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
$\mathrm{M}_{\mathrm{W}}=9.0$ Tohoku earthquake generated coseismic stresses of
$10^2-10^5$~Pa that could have induced changes in motion of small (radius
$\sim100$~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 $\sim3$ mm/yr between $\sim3.75$-year epochs
bracketing this earthquake, consistent with an upper mantle viscosity of $\sim
5\times10^{18}$Pa$\cdot$s, suggesting that great earthquakes may modulate
motions of proximal crustal blocks at frequencies as high as $10^-8$~Hz.
