Seismic Speed Bumps Found in Chile's Earthquake Zone

chile subduction — An image of the subduction zone fault south of Santiago, Chile.

(Image credit: Stephen Hicks, University of Liverpool)

Two blobs of dense rock jammed deep beneath Chile's coastline acted like seismic speed bumps during the magnitude-8.8 Maule earthquake in 2010, according to a new study.

Some of the world's largest earthquakes occur in Chile. The country sits above a subduction zone, where the Nazca tectonic plate dives down under the South America tectonic plate. Where the two plates lock together, a huge amount of strain builds up and is periodically released through earthquakes.

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"The properties of the subduction zone offshore Chile change from south to north, and before now a lot of the focus has been on the role of features on the oceanic plate," said Stephen Hicks, lead study author and a seismologist at the University of Liverpool in the United Kingdom. "I think for the first time we are able to say how the structures in the overriding plate, the South American continent, play a big role."

The structures are most likely made of a type of mantle rock called peridotite, Hicks said. The mantle is the hotter layer of rock beneath Earth's thin, cool crust. The mantle rocks were probably stuck onto the bottom of the South American continent some 250 million to 220 million years ago, when the subduction zone was further east, he said. Directly above the anomalies, on the surface, are outcrops of rocks with similar mafic compositions and ages. "This is where the old volcanoes were located," Hicks told Live Science.

The 2010 earthquake started near the edge of the larger Cobquecura anomaly, the researchers report. Most of the quake's movement, or slip, was along shallower parts of the subduction zone fault. When the fault tried to zip through the zone where the mafic rocks were locked to the Nazca Plate, the rupture suddenly slowed down. Such speed bumps can generate damaging fast shaking during an earthquake (whether the quake is quickening or slowing down). In the case of the Chile earthquake, the slowdown pattern matched up with the areas that felt the fastest shaking.

Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.