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TitleViscoelastic relaxation following subduction earthquakes and its effects on afterslip determination
AuthorSun, T; Wang, K
SourceJournal of Geophysical Research, Solid Earth vol. 120, 2015 p. 1-16,
Alt SeriesEarth Sciences Sector, Contribution Series 20140354
PublisherAmerican Geophysical Union
Mediapaper; on-line; digital
File formatpdf
Subjectsgeophysics; earthquakes; earthquake mechanisms; earthquake studies; viscosity; faults, slip; mantle
Illustrationscross-sections; plots
ProgramWestern Canada Geohazards Project, Public Safety Geoscience
AbstractAfterslip is commonly thought to be the controlling process in postseismic deformation immediately following a great megathrust earthquake and is usually inferred from geodetic observations using purely elastic models. However, observed motion reversal of the near-trench area right after the 2011 Mw 9 Tohoku-oki earthquake demonstrates the dominance of viscoelastic relaxation of coseismically induced stresses. To understand the importance of incorporating viscoelasticity in afterslip determination, we employ biviscous Burgers mantle rheology and use finite element models to explore how viscoelastic relaxation in short-term postseismic deformation is controlled by various geometrical and rheological factors. Our results indicate that immediately after large megathrust earthquakes (Mw>8.0), viscoelastic deformation should always cause opposing motion of inland and trench areas and subsidence around landward termination of the rupture, although the rate of such postseismic motion depends on local conditions such as the age and hence thickness of the slab and transient mantle viscosity values. While elastic models may be adequate for afterslip estimation for earthquakes of Mw<7.5, neglect of viscoelasticity for larger events leads to overestimate of afterslip downdip of the rupture and underestimate of afterslip at shallower depths. Reassessing shallow afterslip following the 2005 Mw 8.7 Nias earthquake using 2-D viscoelastic models suggests that the actual afterslip may be greater than that estimated using an elastic model by more than 50%. Similarly, interpreting trenchward motion of some seafloor GPS sites following the Tohoku-oki earthquake using a viscoelastic model suggests large shallow afterslip outside of the main rupture area.
Summary(Plain Language Summary, not published)
Study of aseismic slip of parts of the megathrust fault following a great subduction earthquake (afterslip) helps to understand frictional behaviour of the fault and provides information for assessing the risk of megathrust earthquakes and tsunamis. A major challenge in this type of study is that postseismic deformation due to the viscoelastic relaxation of the mantle can be confused with that due to afterslip. Using seafloor geodetic observations, we show that the dominant contribution from viscoelastic relaxation can be unambiguously constrained. Numerical models presented in this paper illustrate why and how previously published afterslip estimates for great subduction earthquakes should be revised.