Plate dynamics at the Cascadia subduction zone: What is

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

Dave ChadwellScripps Institution of Oceanography

What is assumed (I):

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.

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)

Question: Where is the locked zone?

What is assumed (II):

Why that is lacking (I):

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.

A = D+C+E

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

What that is lacking (II):

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

Why that is lacking (III):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:

Can address along-strike and across-strikedeformation by establishing new GPS-Acoustic sites using either:

1) ship-based measurements

2) buoy-based GEOCE system under development at SIO. ORION?