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, traveled thousands of kilometres through the Earth, bounced off the core, and returned to the surface, arriving across Japan almost simultaneously 13 minutes after the main shock. While seismologists have studied ScS waves extensively for imaging deep Earth structures, this study offers the first evidence that they can also trigger significant displacement-causing slip after an earthquake.
The Tohoku-Oki quake, occurring at the boundary between the Pacific and North American plates, was one of the largest recorded earthquakes in history, causing widespread devastation and thousands of fatalities. The resulting tsunami exacerbated the damage, leading to meltdowns at the Fukushima Daiichi power station and the release of radioactive caesium into the environment, prompting large-scale evacuations with long-term health impacts.
The earthquake's magnitude exceeded most models' estimates, prompting seismologists to revise existing models of the largest possible earthquakes and their frequencies at tectonic plate boundaries. Instrumented records of this earthquake continue to reveal new information about geophysical processes in these boundaries.
The study, led by Sunyoung Park at the University of Chicago, analyzed shear waves recorded following the earthquake using Japan's dense network of high-rate Global Navigation Satellite System instruments. Along with the expected seismic waves, the team found ScS waves that traveled thousands of kilometres, reflected off the core, and returned to the surface, arriving across Japan almost simultaneously 13 minutes after the main shock.
The researchers, including Park, Hiroo Kanamori at Caltech, and Luis Rivera at the University of Strasbourg, calculated displacements at all instrumented locations for various earthquake rupture configurations. However, these results did not reproduce the observed eastward motion. They then simulated possible slip scenarios across tectonic plate interfaces, suggesting that 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.
What makes this particularly fascinating is the revelation that ScS waves triggered a slow-slip event spread over a large area at intermediate depths of 20–60 km, the range where seismologists commonly observe silent earthquakes. This finding provides new insights into the dynamic processes impacting the earthquake cycle and deep Earth interactions.
Park's broader motivation is a general interest in deep-shallow interaction in the Earth, particularly cases where deep interior processes influence shallow lithosphere faulting and deformation or vice versa. The study's findings, published in Science, highlight the need for earthquake hazard researchers to consider ScS waves to fully understand the consequences of earthquake ruptures on the land and within tectonic plates.
