That hasn’t stopped people from trying to find “anomalies” that reliably precede an earthquake, including animals acting strangely, radon gas seeping from rocks, patterns of precursor earthquakes, and electromagnetic signals from pressurised rocks. None of these have been backed by strong evidence. Studying such “anomalies” may eventually tell us something useful about the physics of earthquakes, but their value towards a predictive test is questionable.
“Groping in the dark to find something like weird animal behaviour or emitted gases –that effort is not worth pursuing,” says Stein. “We have 30 to 40 years of negative results to convince us that this isn’t a good investment of resources.” Hough adds that the field is replete with bad science. “People will go, ‘Here’s an earthquake and here’s a blip beforehand’, but it’s not statistically rigorous.” And those blips, even if they do exist, rarely precede a quake with any consistency. “You can always find those apparent patterns by looking back, but you run the game forward and the methods don’t work.”
This is not a problem that we can data-crunch our way out of, as a group of scientists tried to do in the 1980s. For more than a century, earthquakes had regularly rocked a small part of the San Andreas fault, near Parkfield, California. Anticipating another in 1993, a hundred-strong team of geologists seeded the area with hundreds of seismometers, looking to discover signals that heralded the onset of the next inevitable earthquake. “People said: This is going to be easy,” recalls Stein. “We bet the farm on Parkfield and we put every bloody instrument we had as deep as we could.” When the earthquake finally happened in 2004, the instruments saw nothing. “How is the Parkfield Earthquake Experiment like technology stocks?” joked Stein in 2002. “They both seemed like a great bet at the time.”
The challenge that seismologists face is that there’s five miles of rock between them and what they want to study. “We’ll always be hamstrung by the fact that we’re stuck on the surface, and the surface only goes for the ride,” says Stein. “The action is at depth.” Consider California’s Hayward Fault, which runs parallel to the famous San Andreas. It is over 100 km (60 miles) long, and 10 kilometres (6 miles) deep. To get readings across its entire area would take “hundreds of thousands of drill holes as deep as anything ever drilled for oil and deeper than everything drilled for science,” says Stein. “Maybe then, we could have all the observations we need.”
But while predicting earthquakes may be beyond our grasp, predicting the subsequent aftershocks might be more feasible. Even though the word “aftershock” almost invites you to downplay them, these tremors cannot be ignored. “They’re not a detail,” says Stein. “They can be in some cases as or more dangerous than the main shock.”
Just look what happened in New Zealand. In September 2010, a magnitude-7.1 earthquake shook the ground 40 km (25 miles) away from Christchurch, the country’s second most populous city. The destruction was mild, and no lives were lost. Then, six months later, a magnitude-6.3 aftershock landed six miles from Christchurch’s centre, killing 185 people and causing 20-30 billion New Zealand dollars of damage (US $17-25 billion).
Aftershocks are indistinguishable from main shocks, but they do have some predictable qualities, including their frequency. Ten days after a main earthquake, the frequency of aftershocks falls by a factor of ten; a hundred days later, it falls by a factor of a hundred. But their magnitude stays the same. “They can really turn around and whack you,” says Stein. “They can be very big, and very late.” He and other scientists are trying to understand where aftershocks are most likely to occur, by simulating what happens when faults get distorted by the initial shocks. “That’s a place where we’re making progress. Then, once we’ve had that main shock, we can begin to address what’s more vulnerable and what’s safer.”