Some tectonic plates “oscillate” before major earthquakes

Scientists are obtaining new evidence about seismic activity before large earthquakes along subduction zones by measuring tectonic vibrations in Japan and Chile.

Written by Chelsea Scott, Ph.D., Research Assistant, Arizona State University (ChelseaPScott)

Quote: Scott, C., 2020, Some tectonic plates “oscillate” before major earthquakes, Temblor, http://doi.org/10.32858/temblor.108

The Thoku-Aoki earthquake that struck Japan in 2011, with a magnitude of 9.1, caused widespread damage and triggered a tsunami that engulfed cities along the entire eastern coast. Automated records of an earthquake – one of the largest on record – continue to give scientists new insights into earthquake hazards.

While scientists cannot predict the exact timing and location of these large earthquakes, they are studying past earthquakes to understand how tectonic plate boundaries change prior to their occurrence and whether these changes could be reliable precursors. In a recent study published in the journal Nature, scientists showed that the back-and-forth motion, or wobble, of the Earth’s crust occurred in the months leading up to the Tohoku-Aoki earthquake and the Molly earthquake of magnitude 8.8 in 2010 in Chile. This movement is too small for people to realize, but it helps scientists understand the processes that take place in the crust in the months leading up to major earthquakes.

Cityscape in Sendai, Japan. The Tohoku-Aoki earthquake in 2011 occurred off the coast of Sendai and caused a very devastating tsunami and a severe earthquake along the northern coast of Japan. Credit: Enirac Sum, Public Domain

Slip fault during the Tohoku-Oki earthquake

Off the coast of Japan, the Pacific plate converges towards the North American plate and falls below it. In 2011, the Tohoku-Aoki earthquake of magnitude 9.1 tore the plate boundaries and produced more than 150 feet (over 50 meters) of sudden slip and tsunami with a maximum height of 130 feet (40 meters). In addition to directly studying the large motion produced by an earthquake, scientists also study the much smaller motion before an earthquake to understand the processes that led to it.

Measuring plate movement

Scientists use Global Positioning System (GPS) stations to record the movement of the Earth’s surface. In tectonically active regions like Japan, stations dot the landscape as researchers constantly monitor the amount of movement taking place. Although scientists cannot predict earthquakes, these devices, along with new computational tools, help them understand what happens before, during and after these events.

One of the major developments in this study is the application of a new GPS processing algorithm called “Automatic Greedy Signal Analysis”. This algorithm isolates relatively small tectonic oscillations from other signals recorded in the GPS data. Changes in the amount of water stored in snow and soil change the load on the surface of the earth – when water evaporates or is otherwise removed, the earth bounces from the lower upper weight. This results in GPS signals that fluctuate seasonally and over multiple years, making precise tectonic oscillations impossible to see. This new algorithm assumes that signals like those from tectonic oscillation are rare and separate them from long-term water loading signals.

“With the new developments, scientists can better isolate tectonic motions from non-tectonic signals,” says Catherine Materna, a geophysicist with the US Geological Survey who was not involved in the study.

(A) The Tohoku-Aoki earthquake occurred in which the Pacific Plate (oceanic crust) departs below the North American Plate (continental crust). (B) In the months leading up to the earthquake, the Pacific Plate likely moved faster because the plate became denser. (C) In response, the subduction zone error weakened, the shallow portion of the Pacific plate resisted the downward motion, and the GPS stations recorded a plate oscillation. (D) It is possible that the cause of the large earthquake is the weak fault. Credit: Felipe Jamo, University of Concepcion, Chile

Activity before earthquakes

Through the continuous record of ground motion obtained by GPS stations, scientists rewound and watch the movement of the plate in the years before the Tohoku-Aoki earthquake, according to the study’s lead author Jonathon Bedford, a geophysicist at the German GFZ Research Center for Geosciences in Potsdam, Germany.

Bedford and colleagues note that prior to the Tohoku-Oki earthquake, GPS stations recorded 0.16–0.31 inches (4-8 mm) of back and forth motion of the crust – just under the length of a fingernail – over a seven-month period. GPS data has been used to monitor plate motion for four decades, but this is the first time that such a characteristic wobble has been seen with this dataset.

These oscillations are likely a more common phenomenon. In fact, not only was this fluctuation observed in Japan, but GPS data from the seven months leading up to the Maule earthquake of magnitude 8.8 in 2010 near Santiago, Chile showed a similar pattern. However, in Chile, the observations are noisier due to the presence of much fewer GPS stations, but the oscillation is still observed in an east-west direction perpendicular to the coast prior to the earthquake.

Small fluctuations can have big effects

This oscillation appears to be caused by the movement of the impulse plate, according to the study. Before the oscillations begin, the fused plate releases fluids along the fault. This weakens the error and the deeper submersion plate becomes denser, moves down, and pulls out the shallow portion of the connecting plate. A shallow plate resists pulling down and swaying back and forth.

Each of these processes compress or weaken the subduction zone fault, making a large earthquake more likely. Along mature faults – those that have not experienced a major earthquake recently and are therefore at risk of occurring soon – these changes may be sufficient to induce a major earthquake, such as those seen in Japan, Chile and other subduction regions.

Do plates always sway before earthquakes?

Although recent earthquakes in the subduction zone in Japan and Chile have been preceded by a tectonic vibration, it is not known if similar oscillations always indicate that a major earthquake will occur soon. More research is needed to understand “whether the vibrations only occur before earthquakes or if the oscillations occur during other parts of the earthquake cycle,” says Materna. Bedford agrees and adds that one of the challenges of monitoring large subduction zone earthquakes is that they are relatively rare.

While the Tohoku earthquake had an unexpected magnitude of 9.1, the data recorded before and after the earthquake help scientists understand the processes that led to this type of event. Future research should show whether or not these vibrations are a useful predictor of large subduction zone earthquakes, making these earthquakes less predictable.

Check earthquake hazards in Temblor

Further reading

Bedford, JR, Moreno, M, Ding, Z and others. It oscillates over a range of thousands of kilometers over a period of months before the Great Subduction earthquakes. Nature 580, 628–635 (2020). https://doi.org/10.1038/s41586-020-2212-1

Fujiwara, T, Kodaira, S, No, T, Kaio, Y, Takahashi, N, Kaneda, Y, 2011. 2011 Tohoku-Aoki earthquake: displacement access to the trench axis. Science 334, 1240–1240. https://doi.org/10.1126/science.1211554

Ito, Y, Hino, R, Kido, M, Fujimoto, H, Osada, Y, Inazu, D, Oota, Y, Inuma, T, Uzuno, M, Miura, S, Mishina, M., Suzuki, K., Tsuji, T., Ashi, J., 2013. Accidental slow slip events in the subduction zone of Japan prior to the 2011 Tohoku-Aoki earthquake, Tectonic Physics 600, 14-26. https: // doi.org/10.1016/j.tecto.2012.08.022

Mavrommatis, AP, Segall, P., Johnson, KM, 2014. Trans-node deformation prior to the 2011 M w 9.0 Tohoku-oki earthquake. Geophysics. Precision. Lett. 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. w = 8.8 earthquakes from a co-reflection of the Global Positioning System (GPS) High-speed, remote body waves, InSAR, campaign GPS, and tsunami monitoring: slip in the 2010 Maule earthquake, Chile. Geovese. Precision. Hard Earth 119, 7786-7804. https://doi.org/10.1002/2014JB011340

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