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TitleGeometry and membrane deformation rate of the subducting Cascadia slab
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LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorChiao, L -Y; Creager, K C
SourceThe Cascadia subduction zone and related subduction systems - seismic structure, intraslab earthquakes and processes, and earthquake hazards; by Kirby, S (ed.); Wang, KORCID logo (ed.); Dunlop, S (ed.); Geological Survey of Canada, Open File 4350, 2002 p. 47-54, Open Access logo Open Access
LinksOnline - En ligne
Alt SeriesUnited States Geological Survey, Open-file Report 02-328
PublisherNatural Resources Canada
MeetingIntraslab Earthquakes in the Cascadia Subduction System: Science and Hazards; Victoria; CA; September 18-21, 2000
Documentopen file
Mediaon-line; digital
RelatedThis publication is contained in The Cascadia subduction zone and related subduction systems - seismic structure, intraslab earthquakes and processes, and earthquake hazards
File formatpdf
ProvinceWestern offshore region
NTS92A; 92B
AreaGeorgia Strait; Puget Sound; Juan de Fuca Sound; United States of America; Canada
Lat/Long WENS-126.0000 -122.0000 49.0000 47.0000
Subjectstectonics; structural geology; geophysics; subduction zones; plate tectonics; tectonic elements; tectonic environments; tectonic interpretations; plate motions; subduction; lithosphere; oceanic lithosphere; deformation; geometric analyses; kinematic analysis; modelling; rheology; Cascadia Subduction Zone; Cascadia Margin; Juan de Fuca Plate; Juan de Fuca Slab; Mendocino Transform; SHIPS; Cenozoic
Illustrationstables; sketch maps
Released2002 06 11; 2016 08 31
AbstractThe Juan de Fuca plate subducts along the west coast of North America along a plate boundary that extends some 1000 km from northern California to Vancouver Island. In map view, the plate boundary is nearly straight along its northern and southern regions, but exhibits a 35°
change in orientation offshore from northern Washington State. Landward from this bend is an arch in the shape of the subducting plate, a concentration in Wadati - Benioff seismicity and an anomalously thick Miocene accretionary prism that makes up the two kilometer-high Olympic Mountains. We speculate that all three are related, directly or indirectly, to the fact that the Cascadia slab is subducting into a concave, oceanward corner which creates a geometric space problem analogous to pleats in a tablecloth hanging off the corner of a table. To test this hypothesis, we develop a theory of steady state, incompressible three-dimensional fluid flow that is asymptotically valid for a thin slab with high linear (or non-linear) viscosity relative to the surrounding fluid. Flow in this simple model is constrained by kinematic boundary conditions rather than buoyancy forces. Fixing the slab geometry to the same trench-normal profile along the entire subduction
zone, we invert for the flow field that minimizes the dissipation power of the system and find that strain rates, characterized by along-strike compression, are concentrated along the west coast of Washington, reaching values of 2x10-16 s-1. In a second experiment, we constrain
the same 20° dip along the northern and southern edges of the subducting plate and find that the optimal slab geometry contains an arch in a location similar to the seismologically observed arch, and that the root-mean square of the strain rates are reduced by a factor of five.
Thus, if these strain rates are accommodated by intraplate earthquakes, this arching model provides a geometry that allows a five-fold reduction in seismic moment release rates. The predicted strain rates are consistent with the observed moment release rate associated with one Mw=7 and several Mw>6 earthquakes per century in the Olympic Mountain/Puget Sound region, and a much lower rate to the north and south. Predicted strain rates associated with slab bending and unbending are generally smaller than the predicted membrane strain rates. The anomalously thick accretionary prism associated with the Olympic Mountains can be explained by the critical taper theory as being caused by the shallow dip of the slab along the arch.

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