A 150-Year Tension
Off the coast of northern Chile, the oceanic Nazca plate dives beneath the South American continent at a rate of about 6.5 to 7.9 centimeters per year. This process, known as subduction, is not smooth. For more than a century, a large section of this plate boundary, known as the Northern Chile seismic gap, has been stuck, or "locked". This specific segment, roughly 420 kilometers long, has been accumulating stress since its last major rupture on May 9, 1877.
That event was a megathrust earthquake with an estimated magnitude of 8.5 to 8.8. It generated a devastating tsunami with waves up to 10 meters high that struck 500 kilometers of coastline from Arica to Mejillones. The shaking itself lasted for five minutes in some locations. Since 1877, the plates have continued to push against each other, building up elastic strain. This stored energy will eventually be released in a future great earthquake. The Mejillones Peninsula, a distinct rocky outcrop, marks the southern boundary of this gap and appears to act as a persistent barrier, often stopping earthquake ruptures. Earthquakes in 1995 (M8.0) and 2007 (M7.7) ruptured areas to the south of the peninsula, but the main 1877 segment remains locked.
Measuring a Quake in Waiting
Scientists are waiting. An array of scientific instruments across the Atacama Desert constantly measures the Earth's deformation. The Integrated Plate Boundary Observatory Chile (IPOC), a collaboration established in 2006, operates a network of high-precision GPS stations, seismometers, and other sensors. These GPS stations can detect millimeter-scale movements of the continent as the South American plate is bulged and squeezed by the locked Nazca plate.
This geodetic data allows researchers to calculate the "slip deficit"—the amount of movement that should have occurred between the plates but has been stored as strain. Current estimates suggest a slip deficit of around 8 meters has accumulated since 1877. The 2014 magnitude 8.2 Iquique earthquake ruptured a portion of this gap, but it was not the "big one" and failed to release the majority of the accumulated stress, leaving a large segment still under pressure. Based on the remaining locked area, the next great earthquake here could reach or exceed magnitude 8.5. In response, Chile has developed an Earthquake Early Warning System. Using data from seismic stations, the system is designed to provide an average of 24 seconds of warning before the destructive S-waves arrive, giving a brief window for people to take cover.