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Plate dynamics at the Cascadia subduction zone: What is assumed, why that is lacking, and how to address it. Dave Chadwell Scripps Institution of Oceanography. What is assumed (I):.

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Plate dynamics at the Cascadia subduction zone: What is assumed, why that is lacking, and how to address it.

Dave Chadwell

Scripps Institution of Oceanography


What is assumed i
What is assumed (I): assumed, why that is lacking, and how to address it.

We assume convergence at Cascadia Subduction Zone is defined by Euler pole motion between North America and Juan de Fuca.

NA_JdF = NA_PA + PA_JdF

NA_PA from present-day GPS

PA_JdF from 0.78 geomagentic reversal

In this approach there is no present-day

measurement of JdF plate motion.


What is assumed (II): assumed, why that is lacking, and how to address it.

Question: Where is the locked zone?

Approach: Observe elastic strain with geodetics, assume convergence rate,

assume thrust fault geometry, then solve for the lock zone:

elastic strain = f(convergence rate, fault geometry, lock zone)


Why that is lacking (I) assumed, why that is lacking, and how to address it.:

A = D+C+E

Observe horizontal seafloor motion with centimeter resolution by combing GPS with precision underwater acoustics: GPS-Acoustics.

Seafloor motion vectors in same frame as GPS allows direct comparison to land-GPS vectors and other seafloor vectors separated by 100s km.

e.g., (Gagnon, Chadwell, Norabuena Nature, March 2005)


What that is lacking (II) assumed, why that is lacking, and how to address it.:

Significant deformation (~ 20 mm/yr : white arrow) of JdF plate in direction opposite to convergence.


Why that is lacking (III) assumed, why that is lacking, and how to address it.:

PROPOSED MECHANISM: Plate forces

create region near yield point offshore

central Oregon (Wang et al., 1997), add

backstop of Siletzia basalt and asperity

(Trehu et al.).

OBSERVATIONS:

*GPS-A observation

*onshore vertical (Weldon)

*faults in forearc (Goldfinger)


How to address it: assumed, why that is lacking, and how to address it.

Can address along-strike and across-strike

deformation by establishing new

GPS-Acoustic sites using either:

1) ship-based measurements

2) buoy-based GEOCE system under

development at SIO. ORION?


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