The 2011 Tohoku Earthquake Sent a Seismic Wave to Earth's Core, Bounced Back, and Moved All of Japan 5–6 mm East

Size of the affected area

The secondary fault slip triggered by the ScS wave occurred over an enormous region spanning roughly 3,000 km (1,800 miles)—making it the broadest seismic event ever recorded . It encompassed the entire Japanese archipelago, from Hokkaido in the north through Honshu to Kyushu in the southwest . The energy released in this secondary event was comparable to a magnitude 7.5 earthquake .

Implications for seismic hazard preparedness

A new class of delayed aftershock trigger. Current hazard models account for mainshock-aftershock sequences driven by static stress changes and surface-wave triggering. This finding shows that deep-earth reflected waves can reactivate faults minutes after a major quake, introducing a hazard window that existing early warning and forecasting systems do not explicitly model .

Vastly expanded spatial reach. Core-reflected ScS waves can transmit significant stress changes across continental-scale distances (thousands of kilometers), not just along the fault rupture zone. A large subduction earthquake could therefore induce secondary fault slip across a much wider region than conventional seismic hazard zones assume .

Monitoring and modeling gaps. Seismic networks and GPS arrays already record the signals needed to detect these waves, but the phenomenon is not yet incorporated into operational hazard assessments. Future preparedness may require adding core-reflected wave triggering into real-time aftershock and tsunami models, especially for magnitude 8.5+ events where ScS wave amplitudes are large enough to cause remote fault slip .