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The seismic fault that gave way off the coast of Chile on Tuesday has long been overdue for a big earthquake, experts say. But they can't yet say whether this magnitude-8.2 shock is the "Big One" they've been expecting.
"We're hoping for the sake of the people of northern Chile that this is the Big One, but at the same time we know that even an 8.2 earthquake has not released all the stored energy in that area," Rick Allmendinger, a geologist at Cornell University who specializes in earthquake analysis, told NBC News.
Allmendinger is familiar with Chile's Big Ones, in part because he's a visiting professor at a university in Antofagasta, one of the cities affected by the quake. The last time a major quake hit the area was back in 1877, when an 8.5 tremor triggered 75-foot-high tsunami waves.
Tuesday's quake originated offshore in the Nazca-South American subduction zone — a region where one tectonic plate dives beneath another, giving rise to megathrust earthquakes. "There's about 320 miles of coastline that has not had a big earthquake since 1877," Allmendinger said.
That's been good news for Iquique, a resort city that's home to some of northern Chile's finest beaches. But the good news can't last forever. "The longer a region has been quiet, the bigger the eventual earthquake is likely to be," Allmendinger said. "People get comfortable when there hasn't been a big earthquake for a long time, but in this case they should get nervous."
In retrospect, the magnitude-6.7 quake that struck the Iquique area March 16 looks like a precursor to Tuesday's seismic shock. "It's not 100 percent unusual" to see that pattern, said Purdue seismologist Larry Braile.
The main concern is whether this 8.2 quake is a precursor to an even bigger shock. Allmendinger said he saw "a lot of similarities" to the situation that unfolded in Japan in 2011, when a 7.3 quake turned out to be the foreshock for the 9.0 quake and tsunami that followed two days later.
"This could be it, or there could be a bigger one," Allmendinger said. "I certainly hope there isn't a bigger one."