On Friday, 11 March 2011, an earthquake struck the oceans near Tohoku, a region on Japan’s east coast. With a magnitude of 9.0, it was among the five most powerful earthquakes ever recorded, strong enough to shift the entire planet by several inches along its axis. It triggered a tsunami that killed thousands of people and wrecked the Fukushima-Daiichi nuclear power plant.
A quake that large shouldn’t have happened at Tohoku, at least not to the best of Japanese scientists’ knowledge. The hazard maps they had drawn up predicted that big earthquakes would strike in one of three zones to the south of the country – Tokai, Tanankai, and Nankai. No earthquake has hit these regions since 1975, while several have occurred in “low-probability” zones, such as Tohoku.
Japan isn’t alone. The incredibly destructive earthquakes that hit Wenchuan, China in 2008 and Christchurch, New Zealand in 2010 and 2011 all happened in areas deemed to be “relatively safe”. These events remind us that earthquake prediction teeters precariously between the overly vague and overly precise. At one extreme, we can calculate the odds that big earthquakes will strike broad geographic areas over years or decades – that’s called forecasting. At the other extreme, early warning systems can relay news of the first tremors to people some distance away, giving them seconds to brace themselves. But the ultimate goal – accurately specifying the time, location and magnitude of a future earthquake – is extremely difficult.
In 1977, Charles Richter – the man who gave his name to a now-defunct scale of earthquake strength – wrote, "Journalists and the general public rush to any suggestion of earthquake prediction like hogs toward a full trough… [Prediction] provides a happy hunting ground for amateurs, cranks, and outright publicity-seeking fakers." Susan Hough from the United States Geological Survey says the 1970s witnessed a heyday of earthquake prediction “But the pendulum swung [because of too many false alarms],” says Hough, who wrote a book about the practice called Predicting the Unpredictable. “People became very pessimistic, and prediction got a really bad name.”
Indeed, some scientists, such as Robert Geller from the University of Tokyo, think that prediction is outright impossible. In a 1997 paper, starkly titled Earthquakes Cannot Be Predicted, he argues that the factors that influence the birth and growth of earthquakes are so numerous and complex that measuring and analysing them is a fool’s errand. Nothing in the last 15 years has changed his mind. In an email to me, he wrote: “All serious scientists know there are no prospects in the immediate future.”
Earthquakes start when two of the Earth’s tectonic plates – the huge, moving slabs of land that carry the continents – move around each other. The plates squash, stretch and catch against each other, storing energy which is then suddenly released, breaking and shaking the rock around them.
Those are the basics; the details are much more complex. Ross Stein from the United States Geological Survey explains the problem by comparing tectonic plates to a brick sitting on a desk, and attached to a fishing rod by a rubber band. You can reel it in to mimic the shifting plates, and because the rubber band is elastic, just like the Earth’s crust, the brick doesn’t slide smoothly. Instead, as you turn the reel, the band stretches until, suddenly, the brick zips forward. That’s an earthquake.
If you did this 10 times, says Stein, you would see a huge difference in the number of turns it took to move the brick, or in the distance the brick slid before stopping. “Even when we simplify the Earth down to this ridiculous extreme, we still don’t get regular earthquakes,” he says. The Earth, of course, isn’t simple. The mass, elasticity and friction of the sliding plates vary between different areas, or even different parts of the same fault. All these factors can influence where an earthquake starts (which, Stein says, can be an area as small as your living room), when it starts, how strong it is, and how long it lasts. “We have no business thinking we’ll see regular periodic earthquakes in the crust,” he says.