There is between a 7 and 40 percent chance of a magnitude-9 earthquake striking along the Kuril Trench off Hokkaido within the next 30 years, according to a long-term assessment released by the government's Earthquake Research Committee on Dec. 19.
The assessment marks the first time the committee has released the probability of a mega-quake comparable to the March 2011 Great East Japan Earthquake.
The panel did not rule out simultaneous mega-quakes along the Kuril Trench and the Japan Trench off Honshu's northeastern Tohoku region, which could spell disaster for not only various areas in Hokkaido along the Pacific Coast but also Japan's main island.
"The probability is extremely high. There is a good chance that a quake of the similar magnitude to that of the Great East Japan Earthquake is imminent. It is necessary to take thorough precautions," said Naoshi Hirata, chairman of the committee.
In the early 17th century, a magnitude-8.8 quake is believed to have struck the Pacific coastline of eastern Hokkaido, with the epicentral areas off both Tokachi and Nemuro moving in conjunction. A tsunami over 18 meters high and reaching as far as 4 kilometers inland is thought to have hit the area, causing considerable damage. Although damage to the Tohoku region from the temblor has not been confirmed, the area may once again suffer massive tsunami.
The committee's survey of tsunami deposits on land has shown that up to 18 separate mega-quakes in the same range as the 2011 disaster hit Hokkaido's east coast over the past 6,500 years, with an average interval of 340 to 380 years. The most recent such earthquake occurred some 400 years ago, raising the possibility of an imminent temblor of similar magnitude.
The panel assessed a potential mega-quake in three regions based on different epicenter locations: one each off Hokkaido's Tokachi and Nemuro; and one off Shikotan and Etorofu islands -- two of the four Northern Territories isles held by Russia and claimed by Japan.
Regarding magnitude-8 level quakes, the chances of an M8-8.6 temblor off Tokachi is 7 percent, while the chances of a magnitude-7.8 through 8.5 temblor off Nemuro were calculated at about 70 percent. The committee furthermore estimated the chances of an M7-7.5 quake off both Tokachi and Nemuro at around 80 percent.
The panel has previously released similar long-term assessments for maximum possible quake magnitudes along the Nankai Trough off central and western Japan, and the Sagami Trough off central Japan. According to these reports, the likelihood of a magnitude-8 through 9 quake in the Nankai Trough is somewhere around 70 percent, while that of a magnitude-8 temblor jolting the Sagami Trough is approximately 0-5 percent.
In another long-term assessment for active faults in the Shikoku region in western Japan released the same day, the committee put the probability of a magnitude-6.8 or larger quake striking directly beneath the Median Tectonic Line active fault zone and the Nagao fault zone in Shikoku within the next 30 years at 9 to 15 percent. As the epicenters of many past quakes that hit directly beneath certain areas were shallow, these temblors would likely cause tremendous damage.