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Subducting seamounts blocked a big quake in Chile

 Map of Chile’s coastline showing the locations of the April 2014 earthquake and the seamounts that may have prevented the rupture from propagating farther. Credit: K. Cantner, AGI, adapted from Geersen et al., Nature Communications, 2015. Map of Chile’s coastline showing the locations of the April 2014 earthquake and the seamounts that may have prevented the rupture from propagating farther. Credit: K. Cantner, AGI, adapted from Geersen et al., Nature Communications, 2015.
Chile, which lies above a massive subduction zone fault, is one of the world’s most earthquake-prone countries, experiencing nine temblors of magnitude 7 or greater since 2010. In April 2014, a magnitude-8.1 earthquake struck 95 kilometers northwest of the city of Iquique, but despite its large size, the event failed to release all the stress thought to have built up along that portion of the fault. A new study reveals that a ridge of ancient underwater volcanoes may have blocked the 2014 earthquake rupture from propagating farther, thus limiting the size of the quake.
 
Off the coast of Chile, the Nazca Plate is subducting beneath the South American Plate. Friction between the two plates creates stress along the contact area between them, which is occasionally released in the region’s frequent earthquakes. But the section of the megathrust fault near Iquique has been locked and accumulating stress since the last big event to strike that part of the fault in 1877.
 
“In northern Chile, there is a gap 550 kilometers wide that has not seen a major earthquake since 1877,” says Jacob Geersen, a geophysicist at GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, and lead author of the study, published in Nature Communications. For decades, experts have thought Chile’s next major earthquake would strike in this gap and that it would need to reach a magnitude greater than 9 to release all of the stress accumulated in this region of the fault. Indeed, the April 2014 quake did strike within this gap, but it only reached magnitude 8.1 and it only affected the central part of this gap, leaving the sections to the north and south unruptured. 
 
To investigate why the Iquique quake didn’t rupture the entire gap, Geersen and colleagues combined topographic images of the seafloor around the subduction zone with seismic images showing the deeper structure of the fault. They found that a ridge of seamounts on the downgoing Nazca Plate may be impeding subduction and deforming the interface between the two plates. 
 
The volcanic features, some up to 2.3 kilometers high and 15 kilometers wide, are no longer active, Geersen says. “The seismic data show that several seamounts have already been subducted into the fault zone, while others are pushed up along the interface” — the surface fault trace — “of the two plates, thereby actively deforming the interface and the overlying South American Plate,” Geersen says. 
 
The seamounts on the exposed portion of the Nazca Plate had been previously mapped at low resolution, but the subducted seamounts revealed by the seismic reflection data are a new discovery, says Roland Bürgmann, a geophysicist at the University of California at Berkeley, who was not involved in the new study. “People have thought for a long time that seamounts might affect the subduction earthquake process,” he says. But the data presented in this study make “the nature of this connection, and the apparent arrest of the Iquique rupture, much more concrete.”  
 
“When you bring topographic features into a subduction zone, they tend to fracture the area around the plate boundary so it’s not one continuous fault, but more of a fractured network” of many smaller faults, Geersen says. It seems this fractured network cannot build up or efficiently transmit seismic energy, thus preventing rupture along the entire length of the seismic gap, he says. 
 
But that doesn’t mean that this section of the fault is safe from more damaging quakes, Bürgmann notes. “We should always be careful not to overinterpret and overextrapolate from one such observation,” he says. 
 
In 2014, the Iquique quake broke the middle 150 kilometers of the 550-kilometer-wide gap, leaving sections to the north and south with enough stored seismic energy to produce an earthquake larger than magnitude 8.5, Geersen says. The team is now installing geodesic instruments on the seafloor near the subduction zone to monitor the fault. “The more information we have, the closer we get to being able to manage the risks and mitigate the effects of potentially large earthquakes,” he says.

 

Mary Caperton Morton

Mary Caperton Morton

Morton is a freelance writer and photographer (and EARTH roving correspondent) who makes her home on the back roads of North America, living and working out of a tiny solar-powered Teardrop camper. Follow her travels at www.theblondecoyote.com.

Wednesday, January 20, 2016 - 06:00
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