Six years have passed since the 2011 Great East Japan Earthquake but earthquakes triggered by movements in the Earth’s crust are still occurring in the Tohoku and Kanto areas. Most earthquakes are caused by normal faults, which occur when the rock on one side of the fault is pulled down relative to the other. Experts believe that this makes future major earthquakes highly probable and that Japan should be prepared for the next one.
Earthquakes that occur in eastern Japan are usually caused by reverse faults where bedrocks are compressed and broken. This happens because the Pacific Plate (rock board) that submerges under Japan Trench pushes the Okhotsk Plate, which extends to eastern Japan, to the west. However, the major 9.0 magnitude Tohoku earthquake caused the Okhotsk Plate to shift 65 meters to the west, generating a countervailing force being applied opposite the pulling force from the east.
As a result, the numbers of normal fault earthquakes increased greatly. Out of 10 M7 earthquakes induced by the 2011 Tohoku Earthquake, 6 were caused by normal faults. These include an earthquake that occurred last November off the coast of Fukushima Prefecture which caused tsunamis of up to 1.4 m in height, as well as an earthquake which occurred last month in the northern part of Ibaragi Prefecture measuring 6 or lower on the Japanese Shindo scale.
A magnitude 7.0 earthquake that killed four people occurred in eastern Fukushima Prefecture one month after the 2011 Tohoku Earthquake. Researchers were especially surprised by this incident because it was the largest inland earthquake caused by normal faults and occurred in a region where earthquakes do not usually occur. Two active faults that were thought to have been largely dormant moved, creating a fault protruding above ground by as much as the height of an average adult. Faults that rarely move are often hidden underground, where they may cause unexpected damage.
This earthquake was initially believed to have occurred when the fault being held down in the east was pulled up suddenly in the west. However, Kazutoshi Imanishi, head of the Earthquake Tectonics Research Group at the National Institute of Advanced Industrial Science and Technology (AIST), said that “in this hypothesis, the force suppressing the faults would have to have been very small, which is self-contradictory.”
Imanishi analyzed the mechanisms of small earthquakes which occurred in the past and discovered that normal fault earthquakes have been occurring frequently in this area and that they are even more likely to occur since the 2011 earthquake. This suggests that fault subduction occurred even before the 2011 disaster, which is explained by the fact that the ground under the Tohoku region is even more fractious and broken-apart than realized.
It was not only normal fault earthquakes that increased. Professor Shinji Toda of Tohoku University found that, in five areas of eastern Japan, earthquakes with a magnitude of 2 or higher occurred more than ten times more frequently after the disaster. Toda also discovered that each earthquake had its own characteristics depending on where it occurred.
On the border between Fukushima and Ibaragi prefectures where the earthquake in the eastern part of Fukushima Prefecture occurred as well as in the areas around Choshi city in Chiba Prefecture, the number of normal fault earthquakes increased. On the other hand, the number of strike-slip fault earthquakes increased in the areas around Akita City and Kita-Akita City, while the number of reverse fault earthquakes increased in areas around Kita City, Fukushima Prefecture. This suggested that certain types of earthquakes were more likely to occur in certain areas.
According to Toda, Japanese archipelago was once at the eastern end of the Eurasian continent, but moved to where it is today due to expansion of the Sea of Japan. Many faults were carved out as grooves and striations during this period. He says that the movement of a very large seismic area was one cause of the 2011 Earthquake, which in turn changed the force applied to East Japan over a wide area. “This allowed various types of faults to come to light,” he says.
Such research also helps advance seismology. A series of M7 class earthquakes occurred in the coastal areas of Fukushima Prefecture in 1938. It was long thought that these were reverse fault earthquakes, but it is now believed that they were normal fault earthquakes in the landside plate. If this new interpretation is correct, it may lead to a review of the long-term earthquake forecasts announced by the government.
The increased probability of induced earthquakes can be suppressed relatively quickly in fault areas where stress is applied on a regular basis. However, the probability will continue to increase in areas where stress is not consistently applied, such as in the case of the Nobi earthquake (M 8.0)—one of the nation’s largest active fault earthquakes—which struck Gifu prefecture in 1891. Small induced earthquakes still continue to occur in the surrounding areas even more than a century after the Nobi temblor.
Toda warns that “it will take more than 20 to 30 years, even more than 100 years depending on the location, before the regions where the earthquake probabilities have risen after the disaster go back to their normal state. That is why we need to be prepared.”
Shigeki Harada is a senior staff writer for Sankei Shimbun
Photo is provided by Professor Shinji Toda