Slow motion precursors give earthquakes rapid slip

In a glacier near the South Pole, Earth scientists have found evidence of a quiet slow-motion glide causing strong, fast-sliding earthquakes many miles away, according to Cornell’s research published February 5 in Science Advances.

During an earthquake, rapid slip occurs when energy accumulates underground and is rapidly released along the fault. Lumps of ground slide quickly against each other.

However, on an Antarctic glacier called the Whillans Glacier, Earth scientists have shown that “slow slides” precede dozens of large earthquakes with a magnitude of 7. Lead author Grace Barcheck, an associate researcher in Earth and Atmospheric Sciences at the College of Engineering, said: “We found that there was almost always a precursor“ slow slip ”before an earthquake.

Matthew Siegfried inspects a GPS device, powered by a solar panel, at Whillans Ice Plain.

Barcheck said that these slow precursors – occurring within 20 miles of the epicenter – are directly involved in initiating the earthquake. “These slow slides are remarkably common, and they migrate towards where the rapid earthquake starts,” she said.

Observations before several large earthquakes generated by tsunamis of magnitude 8 and 9 on subduction zone faults indicate a similar process, according to Patrick Fulton, associate professor and Croll Sesquicentennial Fellow in the Department of Earth and Atmospheric Sciences.

Because these faults are often offshore and deep underwater, and because it is difficult to know when or where a major earthquake will occur, it is generally difficult to notice the onset of large earthquakes.

To overcome these challenges, scientists placed GPS sensors atop an ice fissure in the Whillans Ice Plain, where large earthquakes of about 7 times magnitude occur approximately twice daily over 60 miles of the glacier.

Within two months in 2014, the group had captured 75 earthquakes at the bottom of the Antarctic glacier. Data from GPS stations indicated that 73 – or 96% – of the 75 earthquakes exhibited a period of initial slow motion.

Data from GPS tracking stations and surface seismometers allowed the team to determine how the slow passive slip leads to the rapid slip of the earthquake.

“Our group was a bit surprised to see so many precursors,” Barchik said.

“In some cases, we can actually see the front of the earthquake migrate towards the place where the earthquake starts.”

“Before we looked at the data, I thought that if we saw any precursors before earthquakes, they would be rare and in the same place as the epicenter,” she said. “Instead, we found many slow-slip precursors – starting in miles from hotspots and migrating across the rift.”

In addition to Barcheck and Fulton, the co-authors of the research, “Migratory earthquake precursors are dominant on the fault of the ice stream,” Emily Brodsky, professor in the Department of Earth and Planetary Sciences, University of California, Santa Cruz, who was a former visiting professor at Cornell University. Matt King, Professor of Geography and Spatial Sciences, University of Tasmania, Hobart, Tasmania; Matthew Siegfried, Department of Geophysics, Colorado School of Mines, Golden, Colorado; And Slawek Tulaczyk, Professor, Department of Earth and Planetary Sciences, University of California, Santa Cruz.

Fieldwork and analysis for this research was supported by the National Science Foundation.

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