoku region is particularly vulnerable to tsunamis because of the large number of submerged river valleys along the Pacific coast. These form narrow coastal inlets connected to the sea called “ria”that heighten the effects of any tsunamis that occur along this coast.

Living with the Earthquake Threat

Trench-type quakes are not Japan’s only seismic headache, however. “Shallow crustal earthquakes” centered on the active faults beneath the earth’s crust can also cause massive devastation. Recent examples of this type of earthquake include the Great Hanshin-Awaji Earthquake in 1995 and the Iwate-Miyage Nairiku Earthquake in 2008. 

The pressure that results from the subduction of the oceanic plates accumulates not only in the adjacent area but within the continental plates as well. This strain is eventually relieved by earthquakes that result from the shifting of plates up to 20km below the Earth’s surface. Unlike trench-type earthquakes, the intervals of seismic activity for these active faults are extremely long—up to a thousand years. It is estimated that Japan is home to around 2,000 of these active faults. This means that an earthquake can occur almost anywhere, at any time.

For the Japanese, living atop this shifting land, earthquakes are an ever-present danger. Measures to prevent disasters and to minimize the damage caused when an earthquake strikes are taken extremely seriously. In the aftermath of the Great Hanshin-Awaji Earthquake, building standards laws were tightened to make public facilities and other structures more resistant to earthquakes. People in the Tōhoku region , where scientists had long predicted a major earthquake within the next few decades, were particularly well aware of the need to make preparations by securing household furniture and similar steps. Such preparations probably saved lives in many cases. In Shizuoka Prefecture, for example, a magnitude 6.0 earthquake on March 15 only resulted in injuries to a few dozen people.

A full analysis of how effectively Japan’s policies minimized the damage caused by the Tōhoku Pacific Earthquake has yet to be carried out within Japan itself, but analysts in the Western media have already pointed out some of the benefits of Japan’s disaster prevention measures. Stories run by the Associated Press and Agence-France Presse highlighted Japan’s earthquake early warning system, while a Washington Post article explained how strict earthquake-resistance standards were introduced after the Great Hanshin-Awaji Earthquake.

As in the case of that major 1995 earthquake, it seems certain that the Tohoku Pacific earthquake will lead to another concerted effort to raise the level of Japan’s earthquake preparedness.  

Major Earthquakes and Tsunami in Japan

Major Earthquakes and Tsunami in Japan (Based on data from the Japan Meteorological Agency)

* The Japan Meteorological Agency does not have records on seismic intensity for pre-1925 earthquakes

* Seismic intensity measuring level 1 or higher on the JMA scale was not recorded anywhere in Japan during the Great Chilean Earthquake/Tsunami