The news coming out of Nepal, where a powerful earthquake struck near the capital city of Kathmandu on Saturday, is horrifying and tragic. But it isn’t surprising: Disaster experts had been warning for years that Nepal faced some of the highest earthquake risks in the world. Indeed, some of those experts were in Kathmandu just last week to discuss the country’s vulnerability. And earlier this month, GeoHazards International, a California-based nonprofit, wrote on its website that the Kathmandu Valley faced “a serious and growing earthquake risk.”
Nonetheless, Nepal probably wasn’t a place that most Americans thought of as a major earthquake risk. Although it sits in a seismically active region, the country has experienced fewer major quakes in recent decades than Japan, Indonesia or Chile.
But the likelihood of a big quake is only one component of risk. Preparation matters just as much. And Nepal, as the mounting death toll makes clear, was badly unprepared for a disaster of this magnitude. That isn’t necessarily the Nepalese government’s fault: Kathmandu is a large, fast-growing city in the midst of a poor country — a recipe, research has shown, for high vulnerability to natural disasters.
Experts often think about risk according to a rough formula: Risk = Hazard x Vulnerability x Exposure.1 Those terms may sound like different ways of saying the same thing. But in the language of disaster preparedness, they have distinct meanings.
Hazard is the probability of a given event occurring. Cities that lie along active fault lines generally face a high earthquake hazard: They are likely to experience more earthquakes than cities in areas that are less seismically active.2
Vulnerability measures how well-prepared a place is for a given event, regardless of how likely that event is to occur. A city built to modern construction standards is likely to suffer much less damage from an earthquake (or a hurricane, flood or other kind of disaster) than one built to less stringent standards.
Exposure refers to how many people would be affected by a potential disaster. All else equal, an earthquake is going to be much worse if it hits a big city than if it hits a small town.3
Many cities score poorly on two of these three factors. Tokyo, for example, is high on the hazard scale (lots of earthquakes) and high on the exposure scale (lots of people). But because Japan is a rich country with experience dealing with earthquakes, Tokyo is designed to withstand even a major quake. As a result, it’s comparatively low-vulnerability, at least in terms of loss of life. (The economic impact of a major quake in Tokyo would be enormous.)4
New York, on the other hand, is both high-exposure and high-vulnerability (a big city that isn’t built for an earthquake), but because it’s in a seismically inactive area, it’s low-hazard. Alaska’s Aleutian Islands are high-hazard and in some cases vulnerable. However, they are sparsely populated so their exposure is limited.
Kathmandu, however, rates badly on all three measures. As a result, experts have long considered it one of the world’s highest-risk areas for earthquakes, “maybe even number one,” said David Wald, a seismologist with the U.S. Geological Survey in Golden, Colorado.
“From a risk perspective, it was just inevitable that it shakes again, and we know from the past how bad things can get, so it was pretty darn high,” Wald said.
In fact, as bad as Saturday’s earthquake was, it could have been worse. In a 1999 report, GeoHazards International calculated that a quake as big as the one that struck in 1934 could kill about 40,000 people and leave more than half a million homeless. And in 1999, the Kathmandu Valley was home to 1.5 million people; as of 2013, the valley’s population had ballooned to 2.5 million.
Kathmandu’s rapid population growth added to its earthquake exposure. It also may have added to its vulnerability. A 2004 report from the United Nations Development Program found that “the risk of dying in an earthquake was greater in countries with rapid urban growth.” Growth doesn’t necessarily cause higher risk directly, the report cautioned, but inadequate or poorly enforced building codes often can’t keep up with an influx of new residents. That may have been the case in Nepal, which has been struggling with weak economic growth, political instability and corruption.
Still, Kathmandu’s buildings weren’t as vulnerable as those in some other high-risk cities, such as Tehran, said Kishor Jaiswal, a structural engineer with the U.S. Geological Survey. Much of the city’s recent growth has been in reinforced concrete buildings that offer at least some resistance to earthquakes;5 older buildings are more likely to collapse but often have light roofs that make it more likely occupants will survive.
The city has also adopted new building codes since the 1990s. Nonetheless, much of the country’s population lives in buildings made of mud, wood or unreinforced brick.6
Kathmandu isn’t the only big city that ranks high in all three risk categories. Other old, fast-growing cities such as Tehran, Istanbul and Karachi, Pakistan, face similar challenges. Many of the cities on experts’ watch lists are also, like Kathmandu, poor. That not only makes it hard to prepare for a disaster, but also makes it more difficult to respond to one and recover afterward. Haiti has struggled to rebound from the quake that killed tens of thousands of people there in 2010. It will be months before we know whether Nepal is more successful in its recovery than it was in its preparation.