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TitleGeodetically inferred locking state of the Cascadia megathrust based on a viscoelastic Earth model
AuthorLi, S; Wang, K; Wang, Y; Jiang, Y; Dosso, S E
SourceJournal of Geophysical Research, Solid Earth vol. 123, 2018 p. 1-17,
Alt SeriesNatural Resources Canada, Contribution Series 20180105
PublisherAmerican Geophysical Union
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®)
ProvinceBritish Columbia
NTS92B; 92C; 92E; 92F; 92G; 92H; 92I; 92J; 92K; 92L
AreaVancouver; Vancouver Island; Washington State; Oregon; California; Canada; United States
Lat/Long WENS-128.0000 -120.0000 51.0000 40.0000
Subjectsgeophysics; tectonics; subduction zones; models; satellite geodesy; bedrock geology; structural features; faults; stress analyses; stress distribution; deformation; crustal movements; rheology; creep; Cascadia Subduction Zone; geological hazards; megathrust earthquakes; viscoelasticity; fault locking; depth; Global Navigation Satellite System (GNSS); interseismic stress relaxation; inversion models
Illustrationscartoons; geoscientific sketch maps; models; tables; profiles; graphs
ProgramAssessing Earthquake Geohazards, Public Safety Geoscience
Released2018 08 04
AbstractIn a viscoelastic Earth, stresses slowly built up due to fault locking are relaxed concurrently during the entire interseismic period. This interseismic stress relaxation causes crustal deformation much farther away from the locked fault than can be explained using elastic models that neglect the relaxation. Here we develop a viscoelastic geodetic inversion model to address this problem at Cascadia. We invert ~500 horizontal velocity vectors based on continuous and campaign geodetic measurements over the past two decades. Ambiguities arising from long-term rotation of upper-plate crustal blocks are addressed by test-correcting the geodetic velocities with two different block-motion models. Fault back slip (i.e., slip deficit) Green's functions are derived using a Maxwell viscoelastic finite element model with realistic subduction zone structure and megathrust geometry. The preferred model features a narrow and shallow megathrust locked zone, consistent with earlier thermorheological reasoning. For an elastic model to fit the data to the same fidelity, megathrust locking has to extend to much greater depths. However, even with the viscoelastic model, the land-based geodetic data still cannot resolve whether there is some creep (incomplete locking) in the shallowest part of the megathrust far offshore. Neither can the land data fully resolve along-strike variations of the locking state. These ambiguities can be resolved only when adequate seafloor geodetic data are obtained.
Summary(Plain Language Summary, not published)
Understanding interseismic locking of subduction megathrusts is important to assessing the risk of future megathrust earthquakes. The majority of megathrust locking models worldwide based on geodetic observations assume an elastic Earth, missing the contribution of viscoelastic stress relaxation to surface deformation and thus overestimating the maximum depth of locking. Here we use a model of viscoelastic Earth to invert geodetic observations at the Cascadia margin to constrain the megathrust locking state. The results indicate a shallow locked zone, consistent with inferences based on earlier thermal arguments. We also document the ambiguity in resolving the locking state near the deformation front, even with the improved Earth model, and show that obtaining seafloor geodetic observations is the only way to remove the ambiguity.