Scientists are getting new clues about seismic activity before large earthquakes along subduction zones by measuring tectonic wobbles in Japan and Chile.
By Chelsea Scott, Ph.D., Assistant Research Scientist, Arizona State University (@ChelseaPScott)
Citation: Scott, C., 2020, Some tectonic plates ‘wobble’ before major earthquakes, Temblor, http://doi.org/10.32858/temblor.108
The 2011 magnitude-9.1 Tohoku-Oki earthquake in Japan caused widespread destruction and generated a tsunami that submerged cities along the entire eastern coast. Instrumental records from the quake — one of the largest ever recorded — continue to give scientists new insights about earthquake hazards.
While scientists cannot predict the exact timing and location of these large earthquakes, they study past quakes to understand how plate tectonic boundaries change before one occurs and if these changes could be reliable precursors. In a recent study published in the journal Nature, scientists show that back and forth motion, or wobbling, of the Earth’s crust occurred in the months before the Tohoku-Oki and the 2010 magnitude-8.8 Maule quake in Chile. This motion is too small to be perceived by people, but helps scientists understand the processes that occur in the crust in the months before large earthquakes.
Fault slip during the Tohoku-Oki earthquake
Off the coast of Japan, the Pacific Plate is converging towards and subducting under the North American Plate. In 2011, the magnitude-9.1 Tohoku-Oki earthquake ruptured this plate boundary and produced over 150 feet (over 50 meters) of fault 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 up to them.
Measuring plate motion
Scientists use Global Positioning System (GPS) stations to record movement of the Earth’s surface. In tectonically active regions like Japan, stations dot the landscape as researchers continuously monitor how much motion occurs. Although scientists cannot predict earthquakes, this instrumentation, coupled with the new computational tools, is helping them to understand what happens before, during and after such events.
A major advancement in this study is the application of a new GPS processing algorithm called the “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 alter the load on the Earth’s surface — when water evaporates or is otherwise removed, land rebounds from the decrease in overlying weight. This results in GPS signals that fluctuate seasonally and over multiple years, making it impossible to see subtle tectonic wobbles. This new algorithm assumes that signals like those from tectonic wobbling are rare and separates them from the long-term water loading signals.
“With the new advancements, scientists can better isolate the tectonic motions from the non-tectonic signals,” says Kathryn Materna, a geophysicist working at 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 rewound and watched the plate motion in the years leading up to the Tohoku-Oki earthquake, according the study’s lead author Jonathon Bedford, a geophysicist at the GFZ German Research Center for Geosciences in Potsdam, Germany.
Bedford and colleagues observed that prior to the Tohoku-Oki earthquake, GPS stations recorded 0.16-0.31 inches (4-8 millimeters) of back and forth motion of the crust — a little less than the length of a fingernail — over a seven-month period. GPS data have been used to observe plate motion for four decades, but this is the first time such a distinct wobbling has been seen with this dataset.
These wobbles are likely a more widespread phenomenon. In fact, not only was this wobble observed in Japan, but GPS data from the seven months leading up to the 2010 magnitude-8.8 Maule earthquake near Santiago, Chile showed a similar pattern. However, in Chile, 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 wobbles may have big implications
This wobbling appears to be caused by the motion of the subducting plate, according to the study. Before the wobbles begin, the subducting plate releases fluids along the fault. This causes the fault to weaken and the deeper subducting plate becomes denser, moves downward, and pulls on the shallow portion of the subducting plate. The shallow plate partially resists the downward pulling and wobbles back and forth.
Each of these processes stresses or weakens the subduction zone fault, making a large earthquake more likely. Along mature faults — those that have not had a recent large earthquake and therefore are at risk of one occurring soon — these changes may be enough to cause a large earthquake, like those seen in Japan, Chile and other subduction zones.
Do plates always wobble before earthquakes?
Although the recent subduction zone earthquakes in Japan and Chile were preceded by tectonic wobbling, it is unknown if similar wobbles always indicate that a large earthquake will occur soon. More research is needed to understand “if the wobbles happen only before earthquakes or if wobbles occur during other parts of earthquake cycle,” says Materna. Bedford agrees and adds that one of the challenges of observing large subduction zone earthquakes is that they are relatively rare.
While the magnitude 9.1 Tohoku earthquake was unexpected, data recorded before and after the earthquake is helping scientists to understand the processes that lead up to this kind of event. Future research should show whether or not these wobbles are a useful indicator of large subduction zone earthquakes, possibly making these earthquakes less unexpected.
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Bedford, J.R., Moreno, M., Deng, Z. et al. Months-long thousand-kilometre-scale wobbling before great subduction earthquakes. Nature 580, 628–635 (2020). https://doi.org/10.1038/s41586-020-2212-1
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