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TitlePrevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake
AuthorSun, T; Wang, KORCID logo; Iinuma, T; Hino, R; He, J; Fujimoto, H; Kido, M; Osada, Y; Miura, S; Ohta, Y; Hu, Y
SourceNature vol. 514, 2014 p. 84-97,
Alt SeriesEarth Sciences Sector, Contribution Series 20140167
PublisherSpringer Nature
Mediapaper; on-line; digital
File formatpdf
AreaTohoku-oki; Japan
Lat/Long WENS 139.0000 144.4167 40.5000 36.0000
Subjectsgeophysics; tectonics; earthquakes; earthquake studies; deformation; subduction zones; subduction; tectonic interpretations; tectonic setting; tectonic environments
Illustrationslocation maps; cross-sections; plots
ProgramPublic Safety Geoscience National Hazard assessment
Released2014 09 17
AbstractAfter a large subduction earthquake, crustal deformation continues to occur, with a complex pattern of evolution1.This postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture andcontinuing slip (afterslip) or relocking of different parts of the fault2-6. 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 deformation6,7. However, it is difficult to test the validity of this assumption against conventional geodetic data.Herewe showthatnewseafloorGPS(Global Positioning System) observationsimmediately 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 landwardmotion of the trench area, opposing the seaward motion ofGPS siteson land.Usingnumericalmodels of transient viscoelasticmantle rheology, wedemonstrate that the landwardmotion is a consequence of relaxation of stresses induced by the asymmetric ruptureof the thrust earthquake, a processpreviouslyunknownbecause 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.
Summary(Plain Language Summary, not published)
Great subduction earthquakes pose great risk to the west coast of Canada. Understanding the process of strain accumulation and release in these events for hazard assessment requires not only understanding the geology and physics of the megathrust fault, but also the mechanical behavior (or rheology) of the mantle. So far our understanding of the mentle rheology in subduction earthquake cycles is based on land-based observations of crustal deformation. In this work, we use seafloor GPS observations following the great 2011 Tohoku-oki earthquake to study the deformation process near the rupture zone and immediately following the earthquake. The findings show unambiguiously, for the first time, that viscoelastic deformation is predominant right after a great earthquake. Previous models of aseismic fault slip (afterslip) right after a great earthquake assuming an elastic Earth and ignoring the viscous mantle behavor are in error. Our knowledge of fault mechanics based on those models thus needs revision.

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