Deep-Earth Wave Shifts Japan 6mm East After Tohoku Quake

GPS data show an eastward, step-like ground shift of up to 6 millimeters across nearly all of Japan about 15 minutes after the 2011 magnitude-9 Tohoku quake. Researchers say a large ScS seismic wave traveled deep into Earth, reflected off the core, and returned to the surface, reactivating plate boundaries and causing the uniform shift—a first suggested observation of this type of wave influencing the surface and with implications for understanding aftershock hazards across subduction zones.