A new study proposes that a delayed fault slip after the 2011 Tōhoku earthquake was triggered by a seismic wave that traveled to the Earth's core and back.
Observed Delayed Displacement
Analysis of Global Positioning System (GPS) data from Japan recorded a permanent eastward displacement of land of up to 5 to 6 millimeters. This movement occurred approximately 15 minutes after the main earthquake and was observed nearly simultaneously across multiple stations nationwide.
Researchers from the University of Chicago, led by Sunyoung Park, noted that the timing and uniformity of this displacement could not be explained by the main shock itself, its immediate aftershocks, or other phenomena such as submarine landslides or data artifacts.
Proposed Trigger Mechanism
The study attributes the displacement to an ScS seismic wave—a shear wave generated by the main earthquake that traveled down through the mantle, reflected off the boundary between the Earth’s core and mantle at a depth of approximately 2,890 kilometers, and returned to the surface near Japan. The wave’s round trip took about 13 minutes.
The returning ScS wave is interpreted to have acted as a trigger for a subtle, broad slip along the plate interface where the Pacific Plate subducts beneath the plate carrying northern Japan. Modeling suggests this slip released an amount of energy equivalent to a magnitude 7.5 earthquake. Because the slip was distributed over a vast area, it resulted in movements of only millimeters to centimeters and produced minimal additional shaking.
Scientific Context and Implications
ScS waves typically dissipate too much energy during their journey to the core and back to cause measurable surface displacement. However, the Tōhoku earthquake generated an unusually large ScS wave, with a peak-to-peak amplitude exceeding 1 centimeter at some Japanese stations, and was also detected by monitoring stations in China. The study concludes that this is the first documented instance of a fault-slip event triggered by a seismic wave reflected from the core-mantle boundary.
The findings suggest that large earthquakes may have hidden complexity, with the potential for delayed slip events triggered by deep-Earth waves tens of minutes after the main shock. Researchers recommend incorporating this phenomenon into seismic hazard assessments, as it may represent a previously unrecognized source of hazard. Whether this effect occurred in other great earthquakes, such as the 2004 Sumatra-Andaman, 1960 Valdivia, 1964 Alaska, and 2010 Chile earthquakes, would require further examination of GPS data from those events.