Monika Sobiesiak wasn’t expecting the morning of April 2 to start with such an adrenaline jolt. But as she scrolled through a list of earthquakes on her mobile phone, she saw that overnight a series of quakes had rocked the coast of northern Chile — almost exactly where she had installed a seismometer network a few years earlier. “I saw the 8.2,” says the geophysicist, who works at the University of Kiel in Germany, “and I rushed to get to my desk.”

That April 1 quake, which struck offshore near the village of Pisagua, was the largest in Chile since a magnitude-8.8 quake hit farther south in 2010. Although the Pisagua quake was not as big and not particularly damaging, it will still go down in the annals of seismology — as an intensively studied earthquake that upends some assumptions about how and when big quakes happen.

In one sense, seismologists knew it was coming. Northern Chile, near the border with Peru, was the only stretch of the country’s coastline that had not broken in a large earthquake in the past century (see ‘Under pressure’). In 2006, expecting it to go, a German–French–Chilean collaboration blanketed the region with seismometers, tiltmeters and other ground-measuring instruments, creating the Integrated Plate boundary Observatory Chile (IPOC). It captured the Pisagua quake in action, as did Sobiesiak’s network.

But the earthquake was not the ‘Big One’ that seismologists had expected. Only a monstrous earthquake, of around magnitude 9, would have relieved all the geological stress built up in the region. More quakes, on the order of magnitude 8, are still possible, but when they might strike is a mystery. More broadly, the Pisagua event has seismologists rethinking some basic ideas about the risk of earthquakes in similar geological settings elsewhere — places with deep-diving crustal plates, such as Japan and Indonesia.

Over time, earthquakes rupture particular portions of a long fault zone; the unbroken portions are ‘seismic gaps’ considered ripe for future quakes. Officials in these areas are often told to prepare for the worst-case scenario — the biggest possible earthquake in a given seismic gap. But the Pisagua quake shows that this does not always happen, says Susan Beck, a seismologist at the University of Arizona in Tucson. Instead, it underscores that seismic gaps can rupture in all sorts of ways, from lots of smaller quakes to just a few big ones.

Chile is an ideal laboratory in which to study such questions because it lies on the margin of a subduction zone, where the Nazca tectonic plate dives — or subducts — beneath the South American plate. Geological stress builds up and then is released in the occasional massive jolt. Chile is home to the largest earthquake ever recorded — one of magnitude 9.5 in 1960 — and accounts for more than one-quarter of the planet’s total seismic-energy release.

Pisagua had not seen a major earthquake since 1877, when a tremor of around magnitude 9 ripped through the area. Seismic activity began to pick up last August, when a swarm of small earthquakes struck the area. Another set followed over the new year, and a third cluster occurred in March. These three swarms seem to have prepared the subduction zone to rupture in the big 1 April quake, says Onno Oncken, a geophysicist at the German Research Centre for Geosciences in Potsdam.

Until recently, researchers had thought that the next large earthquake in northern Chile would break the entire interface between the Nazca and South American plates, says Jean-Pierre Vilotte, a seismologist at the Paris Institute of Earth Physics. The Pisagua quake and a magnitude-7.6 aftershock two days later, “are a clear counterexample of this simplistic classification”, he says. Together they ruptured just a small portion of the entire region at risk.

Intriguingly, the part of the subduction zone that broke was not the part that had built up the most stress, according to a ground-motion study of northern Chile’s seismic gap by Marianne Métois, a geophysicist at the National Institute of Geophysics and Volcanology in Rome, and her colleagues (M. Métois et al.Geophys. J. Int. 194, 1283–1294; 2013). For some reason, the Pisagua quake released stress in areas that were not the most wound up. “A lot of energy remains to be released in north Chile,” Métois says.

When the next one comes, seismologists plan to be ready. IPOC has added instruments to capture aftershocks from the 1 April quake and whatever might happen next. Dozens of new seismometers and global-positioning stations have been deployed by teams from Chile, Germany and France, says Sergio Barrientos, director of the National Seismological Centre at the University of Chile in Santiago.

For Sobiesiak, every little bit of data helps. Before the 1 April quake, she thought that the northern Chile seismic gap would rupture either to the north or to the south of the Pisagua area, but not right through it. Now she has some fresh thinking to do. “Each of these efforts really does bring us a step forward,” she says.

This article is reproduced with permission from the magazine Nature. The
article was first published on April 22, 2014.