Core-Reflected Seismic Wave Caused Japan to Shift After 2011 Tohoku Earthquake

A study published in Science on June 18, 2021, reveals that a seismic wave from the 2011 Tohoku earthquake caused Japan to shift eastward by several millimeters. Researchers led by Sunyoung Park of the University of Chicago identified the phenomenon as the first recorded instance of a core-reflected S wave triggering fault movement, according to the study.

What Caused Japan’s Ground Shift?

The 2011 magnitude 9.0 Tohoku earthquake generated seismic waves that traveled through Earth’s mantle to the core and reflected back, causing faults to slip. This process, observed via archival seismic and GPS data, resulted in a permanent ground displacement across Japan, as reported by Park and her team. The core-reflected S wave, which traveled 2,900 kilometers to the core’s edge and returned, was detected 15 minutes after the mainshock.

Caltech seismologist Zachary Ross, not involved in the study, noted that the ground shift—measured by hundreds of GPS sensors—indicates the wave caused fault slip rather than merely passing through. The displacement spanned from Hokkaido in the north to Kyushu in the south, suggesting two major plate boundaries ruptured over 3,000 kilometers, the researchers concluded.

Why Is This Discovery Significant?

This event marks the first time a core-reflected S wave has been documented as a trigger for fault movement. Park emphasized that such seismic hazards were previously unrecognized. The rupture length of 3,000 kilometers exceeds that of the 2004 Sumatra earthquake, which had a rupture length of approximately 1,600 kilometers, according to the study.

Purdue University geophysicist Andrea Donnellan, also not part of the research, called the findings plausible, explaining that seismic waves can release built-up tectonic stress over decades or centuries. However, the energy from the Tohoku wave was dispersed over a vast area, making the slippage imperceptible to humans, as noted by the researchers.

What Are the Implications for Future Earthquakes?

The study warns that while the 2011 event was not immediately hazardous, future core-reflected waves could pose greater risks. The researchers stress the need to account for such long-distance seismic interactions in hazard assessments. Park stated, “This is a type of seismic hazard that we didn’t think about before.”

Seismologists are now reevaluating models for earthquake dynamics, particularly in regions with complex plate boundaries. The Tohoku earthquake’s unique characteristics provide critical insights into how deep Earth structures influence surface effects, according to the study.

How Was the Discovery Made?

Researchers analyzed data from Japan’s dense GPS network and seismic records to detect the core-reflected wave’s signature. The wave’s timing and spatial correlation with ground displacement confirmed its role in triggering fault slip. Park’s team used advanced signal-processing techniques to isolate the wave’s distinct pattern from the mainshock’s energy.

The study’s findings highlight the importance of integrating deep-Earth data into seismic monitoring systems. As Park explained, “Understanding these processes helps us better predict how earthquakes interact with the planet’s interior.”