title: Prevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake.
abstract: After a large subduction earthquake, crustal deformation continues to occur, with a complex pattern of evolution. This postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip (afterslip) or relocking of different parts of the fault. When postseismic geodetic observations are used to study Earth's rheology and fault behaviour, it is commonly assumed that short-term (a few years) deformation near the rupture zone is caused mainly by afterslip, and that viscoelasticity is important only for longer-term deformation. However, it is difficult to test the validity of this assumption against conventional geodetic data. Here we show that new seafloor GPS (Global Positioning System) observations immediately after the great Tohoku-oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short-term postseismic deformation. These data reveal fast landward motion of the trench area, opposing the seaward motion of GPS sites on land. Using numerical models of transient viscoelastic mantle rheology, we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake, a process previously unknown because of the lack of near-field observations. Our findings indicate that previous models assuming an elastic Earth will have substantially overestimated afterslip downdip of the rupture zone, and underestimated afterslip updip of the rupture zone; our knowledge of fault friction based on these estimates therefore needs to be revised.
authors.affiliation: School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada.
authors.forename: Tianhaozhe
authors.initials: T
authors.lastname: Sun
authors.affiliation: 1] School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada [2] Pacific Geoscience Centre, Geological Survey of Canada, Natural Resources Canada, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada.
authors.forename: Kelin
authors.initials: K
authors.lastname: Wang
authors.affiliation: International Research Institute of Disaster Science, Tohoku University, Sendai 980-0845, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Iinuma
authors.affiliation: International Research Institute of Disaster Science, Tohoku University, Sendai 980-0845, Japan.
authors.forename: Ryota
authors.initials: R
authors.lastname: Hino
authors.affiliation: Pacific Geoscience Centre, Geological Survey of Canada, Natural Resources Canada, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada.
authors.forename: Jiangheng
authors.initials: J
authors.lastname: He
authors.affiliation: International Research Institute of Disaster Science, Tohoku University, Sendai 980-0845, Japan.
authors.forename: Hiromi
authors.initials: H
authors.lastname: Fujimoto
authors.affiliation: International Research Institute of Disaster Science, Tohoku University, Sendai 980-0845, Japan.
authors.forename: Motoyuki
authors.initials: M
authors.lastname: Kido
authors.affiliation: International Research Institute of Disaster Science, Tohoku University, Sendai 980-0845, Japan.
authors.forename: Yukihito
authors.initials: Y
authors.lastname: Osada
authors.affiliation: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Miura
authors.affiliation: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
authors.forename: Yusaku
authors.initials: Y
authors.lastname: Ohta
authors.affiliation: Berkeley Seismological Laboratory and Department of Earth and Planetary Sciences, University of California, Berkeley, California, California 94720, USA.
authors.forename: Yan
authors.initials: Y
authors.lastname: Hu