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 MW59:0 Tohoku earthquake generated coseismic stresses of 10 –10 Pa that could have induced changes in motion of small (radius 100 km) crustal blocks within Honshu. Analysis of timedependent 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 53 10Pa s, suggesting that great earthquakes may modulate motions of proximal crustal blocks at frequencies as high as 10 Hz.