Dominance of Viscoelastic Relaxation after the 2011 Tohoku Earthquake Revealed By Seafloor GPS Observations

Friday, 19 December 2014: 8:30 AM
Tianhaozhe Sun1, Kelin Wang1,2, Ryota Hino3, Takeshi Iinuma3, Jiangheng He2, Hiromi Fujimoto4, Motoyuki Kido3, Yukihito Osada5 and Yan Hu6, (1)School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada, (2)Pacific Geoscience Centre, Geological Survey of Canada, Sidney, BC, Canada, (3)International Research Institute of Disaster Science, Tohoku University, Sendai, Japan, (4)National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan, (5)Department of Geophysics, Tohoku University, Sendai, Japan, (6)Department of Earth and Planetary Sciences, University of California, Berkeley, CA, United States
Megathrust rupture is mostly offshore. Using land-based GPS data only, it is difficult to tell whether viscoelastic relaxation, when compared to afterslip, is important in driving post-seismic deformation immediately after the earthquake. GPS/acoustic observations, which measure the displacements of seafloor precision transponder arrays using sea surface platforms equipped with GPS antennas and acoustic transducers, were made by Japan Coast Guard and Tohoku University during and after the 2011 M 9 Tohoku earthquake. Five seafloor sites near the main rupture were reoccupied 4-10 times over the first two years after the earthquake and provide a unique opportunity to study the importance of viscoelastic relaxation. After the large coseismic seaward motion (up to ~ 31 m), the near-trench sites immediately reversed their direction to move landward, opposing the continuing seaward motion of all land sites. This landward motion (~ 30-50 cm over the first year) is faster than the plate convergence rate (8.3 cm/year) and therefore cannot be explained by the relocking of the subduction fault. Using numerical models with transient mantle rheology, we demonstrate that the fast landward motion is caused by viscoelastic relaxation of stresses induced by the asymmetric rupture of the thrust earthquake. For shallow megathrust events, the hanging wall has a lower stiffness and hence always exhibits larger coseismic seaward motion than the landward motion of the footwall. Greater tension is thus induced landward of the rupture area. As the mantle wedge undergoes viscoelastic relaxation, this tension in the upper plate drives the observed landward motion of the trench area. Tests on the effects of mantle rheology, coseismic slip distribution, and plate thickness suggest the wide presence of viscoelastic landward trench motion following large subduction earthquakes. This work indicates that afterslip models assuming a purely elastic Earth substantially overestimated or underestimated afterslip downdip or updip of the rupture zone, respectively. Post-seismic fault behavior has to be studied in the context of a viscoelastic Earth even immediately after the earthquake.