Japan achieved its postwar recovery and rapid growth in an era in which it experienced no major earthquakes, but that period of respite, brief in geological terms, now appears to have passed.
Since the Great Hanshin Earthquake in 1995, no less than nine temblors have been powerful enough to earn official names from the Japan Meteorological Agency. They have included the Iwate-Miyagi Inland Earthquake, the Noto Peninsula Earthquake, the Niigata Chuetsu Earthquake, the Geiyo Earthquake, and the Great East Japan Earthquake.
For people living in Japan, the obvious question is whether this traffic jam of catastrophe has just been bad luck, or whether the country has actually entered a period of heightened seismologic activity. Can the country or particular parts of it expect more disasters like those of the past 17 years?
"We do not see any clear active and quiet periods in Japan as a whole, but we have seen differences in certain areas," says Yoshimitsu Okada, president of the National Research Institute for Earth Science and Disaster Prevention.
Adding up the number of earthquakes that hit the Japanese islands each year in the 20th century, it becomes clear that earthquakes of magnitude 7 or less actually occurred at a fairly steady rate, with no clearly defined periods of lessened or heightened activity.
However, the picture changes when one focuses on the Tokyo area and the Kansai region. In the decades prior to and the 10 or so years following magnitude-8 ocean trench earthquakes, inland magnitude-7 temblors show a marked increase in frequency.
There were a string of magnitude-7 earthquakes before and after the magnitude-7.9 Great Kanto Earthquake that occurred in the Sagami Trough in 1923, including the Tokyo Earthquake of 1894 (M7) and the Tanzawa Earthquake of 1924 (M7.3).
Giant earthquakes have occurred every 100 to 150 years in western Japan along the Nankai Trough. The last great earthquakes to strike there were the Tonankai Earthquake of 1944 (M7.9) and the Nankai Earthquake of 1946 (M8), which were in turn associated with the 1925 Tajima Earthquake (M6.8), the 1927 Kita-Tango Earthquake (M7.3), the 1943 Tottori Earthquake (M7.2) and the 1945 Mikawa Earthquake (M6.8).
The fear hanging over the Tokyo area is that the recent increase in magnitude-7 inland earthquakes may have prefigured a major earthquake striking directly under the Tokyo area and that risk is being taken seriously at the highest levels of the Japanese government.
The Cabinet Office recently underlined "the risk of another Tonankai and Nankai earthquake occurring in the first half of this century, preceded by greater seismic activity inland."
It said: "Magnitude-8 earthquakes strike the Tokyo area every 200 to 300 years, but they are preceded by the risk of magnitude-7 earthquakes."
It is believed that magnitude-8 earthquakes have a critical role in determining the pattern of forces working on the Earth's crust in a quake-prone area like Japan. Their effects are quite extensive, reaching hundreds of miles inland away from major offshore faults. But these effects are complex, not only activating active faults under the Japanese mainland but also deactivating others. It is that complexity that may explain the phenomenon of magnitude-7 earthquakes occurring both in the run-up to and in the aftermath of great quakes.
For some time after a major quake, the activated faults will go through periods of increased activity. The active faults that were actually deactivated by the giant earthquake, on the other hand, experience very little seismic activity, thus heralding a period of calm. Earthquakes will again start to occur along these faults after a few decades once the effect of the huge earthquake fades.
An active period can therefore be understood as a three-stage process: 1) Earthquakes start to happen on active faults that were suppressed by a previous major earthquake; 2) A major earthquake occurs; 3) The major earthquake spawns aftershocks and induces other quakes.
Nagoya University professor Takeshi Sagiya, an expert in crust movement, says that although the forces acting on the Japanese archipelago are complex, we can speculate that the "effect on active faults is not small."
Even if a major earthquake only exerts a small force on a fault, he says, the force may be equal to that normally applied every few decades or centuries, forces that would only ordinarily be enough to trigger an earthquake every 1,000 or 10,000 years.
Takane Hori, a member of the data analysis group at the Japan Agency for Marine-Earth Science and Technology, part of the Earthquakes and Tsunami Research Project for Disaster Prevention, calculated the effects of large earthquakes along the Nankai Trough on active faults in western Japan.
He found that before the last Tonankai and Nankai earthquakes, faults suppressed by earlier major earthquakes experienced seismic activity, and that there was evidence of other faults becoming more active as a direct result of the major events.
Hori says many active faults in western Japan actually appear to be suppressed by large Nankai Trough earthquakes.
"There have been a few more magnitude-5 quakes on the suppressed faults since the 1990s, so the region may be transitioning to a more active period," he said.
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