Seismic waves reflected from Earth’s core moved parts of Japan 5 mm east
A previously unrecognized source of seismic hazard caused parts of Japan to move eastwards by up to 5 mm within minutes of the 2011 Tohoku-Oki earthquake. This phenomenon, known as ScS waves, has been extensively studied for imaging the deep Earth, but this is the first evidence that it can also trigger significant displacement-causing slip after an earthquake. The study, led by Sunyoung Park at the University of Chicago, US, examined shear waves recorded following the earthquake in Japan’s dense network of high-rate Global Navigation Satellite System instruments.
The Tohoku-Oki quake, which occurred at the boundary between the Pacific plate and the North American plate, was one of the largest recorded earthquakes in history. It caused widespread devastation, including thousands of fatalities and the meltdown of three out of six nuclear reactors at the Fukushima Daiichi power station, releasing radioactive caesium into the air and water and prompting a large-scale evacuation with long-term health impacts.
The earthquake was bigger than most models estimated, which compelled seismologists to examine and revise existing models of the largest possible earthquakes and their frequencies in the boundaries of tectonic plates. Instrumented records of this earthquake are still revealing new information about geophysical processes in the boundaries of tectonic plates.
Surface to core and back
The team found ScS waves that travelled thousands of kilometres through the Earth, bounced off the core and travelled back to the surface, arriving across Japan almost simultaneously 13 minutes after the main shock. The name ScS reflects that path: S[hear] through the mantle, c for reflection at the core-mantle boundary, and then S again on the return path.
The study offers the first evidence that ScS waves can also trigger significant displacement-causing slip after an earthquake, meaning that earthquake hazard researchers may need to consider these waves to fully capture consequences of earthquake ruptures on the land as well as deep within tectonic plates.
A high-quality, high-resolution dataset
The Tohoku-Oki earthquake was an incredible opportunity to study the role of ScS waves in triggering a slip event. The nature of the earthquake itself, combined with the quality and resolution of data, allowed researchers to look for small, coherent signals that would likely be invisible elsewhere.
The researchers simulated possible slip scenarios across the tectonic plate interfaces, hoping to better characterize the levels and spatial extent of the slip that caused the eastward displacement. The inferred slip event likely involved interfaces where the Pacific plate subducts beneath the Okhotsk Plate, and where the Philippine Sea plate subducts beneath the Eurasian plate.
A slow-motion slip
The ScS waves from the Tohoku-Oki quake appear to have set off a slow slip event spread over a very large area at intermediate depths of 20–60 km. This is the depth range where seismologists commonly observe silent earthquakes, and it provides avenues for understanding the dynamic processes that impact the earthquake cycle.
The researchers report their findings in Science (https://www.science.org/doi/10.1126/science.aec4190).
In my opinion, this study is fascinating because it reveals a previously unknown mechanism for earthquake hazard. It also highlights the importance of high-quality, high-resolution data in seismology. The fact that ScS waves can trigger significant displacement-causing slip after an earthquake means that earthquake hazard researchers may need to reconsider their models and focus more attention on these waves.
What makes this particularly fascinating is the idea that the Earth’s core can influence the surface in such a dramatic way. It raises a deeper question about the interconnectedness of the Earth’s systems and the potential for unexpected consequences from seemingly minor events. This study also underscores the importance of continued research into the deep Earth and the potential for new discoveries that could change our understanding of earthquake hazard and risk.
