The discovery could potentially alert scientists to the danger of a major earthquake brewing in seismically active parts of the world. According to scientists at The Ohio State University, one such wobble preceded the biggest earthquake to ever strike Japan – a magnitude 9 cataclysm that killed more than 15,500 people in 2011.
In the months leading to the so-called Tohoku-oki earthquake, the country’s land masses shifted from the west to the east and back again.
A similar event occurred before the 2010 Maule earthquake in Chile that peaked at magnitude 8.8 and killed at least 525 people.
Both events struck along the Pacific Rim, which overlaps with the Pacific Ring of Fire – a hotbed of volcanic activity and earthquakes triggered by plate tectonics.
The findings were published on April 30 in the journal Nature.
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Michael Bevis, study co-author and professor of earth sciences at The Ohio State University, said: “What happened in Japan was an enormous but slow wobble – something never observed before.
“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.”
The wobbles may have been too weak for anyone to notice.
But a network of more than 1,000 GPS stations spread across Japan collected enough data to show movement of about five millimetres per month in the months before the deadly March 11 earthquake.
We really need to be monitoring all major subduction zones
Jonathan Bedford, GFZ German Research Center for Geosciences
The Tohoku-oki earthquake struck about 43 miles (70km) east of Tohoku region on Japan’s Honshu Island.
The earthquake caused parts of Japan’s coastline to sink, collapsed more than 120,000 buildings, and produced a tsunami that led to the Fukushima nuclear disaster – the biggest nuclear accident since Chernobyl in 1986.
Japan’s Prime Minister Naoto Kan said at the time: “In the 65 years after the end of World War II, this is the toughest and the most difficult crisis for Japan.”
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Although the Earth’s land masses are always on the move, Professor Bevis said the wobbles stand out from Earth’s natural cycles.
He said: “The world is broken up into plates that are always moving in one way or another.
“Movement is not unusual. It’s this style of movement that’s unusual.”
The research team, involving scientists from Germany, Chile and the US, found Japan’s land mass shifted between four to eight millimetres to the east, then to the west and back to the east again.
Although it remains to be seen whether these wobbles will happen again before the next major earthquake, there is a chance they could be used to predict future cataclysms.
However, Jonathan Bedford from the GFZ German Research Center for Geosciences, who led the study, said the findings cannot be applied to other parts of the world without comprehensive GPS networks in place.
Before the Chile earthquake struck, for instance, a similar wobble took place.
But the data collected was “only just good enough to capture the signal”.
Dr Bedford said: “We really need to be monitoring all major subduction zones with high-density GPS networks as soon as possible.”
In their study, the scientists proposed installing stations around the globe that will monitor signals from the Global Navigations Satellite System (GNSS).
The network could allow seismologists to “track non-steady surface motion” of tectonic plate movements before cataclysm strikes.
They wrote: “Modelling of the surface displacement reversal that occurred before the Tohoku-oki earthquake suggests an initial slow slip followed by a sudden pulldown of the Philippine Sea slab so rapid that it caused a viscoelastic rebound across the whole of Japan.
“Therefore, to understand better when large earthquakes are imminent, we must consider not only the evolution of plate interface frictional processes but also the dynamic boundary conditions from deeper subduction processes, such as sudden densification of metastable slab.”
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