In newspaper headlines in January the warning went out: a 70 percent chance within four years of a magnitude-7 earthquake striking beneath Tokyo.
That was based on predictions compiled by the University of Tokyo's Earthquake Research Institute.
Compared with the "70-percent-chance-in-30-years calculation" provided by the national government's Headquarters for Earthquake Research Promotion in 2004, the new prediction brought a real sense of urgency, causing an uproar as it featured prominently on variety shows and in weekly news magazines.
Naoshi Hirata, 57, the professor leading the university research team, was making television appearances on what seemed like a daily basis.
Is the imminence of a major earthquake really increasing?
The new figure was calculated based on the nature of aftershocks and the rate of occurrence of small and large earthquakes.
In general, large earthquakes occur less frequently than smaller ones, with the frequency dropping by 90 percent for each one point increase in magnitude. Based on the number of magnitude 3 or greater earthquakes occurring after last year's Great East Japan Earthquake, the chance of a magnitude-7 class quake striking was statistically calculated last September at 70 percent within four years.
For the half year preceding the March 11 quake, the number of earthquakes registering magnitude 3 or greater in the southern Kanto region was 47. For the six-month period after the great quake, however, the number increased roughly sevenfold to 343.
Though he was also criticized for "stirring up anxiety," according to Hirata, "The fact that the occurrence of smaller earthquakes is increasing to this extent means the risk of a magnitude-7 class earthquake occurring is also increasing. That is what I wanted to say."
However, since September of last year, the number of earthquakes has been decreasing, and in January of this year the probability of a magnitude-7 class quake occurring was lower, calculated to be 25 to 60 percent within five years. The number of earthquakes occurring at a specific time causes the prediction to change accordingly.
For the record, the forecast provided by the government's Headquarters for Earthquake Research Promotion of 70 percent within 30 years is based on the frequency of magnitude-7 class earthquakes that have occurred in the past.
From 1885 to 2004 five earthquakes of that class have occurred in southern Kanto. This frequency of one every 23.8 years was used to calculate the probability of a magnitude-7 class earthquake striking over the next 30 years.
"Though the impression given by the calculations is quite different, their meaning is the same. We will definitely experience a magnitude-7 class earthquake while we are still alive," said Hirata.
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Four major plates of the earth's crust greatly affect Japan. The North America and Eurasian plates upon which the Japanese archipelago sits, the Philippine Plate, which dives beneath those two plates from the south, and the Pacific Plate, which sinks even lower beneath the Philippine Plate.
"The subsurface structure of southern Kanto is very complex, so much so that there are no other examples like it in the world, there are also many places where earthquakes can occur," said Kunihiko Shimazaki, chair of the Coordinating Committee for Earthquake Prediction, Japan.
As such, there are various types of earthquakes that could possibly strike Tokyo, including interplate quakes, which occur at the boundary between two plates, intraplate quakes, which occur in the interior of the plate, and active fault earthquakes.
So, which type of earthquake is going to occur?
Most of the magnitude-7 class earthquakes that have occurred between 1885 and 2004 have been of the intraplate type, and this is the kind most likely to strike in the future. However, the risk of occurrence for both interplate and active fault types is said to also be increasing due to the catalytic effects of the March 11 quake.
Hirata and his earthquake research team from the University of Tokyo examined about 300 locations in the greater Tokyo area and found the occurrence of earthquakes to be noticeable on plate boundaries in the eastern part of Chiba Prefecture, southern Ibaraki Prefecture and northern Tokyo Bay.
"Due to the effects of the Great East Japan Earthquake, movement at plate boundaries in southern Kanto is quickening," said Hirata, pointing out concern of the possibility of an interplate quake also occurring.
Active faults inland have also become more susceptible to movement due to the force applied by the Great East Japan Earthquake. Last year, the government's Earthquake Research Committee released the names of active faults whose risk of causing a quake may have increased due to the Great East Japan Earthquake.
For the greater Tokyo area, the list of names included the Miura Peninsula Fault and the Tachikawa Fault, among others. The chance of the Miura Peninsula Fault generating a magnitude-6.6 class earthquake within the next 30 years is up to 11 percent, the highest among all active faults in the country.
Active faults are thought to move over long cycles ranging in length from thousands to tens of thousands of years. However, University of Tokyo Earthquake Research Center professor Hiroshi Sato said, "After the occurrence of the huge magnitude-9 deformation of the earth's crust, the crust became unstable and is now continuing to move in order to adjust itself. Traditional assumptions may no longer apply."
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When considering major earthquakes of the greater Tokyo area, the Great Kanto Earthquake of 1923 (magnitude 7.9) comes to mind. The quake epicenter was the Sugami Trough, where the seaward side plate subducts under the landward side plate.
Two hundred and twenty years prior, in 1703, the Genroku Kanto Earthquake (magnitude-8 class) occurred, and it is thought that large-scale earthquakes of a magnitude-8 class occur repeatedly in the Sugami Trough in cycles of between 200 and 400 years. If this cycle is maintained then the chance of a large quake striking now, only about 90 years after the Great Kanto Earthquake, is quite small.
However, the 80- to 90-year period prior to the occurrence of a big earthquake is said to be the active phase. Before the Great Kanto Earthquake a lot of magnitude-7 class earthquakes occurred, including the Ansei Edo Earthquake in 1855 and the 1894 Meiji Tokyo Earthquake.
According to Shimazaki, Coordinating Committee for Earthquake Prediction, Japan chair, "From before it has been thought we would soon be entering an active-phase period; however, it could be sooner due to the effects of the Great East Japan Earthquake."
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