Block motion changes in Japan triggered by the 2011 great Tohoku earthquake

Plate motions are governed by equilibrium between basal and edge forces. Great earthquakes may induce differential static stress changes across tectonic plates, enabling a new equilibrium state. Here we consider the torque balance for idealized circular plates and find a simple scalar relationship for changes in relative plate speed as a function of its size, upper mantle viscosity, and coseismic stress changes. Applied to Japan, the 2011 MW=9.0 Tohoku earthquake generated coseismic stresses of 102–105 Pa that could have induced changes in motion of small (radius ∼100 km) crustal blocks within Honshu. Analysis of time‐dependent GPS velocities, with corrections for earthquake cycle effects, reveals that plate speeds may have changed by up to ∼3 mm/yr between ∼3.75 year epochs bracketing this earthquake, consistent with an upper mantle viscosity of ∼5 × 1018Pa ·s, suggesting that great earthquakes may modulate motions of proximal crustal blocks at frequencies as high as 10−8 Hz. Key Points Analytic theory for block motion changes from earthquakes GPS constrained block motions reveal select block motion changes before and after the 2011 Tohoku earthquake