Harbor destruction. The magnitude-8.2 quake that struck off the northern coast of Chile Tuesday, 1 April, pounded the port city of Iquique with a 2-meter tsunami.

Harbor destruction. The magnitude-8.2 quake that struck off the northern coast of Chile Tuesday, 1 April, pounded the port city of Iquique with a 2-meter tsunami.

Hector Merida/Reuters

Explainer: Chilean Earthquake Rocks Formerly Quiet Zone

In the early evening of Tuesday, 1 April, a magnitude-8.2 quake occurred off the coast of northern Chile, killing six people, generating 2-meter-high waves, and prompting the evacuation of nearly 1 million coastal residents. According to the U.S. Geological Survey, the temblor was centered about 95 kilometers northwest of the port city of Iquique and originated at a depth of about 20 kilometers. The location of the quake, as well as data from seismometers indicating in which direction the ground moved, suggests it occurred along the boundary between the Nazca and South American tectonic plates.

Was the quake expected?

Looking back, it’s not surprising. First, the region had recently experienced increased seismic activity, including a magnitude-6.7 quake 2 weeks ago and a series of aftershocks that included three with a magnitude of 6.2, 26 with a magnitude of 5 or greater, and more than 60 with a magnitude of 4 or more. During this 2-week period, seismic activity had, in general, moved northward; Tuesday evening’s magnitude-8.2 temblor was centered near the northern end of this seismic sequence.

Isn’t this a seismically active region?

The subduction zone along the western edge of South America, where the Nazca plate slides eastward beneath the South American plate at an average rate of about 6.5 centimeters per year, can indeed generate massive quakes. But Tuesday’s temblor occurred along a stretch of tectonic boundary that hadn’t slipped to produce a major quake in nearly 150 years. The last large quake to strike there happened in 1877 and had a magnitude of 8.8. Since then, the region had been so quiet that scientists dubbed it “the Iquique seismic gap” and referred to it as “locked.”

Despite the lack of large quakes in the gap, some researchers had suggested the region might be overdue for a large temblor. That’s because the 1877 event followed another magnitude-8.8 quake that occurred just to the north along the same tectonic boundary in 1868. The most recent quake along that section of tectonic boundary, which lies mostly off the southern coast of Peru, was a magnitude-8.4 temblor that struck in June 2001. If seismic history repeats itself, then the Iquique region was slightly overdue for a large quake like Tuesday’s.

But how repeatable are such patterns of quakes?

It’s difficult to say, simply because the intervals between quakes of that magnitude are so long and historical records in many regions are so short. Plus, before seismic instruments were first deployed in the early 1900s, earthquake magnitudes had to be estimated rather than measured. In areas with sparse populations, such as along the remote coast of Chile, records are particularly spotty and could be inaccurate.

So, can large quakes be predicted?

Not when it comes to the specific times and places of earthquakes. Quakes can be “forecast” only over large seismic areas and long time scales. In a 2006 paper in Science, researchers—including David Robinson, then a seismologist at the University of Oxford in the United Kingdom—noted that the June 2001 in Peru quake ruptured only 400 km of the 1000-km-long seismic gap, leaving 600 km of the plate boundary largely locked. A quake rupturing the full length of the still-locked section might reach magnitude 9, Robinson says. Because Tuesday’s quake wasn’t nearly that strong, it suggests that a portion of the gap remains locked. But that’s not necessarily a sign that the locked-up section is ready to let loose, Robinson notes. It’s entirely possible that recently slipped areas of the plate boundary will be able to absorb any additional stress, thereby delaying a temblor in the still-locked section. For now, it’s impossible to say when Chile should expect its next major quake—though as one of the world’s most seismically active countries, it probably won’t be long.

*Update, 3 April, 11:05 a.m.: On the evening of 2 April, after this story was posted, two more large quakes occurred within the Iquique seismic gap. One, a magnitude-6.5 temblor, happened just before 9:00 p.m. local time and was centered 43 kilometers west of Iquique, Chile. The other, a magnitude-7.6 event, struck about 45 minutes later and originated about 19 km south of Iquique.