1. 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

2. 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.

3. 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):

4. 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)

5. What that is lacking (II) : Significant deformation (~ 20 mm/yr : white arrow) of JdF plate in direction opposite to convergence.

6. 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)

7. 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?