by Nick Carne

April 30, 2020

from CosmosMagazine Website

The Tohoku-oki earthquake in 2011

devastated much of the east coast

of Japan's Honshu island.
Enase / Getty Images

 

It's not clear, though,

whether this will happen again...
 

The earth "wobbled" before two of the largest earthquakes in recent history, according to a new study, but it's not clear whether this is likely to happen before the next one.

Geoscientists investigated recorded signals capturing the movement of global navigational satellite systems (GNSS) stations before the 2010 Maule earthquake in Chile (magnitude 8.8) and Japan's Tohoku-oki earthquake in 2011 (9.0) and noted a strange reversal of ground motion.
 

The Tohoku-oki earthquake led to a devastating tsunami and the Fukushima nuclear meltdown.
 

Both events occurred at the Pacific Rim where oceanic plates dive beneath the continental crust in a process called subduction.
 

The researchers say geodetic analyses revealed that a 1000-kilometre-scale region of the Earth's surface close to the plate boundary alternated its sense of motion over a period of several months directly leading up to both earthquakes.
 

The study was led by Jonathan Bedford from GFZ German Research Centre for Geosciences and included scientists from Germany, Chile and the US.

The findings (Months-long thousand-kilometer-scale Wobbling before great Subduction Earthquakes) are reported in the journal Nature.

"What happened in Japan was an enormous but very slow wobble - something never observed before," says co-author Michael Bevis, from Ohio State University, US.

 

"But are all giant earthquakes preceded by wobbles of this kind?

 

We don't know because we don't have enough data. This is one more thing to watch for when assessing seismic risk in subduction zones like those in Japan, Sumatra, the Andes and Alaska."

What the research does show, says Bedford, is that subduction zones are more dynamic on the observable timescale than previously thought.

"It is a common assumption that deeper subduction proceeds at a fairly constant speed in between large earthquakes," he says.

 

"Our study shows that this assumption is an oversimplification. In fact, its variability might be a key factor in understanding how the largest earthquakes nucleate."

Michael Bevis says the Japanese wobble would have been imperceptible to people but was obvious in data recorded by more than 1000 GPS stations distributed throughout Japan, which are tracked by GNSS.

Chile's network is not as dense but tracks most of the deforming continental plate.
 

Normally, the stations on land move away ever so slightly from the subduction trench as the continental crust is squeezed and thus shortened.
 

Studying the time series of GNSS signals, the researchers found a reversal of direction:

the stations suddenly moved towards the subduction trench (towards the open ocean) then returned to their normal movement.

Very shortly after this second reversal, the underground ruptured and the immense earthquakes occurred.
 

The authors propose that these reversals capture periods of enhanced pulling caused by rapid, densifying compositional changes in the oceanic plate as it subducts.
 

Accordingly, they suggest that these periods of enhanced tugging accelerated the inevitable failure at the shallower, frictionally-stuck segments of the subduction zone.

Bedford et al. 2020,

Nature