After a subduction earthquake like the 11 March 2011 Tohoku-Oki earthquake, Earth’s crust continues to deform. Scientists have been monitoring this deformation near the earthquake’s rupture zone to estimate further seismic hazards.
The Japan Coast Guard (JCG) has been measuring the precise position of the seafloor above the earthquake’s rupture zone using GPS and acoustic observation using a series of transponders installed on the seafloor. The transponders respond to sound signals emitted from the survey ship and return them to the ship; by measuring travel time from the ship to the transponders, the JCG can detect the movements of the seafloor with an accuracy of centimeters. After 9 to 15 campaign observations at each transponder site over the last 3 years, Watanabe et al. report that the seafloor has moved and continues to move in two ways: through afterslip and viscoelastic relaxation.
Afterslip is additional fault slip that occurs following an earthquake; viscoelastic relaxation is the deformation of the Earth’s interior due to stress relief. The researchers discovered westward-downward movements on the seafloor at study sites above the main rupture zone, which they attribute solely to viscoelastic relaxation. South of the rupture zone, they measured an eastward-downward movement that they attribute to a possible combination of both afterslip and viscoelastic relaxation, but they need to continue their research to be sure.
The Japan Trench is part of the Pacific Ring of Fire, where 90% of the world’s earthquakes occur. Monitoring the state and deformation of Earth’s crust following a major event like the Tohoku-Oki earthquake will lead to a better understanding of the postseismic process and strain restoration of the trench in the future, the authors write. (Geophysical Research Letters, doi:10.1002/2014GL061134, 2014)
—Jessica Orwig, Writer
Citation: Orwig, J. (2014), Seafloor changes above the Tohoku-Oki earthquake rupture zone, Eos Trans. AGU, 95(50), 484, doi:10.1002/2014EO500009.