Japan’s capital city, Tokyo, has a high probability of experiencing a magnitude-7 earthquake within just a few years, according to Japanese scientists.
Researchers at the University of Tokyo’s Earthquake Research Institute announced Monday there was a 75 percent chance that a 7.0 magnitude quake could strike the region within four years, and a 98 percent probability of one striking in the next 30 years.
The announcement coincided a magnitude 5.1 quake that rocked large parts of north-east Japan, which was already suffering from a March 11, 2011 magnitude-9 earthquake. There were no reports of death or injuries from the quake, but train lines were briefly shut down as a precaution. Tokyo Electric Power Co. told reporters the tremor caused no further damage to the Fukushima Daiichi nuclear plant ravaged from last year’s tragedy.
The region has been rocked by nearly 700 aftershocks of magnitude 5 or greater following last year’s quake, according to the government.
Dubbed the Tohoku quake, last year’s event was the fourth strongest quake in recorded history. It caused abrupt stress changes in the upper plate and triggered widespread seismic activity throughout the Japanese island. The northern part of Ibaraki Prefecture, in particular, saw a significant increase of shallow seismicity after the quake compared with the extremely low rate eight years prior, the researchers said.
The last time Tokyo itself was hit by a big earthquake was in 1923, when a magnitude 7.9 temblor killed more than 100,000 people, many in fires that broke out. Researchers say that since the March 2011 quake, seismic activity in Tokyo has increased dramatically, which leads to a higher probability of a major earthquake.
While basing their calculations on data from Japan’s Meteorological Agency, the research institute says it is still “hard to predict” the casualty impact of a major quake on Tokyo, but government and individuals should be prepared for it all the same.
The Central Disaster Management Council has estimated that a large-scale earthquake in the next few decades could result in 11,000 deaths and an economic loss of 112 trillion yen ($1 trillion US). Its goal is to halve the estimated death toll and reduce the estimated economic loss.
The Japanese government’s Headquarters for Earthquake Research Promotion puts the magnitude-7 earthquake probability at 70 percent for the Kanto region, which includes Tokyo and Yokohama, within 30 years.
The government’s model estimates that a magnitude 7.3 quake beneath Tokyo Bay could kill 5,600 people in the city and injure more than 145,000, seriously injuring about 15 percent of those.
The model shows that most of the 23 wards of the city would experience an intensity above level 6, with buildings along the rivers that lead inland from the bay susceptible to liquefaction and others in densely populated areas succumbing to fire. More than 25 percent of all buildings in the 23 wards would be destroyed, accounting for nearly 500,000 buildings, according to the model.
Tokyo was fortunate to escape the worst of last year’s earthquake, although the quake still measured an intensity level of five and the tsunami reached 5 feet high in Tokyo Bay. Seven deaths and 116 injuries were attributed to the quake, with around 3,500 buildings being damaged, 13 of which collapsed.
Gary Gibson, a seismologist at Australia’s Seismology Research Center, said he wasn’t surprised by the research institute’s findings, given the level of seismic activity and their model used.
“Seismologists cannot predict with certainty when, where and how large the next big earthquake will be,” Gibson told CNN by email. “However, it is possible to forecast the probability of an earthquake in a particular time range (e.g. the next four years), location area (e.g. the Tokyo region), and magnitude range (e.g. greater than magnitude 7.0).”
“Long-term forecasts use average activity levels from all known past earthquakes, geological data from fault displacements, and data about plate movement and deformation from very precise GPS measurements and other methods to determine these probabilities,” he wrote.
“A magnitude 7.0 earthquake is very much smaller than the magnitude 9.0 Tohoku earthquake,” Gibson wrote. “It would need 1,000 magnitude-7 earthquakes to release as much strain energy as the magnitude-9. The fault rupture size is tens of kilometers, rather than hundreds of kilometers, and the fault displacement two to three meters rather than 20 to 30 meters.”
And depending on location (offshore or onshore) and depth, damage could either be heightened or limited. A shallower quake, with 6 miles of the surface, would result in more localized damage, Gibson added.