Along-trench variation in seafloor displacements after the 2011 Tohoku earthquake.

The 2011 Tohoku-oki earthquake was the largest earthquake ever observed with seafloor geodetic techniques in and around its source region. Large crustal deformation associated with both the coseismic rupture and the rapid postseismic deformation has been reported. However, these observations are insufficient to describe the postseismic deformation processes occurring around the broad rupture area. We report the first results of seafloor Global Positioning System and acoustic ranging (GPS-A) observations based on repeated campaign surveys conducted over nearly 4 years using the extended GPS-A network deployed along the Japan Trench in September 2012. The observed postseismic displacement rates (DRs) show evident spatial variation along the trench: (i) distinct landward DRs in the large coseismic slip area [primary rupture area (PRA)], evidencing the predominance of viscoelastic relaxation; (ii) remarkable trenchward DRs in the south of the PRA, indicating rapid afterslip; and (iii) slight trenchward DRs in the north of the PRA. These features provide great insights into constructing a more complete model of viscoelastic relaxation, and they also indicate spatial variation of afterslip and fault locking along the plate interface with clear spatial resolution, providing invaluable information for the improvement of seismic hazard assessment.