# Block Motion Changes in Japan Triggered by the 2011 Great Tohoku Earthquake

## TL;DR

The paper models how the 2011 Tohoku earthquake affected local plate motions in Japan, showing that large earthquakes can induce measurable changes in crustal block velocities through stress alterations.

## Contribution

It introduces a scalar relationship linking earthquake-induced stress changes to relative plate speed variations, supported by GPS data analysis in Japan.

## Key findings

- Plate speeds changed by up to 3 mm/yr after the earthquake.
- Coseismic stresses of 10^2-10^5 Pa can influence small crustal blocks.
- Upper mantle viscosity estimated at 5×10^18 Pa·s.

## Abstract

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.