Outside of an asteroid hitting a densely populated area, the world’s biggest catastrophe will likely happen in Tokyo, Japan. It’s not a question of if, but when:

Thinking and writing about such matters is unsettling, but we have learned two great lessons from many modern disasters: 1) our response to them is always initially more chaotic and less effective than envisaged in model scenarios; but 2) a higher degree of preparedness can make a substantial difference, both in avoided death and injury and in property damage.

Earthquakes cannot be predicted, but probabilistic appraisals are another matter. Bozkurt et al. used more than 10,000 observations of earthquake intensity accumulated since 1600 to estimate that the probability of severe shaking in Tokyo is 30 to 40 percent during an average 30-year period. In 2006, the Earthquake Research Committee of Japan estimated the probability of an earthquake with the epicenter beneath northern Tokyo Bay at 70 percent in the next 30 years, with the area including Tokyo, Chiba, Saitama, and Kanagawa experiencing shaking of at least magnitude 6, and 25 million people, or 20 percent of Japan’s total population, affected—an event unprecedented in global history.

In the constructed scenario, the earthquake occurs at 6 p.m. on a winter day and damage was calculated for two wind speeds: about 22 km/hour and for a very windy evening with 54 km/hour (the latter average being an unlikely maximum based on the wind speed during the Great Kanto Earthquake of 1923). This was the first time estimates were made for many types of damage a major earthquake would trigger in the world’s largest metropolis. According to this scenario, there would be about 6,400 instant deaths (more than half of them due to fires) and more than 160,000 severely injured people, many of whom would die because they could not be rapidly transported to hospitals, and emergency wards would be taxed far beyond their capacity. Even with the city’s extraordinary advances in earthquake-proof construction, the scenario expects about 462,000 damaged buildings (no tsunami would reach the city in this scenario).

Much larger numbers of people would be affected in many other ways, lasting hours to months. The city that is the global paragon of long-distance urban commuting would see both its highways and railways cut at more than 600 sites and would lose almost a fifth of its electricity supply (all subways and railways are electrified), and hence nearly 4.5 million people would not be able to return home. Even a day later, their number would be still just shy of 4 million. Should the earthquake happen during the coldest part of the year, the city would face a globally unprecedented task of finding emergency shelters for 4 million people who would also be thirsty and hungry.

The most pressing structural challenge would be to restore utilities. At least a third of water supply infrastructure would be damaged, as would close to 20 percent of natural gas flows and more than 20 percent of sewage facilities. The city would face the task of removing about 40 million tons of debris, mostly a jumble of concrete and metals. After 2011’s Tohoku earthquake, the Cabinet Office for Disaster Management revisited these estimates, putting the death toll at 11,000 people, injuries at 210,000, collapsed buildings numbering more than 850,000 (650,000 due to fires), and the total economic loss exceeding ¥112 trillion.

With Japan’s GDP in 2010 being about ¥540 trillion, the damage would be equivalent to at least 20 percent of the country’s annual economic product. As already noted, the country’s GDP (all figures adjusted for inflation) fell by less than 5 percent in 1923, the year of Tokyo’s last great quake, and the war-induced GDP decline was on the order of 50 percent (no value is available for 1945 but in 1946 the economic product was 45 percent below the 1944 level). By any measure, a virtually instant loss of 20 percent of economic product in the world’s third-largest economy would be a disaster of historic, and unprecedented, significance.

We can only do so much to prepare against Mother Nature’s wrath…

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(via Farnam Street)