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TitleFinding simplicity in the complexity of postseismic coastal uplift and subsidence following great subduction earthquakes
AuthorLuo, HORCID logo; Wang, KORCID logo
SourceJournal of Geophysical Research, Solid Earth vol. 127, 2022 p. 1-21,
Alt SeriesNatural Resources Canada, Contribution Series 20220168
PublisherAmerican Geophysical Union
Mediapaper; digital; on-line
File formatpdf
Subjectstectonics; subduction zones; subduction; earthquakes; earthquake studies; seismology; deformation; aftershocks; modelling
Illustrationsschematic diagrams; schematic sections; models
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2022 10 10
AbstractFollowing great subduction earthquakes, postseismic deformation of coastal areas shows consistent seaward motion but complex vertical deformation. Understanding both the horizontal and vertical components in the same geodynamic framework presents challenges. Here, by modeling short-term (a few years) postseismic viscoelastic relaxation (VER) and afterslip following synthetic and real subduction earthquakes, we demonstrate that the complexity of the vertical deformation can be explained in simple terms. Along a margin-normal profile, VER results in an up-down-up trisegment, long-wavelength pattern common to most megathrust earthquakes, including near-trench uplift, midway subsidence, and near-arc uplift, with locations controlled by coseismic fault slip. The magnitude of the first two segments is controlled mainly by oceanic mantle viscosity, and the third by mantle wedge viscosity. In contrast with VER, afterslip results in an up-down bimodal pattern of variable wavelengths specific to individual earthquakes. Its site-specific and heterogeneous nature is primarily responsible for the complexity in vertical deformation, but its effect can be adequately modeled using a simple elastic model. If the coast is near the megathrust rupture zone, variable combinations of the VER and afterslip effects lead to either uplift or subsidence. If the coast is in the near-arc segment of VER deformation, uplift usually occurs. Modeling the common VER process enables the identification of site-specific afterslip, which helps to understand the mechanism of afterslip in the context of the broad spectrum of fault slip behavior. Our results also have important implications to deciphering coastal paleoseismic records to constrain coseismic versus postseismic deformation of ancient earthquakes.
Summary(Plain Language Summary, not published)
Land level change along the coast happens not only suddenly during a megathrust earthquake but also slowly after the earthquake. However, observed postseismic coastal vertical deformation worldwide shows a very complex pattern. In this work, we synthesize global observations and construct conceptual and numerical models to identify the common physics underlying the complex pattern. For the first time, we are able to explain the complex observations in simple terms. We show that the complexity is due to a combination of the viscoelastic behaviour of the mantle and continuing aseismic slip of the megathrust fault compounded by different locations of the coastal area relative to the earthquake rupture zone. The findings have broad implications to predicting relative sea level change associated with subduction earthquakes, understanding the dynamics of megathrust earthquake cycles, and interpreting paleoseismic coastal observations.

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