Here in San Francisco, we live under constant threat of a catastrophic earthquake. It's a subconscious worry, perhaps only creeping to the front of our minds as we drive over the Bay Bridge or step out of the shower (would you grab a pair of shorts before bolting for the nearest doorway?).
We know it's coming. Might be later today. Might be in 100 years. Either way, it'll be bad. But this is a first-world city with first-world infrastructure, better prepared than most places around the world.
And as populations rise in every corner of the earth, so do the number of people living under constant threat of a catastrophic earthquake. A new study by the U.S. Geological Survey, published in this month's Earthquake Spectra, has quantified what this means: An estimated 3.5 million people will die from earthquakes this century, more than double the total of the last one.
The number of catastrophic quakes -- defined as tremors with death tolls higher than 50,000 -- will triple from seven in the 20th century to 21 in the 21st century, according to the study, authored by USGS engineering geologists Thomas L. Holzer and James C. Savage, who work out of the agency's Menlo Park office.
It's not that there will be more earthquakes. Rather, more and more people are living in areas prone to earthquake destruction, particularly in the developing world. The study notes that nearly two-thirds of the world's population live in regions where earthquakes pose a significant hazard.
"There are places, like along the front of the Himalayas, that are just waiting for another disaster," Holzer said, according to The Weather Channel. "China, the Middle East and many of the cities in these places just don't design to resist earthquakes. If we don't address this, we're going to see many more catastrophes than we've seen historically, and humanitarian aid efforts are going to be stressed even more over this century. We're going to see more Haiti-type situations."
To reach their estimates, the researchers compared death tolls from past earthquakes -- as far back as the ninth century -- with contemporary populations rates, then measured that correlation against future population projections (the United Nations predict that our world population will hit 10.1 billion by 2100).
Modern global earthquake policy has focused on sending resources after the disaster -- Red Cross tents and volunteers and food packages dropped from helicopters. But, as the authors suggest, preventative measures are also essential.
"Without a significant increase in seismic retrofitting and seismic-resistant construction in earthquake hazard zones at a global scale, the number of catastrophic earthquakes and earthquake fatalities will continue to increase and our predictions are likely to be fulfilled," Holzer said in a statement.
Before 1900, there was one catastrophic (resulting in at least 50,000 deaths) earthquake per century. Between 1900 and 2000, there were seven. In the 12 or so years since, there have been four. If anything, the researchers may have presented conservative numbers. According to the USGS, the world has already seen more than 700,000 earthquake fatalities in this young century.