These earthquakes are among the 10 largest ever measured: The 2011 Tohoku-oki earthquake off the coast of Japan registered 9.0 on the magnitude scale and triggered the devastating tsunami that destroyed the Fukushima nuclear power plant. It was the fourth strongest quake in the past 100 years. The 2010 earthquake off the coast of the Chilean region of Maule ranks sixth on the list.
Researchers at the Geoforschungszentrum Potsdam (GFZ) as well as employees from Chile and the USA discovered astonishing similarities in the months before the two quakes: several strange reversals of the ground movement – an extremely slow “wobbling” of the continental plate, so to speak.
The lead author, geophysicist Jonathan Bedford, along with a team of geodesists, geologists and seismologists, evaluated the movements of Global Navigation Satellite System (GNSS) ground stations.
The massive tsunami after the severe earthquake in Japan brought everything with it and cost thousands of lives
These ground stations continuously track the distances to several GNSS satellites orbiting the Earth in elliptical orbits. Once these distances have been determined and the satellite position estimates are available, geodesists can define the movement of points on Earth within a terrestrial reference frame.
There is a very dense network of ground stations in Japan. Although the network in Chile was not that dense in 2010, it was enough to get the data.
Continue reading: Moving hotspots: Scientists explain the mysterious bend of the Hawaii Emperor volcanic chain
Entire continental plate “wobbles”
Bedford and his colleagues analyzed how the ground stations in Japan and Chile had moved in the five years before the two quakes. They found that the movement of the continental plate on which the stations are located had been reversed several times in the past five months before the quake (in the case of Japan) and seven months (in the case of Chile). The researchers published their results in the journal Nature.
Both plate boundaries are subduction zones. Subduction is the process by which one plate is submerged under another. In both regions of the study, the oceanic plates subtract from the continental plates and meet at a location called the trench, which is under the ocean in both cases. Usually the continental plate is pressed by the ocean plate and thus pushed away from the trench. However, geophysicists have now found that this movement was first reversed towards the trench, then away from the trench and then back to the trench.
Continue reading: Volcanoes and earthquakes: the Pacific ring of fire
Same movement over thousands of kilometers
They call this phenomenon “wobble” because of its appearance in the GNSS time series. The amplitude of this movement is not particularly large – it was only between 4 and 8 millimeters – but Bedford points out that this is significant compared to the relative plate movement, which can be a few centimeters per year. In addition, the spatial extension of the signal spanned thousands of kilometers along the plate boundaries.
“It’s a common assumption that deeper subduction between major earthquakes occurs at a fairly constant rate,” Bedford says. “Our study shows that this assumption is an oversimplification. In fact, its variability could be a key factor in understanding the nucleation of the largest earthquakes.”
As global satellite positioning continues to improve and accurate data has been available for the first time in decades, earthquake researchers can make such observations more and more frequently.
“We can now trace movements back decades,” says Bedford. “In the next phase, we want to monitor the changes almost in real time.”
Despite all the investigations, a short-term earthquake warning is not yet possible
Not suitable as an early warning system
So far, seismologists have been able to say more Where It is likely that major earthquakes will occur – less can say when. By simply calculating the magnitude of the last major earthquake in a region and knowing the average relative disk speed, it can be estimated when this error is mature enough to sustain a repeat event, although this approach has great uncertainties. This is because sometimes an error occurs only over a smaller area (e.g. with a “large” size 8 event) and sometimes over many areas of size 8 simultaneously (e.g. in a “mega” event of the Size 9) breaks.
Could an observed atypical plate movement better warn us of an impending earthquake? Not really.
“It would not be advisable for a geophysicist to issue such a warning,” is Bedford’s sobering answer. “The signals observed in this study are not necessarily preliminary movements of a severe earthquake.”
Further research is needed and people in known earthquake areas should not give up their vigilance.
“The general public should always be prepared,” warns Bedford.