Damaged buildings lean to their sides in Kathmandu, Nepal, on April 27. A magnitude 7.8 earthquake shook Nepal’s capital and the densely populated Kathmandu Valley on April 25, causing extensive damage, with toppled walls and collapsed buildings. (Wally Santana/AP)

Writing about seismic risk is frustrating: The experts know that big earthquakes are going to happen, but not exactly when and where. The ensuing articles are laced with hypotheticals. There is an airy, foamy quality to these stories; as a writer, you long for solid facts and certainties, and wonder whether any of this stuff makes a difference on the ground, in lives of actual human beings who are at risk. For example, here’s a story I wrote five years ago:

The next Big One could strike Tokyo, Istanbul, Tehran, Mexico City, New Delhi, Kathmandu or the two metropolises near California’s San Andreas Fault, Los Angeles and San Francisco. Or it could devastate Dhaka, Jakarta, Karachi, Manila, Cairo, Osaka, Lima or Bogota. The list goes on and on …

For years, earthquake scientists have shouted their warnings about the strong likelihood that a major quake would level an impoverished city and kill hundreds of thousands of people. They have said, for example, that Kathmandu, where masonry structures expand so haphazardly that some eventually cantilever over narrow city streets, is every bit as vulnerable as the surrounding Himalayas are majestic.

For years, scientists have talked to me about the very high likelihood of a Kathmandu earthquake. It happened Saturday. The experts called it. But being right hasn’t made them any less horrified and saddened by this disaster. In fact, they sounded dismayed, as if overcome with a sense of powerlessness. Tectonic forces are massive and implacable, and human societies are often poor and fragile.

India is crashing into Eurasia; the result is the Himalaya Mountains. People live in valleys where the softer sediments amplify shaking; see Kathmandu. Urbanization is a long-term global trend that has accelerated in Nepal due to political unrest in the countryside. You can talk all day long about how it is dangerous to live and work and pray in certain kinds of masonry structures or in buildings with a “soft” first floor, but in poor countries people are just trying to get by day to day. They have no choice but to play the lottery when it comes to a hypothetical natural disaster that might come along once or twice a century.

I talked about this on Science Friday a few years ago:

If you go back to 1800, the year 1800, there was one city in the world that had a million people, and that was Beijing. The most recent count I’ve come across is that there are 381 cities with at least a million people. Of those cities, a lot of them are in seismically very hazardous places like Caracas, Venezuela, or Mexico City or Katmandu.

And if you’re living in a city, and you are poor, you’re not going to worry about events that don’t happen but once every 200 years or so.

Earthquakes are innately unpredictable, and don’t let anyone tell you otherwise. Skittish birds and snakes are not a reliable indicator. No one knows why a small earthquake can keep breaking and turn into a 1906-style catastrophic rupture. Sometimes they stop, sometimes they keep going. The 2011 Japan earthquake was a 9.0 on a fault that, according to the scientific consensus, could generate only magnitude 7 and 8 earthquakes. On a logarithmic scale, a 9 is 10 times more powerful than an 8 and 100 times more powerful than a 7. (One Japanese scientist did manage to figure out, a few years before the earthquake, that a 9 was possible; being correct did not make him feel any better when the horrible news came.)

Japan, to its credit, has prepared for earthquakes, and retains cultural memory of the 1923 earthquake and fire that devastated Tokyo. But what happened in 2011 defied the received wisdom in Japan about where the next big one would be. Here’s a brief excerpt of my 2006 story in National Geographic on earthquake prediction:

In Japan, government scientists say they have settled the question. Earthquakes are not random. They follow a pattern. They have detectable precursors. The government knows where Japan’s big one will most likely strike. This is a country where the trains run on time, and earthquakes are supposed to do the same. “We believe that earthquake prediction is possible,” says Koshun Yamaoka, a scientist at the Earthquake Research Institute of the University of Tokyo.

Japan has already named its next great earthquake: the Tokai earthquake. The government has identified and delineated by law the precise affected area — a region along the Pacific coast about a hundred miles (160 kilometers) southwest of Tokyo. After a series of small quakes in the Tokai area in the 1970s, scientists predicted that a major quake might be imminent there. The Japanese government passed a law in 1978 mandating that preparations begin for the Tokai earthquake.

Scientists have estimated a death toll of between 7,900 and 9,200 for a quake striking without warning in the wee hours. Estimated property damage: up to 310 billion dollars. At the Tokai earthquake preparedness center in Shizuoka, a map pinpoints 6,449 landslide locations. Another map shows where 58,402 houses could burn in quake-related fires. It’s all remarkably enumerated. The only thing left is for the earthquake to happen.

It hasn’t happened. The Big One turned out to be hundreds of miles to the north.

Here’s a prediction: There will be a huge earthquake in California. The San Andreas Fault will rupture and produce something close to a magnitude 8.0. If you had to bet, you’d wager it would be in the southernmost section, which hasn’t ruptured since about 1680. But who knows?

The seismic hazard maps are useful in designing building codes, and giving us approximations of the likelihood of major ground motion. The maps, however, are limited in their ability to tell us what will happen, and when, and where, exactly. Northridge 1994 (6.7 magnitude) happened on an unmapped fault. The world (and the Los Angeles area in particular) is undergirded by countless unmapped faults. One earthquake can shift strain to another fault that might break in an even more violent way. The Earth doesn’t operate on human timetables. Very large earthquakes can occur on hidden faults with extremely long repeat times of several thousand years or more. Just because a place doesn’t have very many earthquakes doesn’t mean it can’t have a 7.0 magnitude quake or even something stronger.

[Update: The Seismological Society of America on Tuesday released a collection of research papers that could be of use for anyone studying or writing about the Nepal earthquake. Here is the collection. I’ve been skimming through the material, and this line, from a paper titled “Superquakes and Supercycles,” summarizes the problem with “earthquake prediction”:

“It is becoming increasingly clear that our short instrumental and historical records are inadequate to characterize the complex and multiscale seismic behavior of subduction zones and other major fault systems. The recurrence intervals for superquakes and supercycles may be very long, potentially biasing the record from the start. The data presented here suggest that the types and sizes of large earthquakes a given fault system is capable of producing may be unknown for most major fault systems. This too is a function of the short histories available. This bias toward what we have observed or recorded directly has shaped the development of conceptual models [italics added] such as the characteristic earthquake model, the seismic gap theory, the relationship between plate age and convergence velocity, and time and slip predictability.”]

I wrote about this in 2012 on the anniversary of the Japan earthquake/tsunami:

Since the development of the theory of plate tectonics in the 1960s, scientists have a better understanding of why earthquakes occur along plate boundaries. The 1906 San Francisco earthquake, for example, released the strain that had built up as the North American plate and Pacific plate fitfully tried to slide past each other along a plate boundary, the San Andreas Fault. Scientists have tried to monitor the motion of tectonic plates and create maps showing where strain is building at a locked plate boundary.

But earthquakes remain fundamentally unpredictable and eccentric. Scientists were surprised by the location of the 9.1 magnitude earthquake in the Indian Ocean on Dec. 26, 2004, which generated a huge tsunami and took 230,000 lives. That portion of the subduction zone near Sumatra had been considered an unlikely source of a great earthquake.

Since then, more surprises. The 2008 earthquake in China’s Sichuan province, which killed 68,000 people, was in an area that “did not look like a very active region,” American geologist Peter Molnar said afterward. The 2010 earthquake in Haiti occurred along a fault generally considered less dangerous than another fault to the north. And New Zealand has had two significant earthquakes on unmapped faults in the past year and a half.

Surprises may be the norm for the seismic future, even in places considered hazard-free. The Virginia earthquake in August, which damaged the Washington Monument and the National Cathedral, is a reminder that the East Coast could be more vulnerable to tremors than most people realize.

I’m not saying everyone should be nervous about an imminent earthquake. But everyone should understand the need for societal resilience. We have put more people and more infrastructure in the path of natural forces that have always been around, and always will be part of our lives.

I’d like to think that science and communication can make a huge difference when it comes to disasters. It may turn out that the many years of hard work in Nepal on earthquake preparedness and disaster mitigation — work by nonprofit organizations, such as GeoHazards International, and by Nepalese officials, scientists and educators — have saved thousands of lives that otherwise would have been lost Saturday.

Further Reading:

My news story Saturday reporting that scientists had predicted the Kathmandu earthquake.

A story from Columbia University on New York City’s vulnerability to an earthquake.

Our narrative on the March 11, 2011 earthquake and tsunami in Japan