Photo: Yuriko Nakao/Reuters

ALL FALL DOWN: Emergency workers in Machida, just west of downtown Tokyo, practice rescue efforts as part of Disaster Prevention Day drills.

September in Japan always begins the same way: Disaster Prevention Day. Across the country, schoolchildren don protective headgear and take cover under sturdy desks, the better to avoid falling debris; public-safety workers conduct mock searches and rescues for victims stuck under rubble; and medical ­personnel attend to the faux injured.

The annual ritual underscores a simple fact of life for the Japanese: earthquakes happen. Tokyo, the gleaming megalopolis of 35 million, lies in a particularly vulnerable area. Three tectonic plates converge 300 kilometers east of the city, while a chain of active volcanoes stretches 100 km to the west. Tokyoites experience the odd tremor every other week on average, and the city has been rocked by five major quakes in the past three centuries.

The country’s annual disaster exercise occurs on the anniversary of the deadliest of these episodes, the Great Kanto Earthquake of 1923. In that catastrophe, 140 000 people perished, and much of Tokyo and Yokohama was leveled.

The likelihood of something similarly calamitous taking place by the middle of this century is uncomfortably high. A group of U.S. and Japanese earthquake experts, funded by Swiss Re (a Zurich-based ­company that insures insurance companies), recently put the probability of a magnitude 7.3 earthquake hitting greater Tokyo in the next 30 years at 35 percent. And the numbers only get worse the further out you look.

Put another way, the odds that a massive earthquake won’t strike Tokyo at some point are, essentially, zero.

“We can try to pin down the exact probabilities, but in a sense it’s not worth talking about,” says Thomas Heaton, a professor of geophysics and an earthquake specialist at the California Institute of Technology. “Because if you build in Tokyo, you have to assume that large earthquakes will happen.”

The costs of a major quake will be enormous. According to Japanese government estimates, if a magnitude 7.3 event were to hit during the evening rush hour, say, with high winds, greater Tokyo would suffer 11 000 deaths, 210 000 injuries, and the destruction of 840 000 buildings. The projected monetary loss would approach US $1 trillion, or about 130 percent of Japan’s entire annual budget. Fire would account for most of the casualties and property damage, just as it did in the Great Kanto earthquake. But back in 1923, the Japanese capital was not a world center of commerce and culture. It is now. Although other large cities, even megacities, are vulnerable to earthquakes—San Francisco, Istanbul, and Tehran come to mind—none holds Tokyo’s pivotal place in the international economy. A demolished Tokyo this time around could prove a global catastrophe.

Mindful of the statistics, the Japanese government has spent the last five years constructing an automated earthquake early-warning network. It’s the first national system of its kind and by far the most sophisticated. Consisting of more than 1000 seismometer stations scattered around the country, the network is designed to detect the first tremors of an earthquake, calculate the likely source and magnitude, and then broadcast an alert—all within seconds. The alert would offer only a brief window of warning—a few tens of seconds at most—before the earthquake’s full force struck. But even that short a lead would give automated systems installed at railroads, power stations, hospitals, schools, and the like sufficient time to take action and save lives and property. At least that’s the hope.

Twelve years ago, a massive earthquake flattened large swaths of Kobe and surrounding areas and killed some 6400 people. That it occurred at all came as a profound shock, because the area had been considered geologically stable, at least by Japanese standards. The amount of devastation, as well as the slow response in rescuing survivors, served as a wake-up call.

Before the Kobe quake, the Japanese government had been most concerned about a major earthquake striking in the Tokai region, just west and south of Tokyo, and much of the funding for earthquake research focused on that area, says Yukio Fujinawa, senior managing director of a nongovernmental group called the Real‑Time Earthquake Information Consortium (REIC).