Scientists get new clues about seismic activity before major earthquakes along subduction zones by measuring tectonic wobblers in Japan and Chile.

Due to Chelsea Scott, Ph.D., Assistant Research Scientist, Arizona State University (@ChelseaPScott)

Quote: Scott, C., 2020, Some tectonic plates ‘wobble’ before major earthquakes, Temblor, http://doi.org/10.32858/temblor.108

The 2011-9.1 Tohoku-Oki earthquake in Japan caused widespread destruction and generated a tsunami that swept cities along the entire east coast. Instrumental records from the quake – one of the largest ever recorded – give scientists new insights into the danger of earthquakes.

Although scientists can not predict the exact timing and location of these large earthquakes, they study spheres in the past to understand how flat tectonic boundaries change before one happens and if these changes could be reliable precursors. In a recent study published in the journal Nature, scientists show that forward and forward motion, like wobbling, of the Earth’s crust occurred in the months before the Tohoku-Oki and the 2010 scale of 8.8 Maule in Chile. This motion is too small to be observed by humans, but helps scientists understand the processes that occur in the crust during the months before major earthquakes.

Fault slip during the Tohoku-Oki earthquake

Off the coast of Japan, the Pacific Plate is converging towards and subordinate to the North American Plate. In 2011, the magnitude 9.1 earthquake of Tohoku-Oki broke this plate boundary and produced more than 150 feet (more than 50 meters) of false slip and a tsunami with a maximum height of 130 feet (40 meters). In addition to directly studying the large motion produced in the earthquake, scientists also study the much smaller motion prior to the earthquake to understand the processes that lead to it.

Measure plate movement

Scientists use GPS stations Global Positioning System (GPS) to record movement of the earth’s surface. In tectonically active regions such as Japan, stations support the landscape as researchers continuously monitor the frequency of movement. Although scientists cannot predict earthquakes, this instrument, along with the new computer instruments, helps them to understand what happens before, during and after such events.

An important advancement in this study is the application of a new GPS processing algorithm called ‘Greedy Automatic Signal Decomposition.’ This algorithm isolates the relatively small tectonic wobbles from other signals recorded in GPS data. Changes in the amount of water stored in snow and soil change the charge on the earth’s surface – when water evaporates or is otherwise removed, rebounds land back from the decrease in surface weight. This results in GPS signals fluctuating seasonally and over several years, making it impossible to see subtle tectonic wobblers. This new algorithm assumes that signals such as those of tectonic oscillation are rare and separates them from the signals for long charge of water.

“With the new advances, scientists can better isolate the tectonic motions from the non-tectonic signals,” said Kathryn Materna, a geophysicist who worked on the U.S. Geologic Survey who was not involved in the study.

Activity before earthquakes

With a continuous record of ground motion obtained by GPS stations, scientists revived the plate motion in the years following the Tohoku-Oki earthquake, according to lead author Jonathon Bedford, a geophysicist at the GFZ German Geoscience Research Center in Potsdam, Germany.

Bedford and colleagues noted that prior to the Tohoku-Oki earthquake, GPS stations took 0.16-0.31 inches (4-8 millimeters) of backward motion of the crust – slightly less than the length of a fingernail – over a sand month period. GPS data has been used for four decades to observe plate motion, but this is the first time such distinct wobbling with this dataset has been seen.

These wobbles are probably a more widespread phenomenon. In fact, this wobble was not only observed in Japan, but GPS data from the seven months following the magnitude 8.8 earthquake in Maule in 2010 near Santiago, Chile, showed a similar pattern. In Chile, however, the observations are noisier because there are far fewer GPS stations, but the wobble was still observed in the east-west direction perpendicular to the coastline before the earthquake.

Small wobblers can have big implications

This wobble appears to be caused by the movement of the subductive plate, according to the study. Before the wobbles begin, the subducting plate throws fluids along the fault. This causes the fault to weaken and the deeper subducting plate becomes denser, moves downwards, and attracts to the shallow part of the subducting plate. The shallow plate is partially resistant to pulling down and wobbling back and forth.

Each of these processes stresses or attenuates the fault of the subduction zone, making a major earthquake more likely. Along with adult failures – those that have not had a recent major earthquake and therefore are at risk of one getting there soon – these changes may be enough to cause a major earthquake, as seen in Japan, Chile and other subduction zones.

Do plates always weigh before earthquakes?

Although the earthquakes in the recent subduction area in Japan and Chile were preceded by tectonic wobble, it is not known if similar wobbles always indicate that a major earthquake will occur soon. More research is needed to understand “if the wobbles occur only before earthquakes or if wobblers occur during other parts of earthquake cycle,” says Materna. Bedford agrees, adding that one of the challenges of observing large-scale subduction zone earthquakes is that they are relatively rare.

While the magnitude of Tohoku’s 9.1 earthquake was unexpected, data recorded before and after the earthquake helps scientists understand the processes that lead to this type of event. Future research should show if these wobblers are a useful indicator of earthquakes for large subduction zone, though these earthquakes may make them less unexpected.

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Bedford, JR, Moreno, M., Deng, Z. et al. Month-long thousands of miles of rocking before major earthquakes of subduction. Nature 580, 628–635 (2020). https://doi.org/10.1038/s41586-020-2212-1

Fujiwara, T., Kodaira, S., No, T., Kaiho, Y., Takahashi, N., Kaneda, Y., 2011. The Tohoku-Oki Earthquake for 2011: Adjustment of the Troubles. Science 334, 1240–1240. https://doi.org/10.1126/science.1211554

Ito, Y., Hino, R., Kido, M., Fujimoto, H., Osada, Y., Inazu, D., Ohta, Y., Iinuma, T., Ohzono, M., Miura, S., Mishina, M., Suzuki, K., Tsuji, T., Ashi, J., 2013. Episodic slow slip events in the Japan subduction area before the Tohoku-Oki earthquake of 2011. Tectonophysics 600, 14-26. https://doi.org/10.1016/j.tecto.2012.08.022

Mavrommatis, AP, Segall, P., Johnson, KM, 2014. A decadal scale change transient prior to the Mw 9.0 Tohoku-oki earthquake in 2011. Geophys. Res. Easy. 41, 4486–4494. https://doi.org/10.1002/2014GL060139

Yue, H., Lay, T., Rivera, L., An, C., Vigny, C., Tong, X., Báez Soto, JC, 2014. Localized error slip to the ditch in the Maule of 2010, M w = 8.8 Earthquake by Joint Reversal of High-Rate GPS, Teleseismic Body Waves, InSAR, GPS Campaign, and Tsunami Observations: Slip in the Maule Earthquake in 2010. J. Geophys. Res. Solid Earth 119, 7786–7804. https://doi.org/10.1002/2014JB011340