BY TAKASHI SOEDA, THE ASAHI SHIMBUN
The heavily populated Tokai region, which experts say is due for a major earthquake within 30 years, has experienced at least three “super” temblors of unimaginable destruction during the past 5,000 years, according to a study.
Unlike so-called Tokai earthquakes that occur every 100 years or so, a “super” earthquake is one that causes dramatic change to the landscape through shifts in the Earth’s crust.
The envisaged Tokai quake, whose epicenter would be in the Bay of Suruga facing Shizuoka Prefecture, has an anticipated 87-percent likelihood of hitting by 2037, according to a government taskforce.
It would be in the region of magnitude 8. But some seismologists say the coming quake could actually be a “super” jolt that occurs every millennium or so.
If seismologists are correct in their assessment of risk, a “super” Tokai quake could cause crustal movement three times as big as that of an ordinary Tokai earthquake.
Katsuhiko Ishibashi, a professor of seismology at Kobe University who theorized that a major, recurring earthquake is imminent in the region, called for additional preparedness. His hypothesis prompted the central government to draw up emergency measures to deal with such a disaster.
“It is almost certain that a ‘super’ Tokai earthquake is far mightier than the Tokai quake that we have been anticipating,” Ishibashi said. “The problem is that we are clueless as to its mechanism.
“The next Tokai quake may be of this type and we need to be prepared for this.”
The findings were based on bore surveys conducted in the vicinity of Chubu Electric Power Co.’s Hamaoka Nuclear Power Plant in Omaezaki, in the prefecture, in 2005 through this year.
The findings were reported at a conference held in Kobe through Sunday of the Japan Association for Quaternary Research. The group is devoted to the study of geology and other disciplines over the past 2.6 million years.
The bore surveys were led by a tam including Osamu Fujiwara, a researcher with the Active Fault Research Geological Survey of Japan at the Advanced Industrial Science and Technology, an independent administrative agency, and Kazuomi Hirakawa, a seismologist at Hokkaido University.
The team drilled more than 10 meters at eight sites in an area about 2 kilometers east of the Hamaoka Nuclear Power Plant. The survey was done to find out crustal movements by dating fossilized plants and shells in the ancient sediment.
The researchers confirmed that a Tokai earthquake has jolted the region periodically every 100 to 200 years for the past 8,000 years ago. They also found traces of much more powerful quakes that shook the region about 4,800 years ago, between 3,800 and 4,000 years ago and 2,400 years ago.(IHT/Asahi: September 5,2007)