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):.
Plate dynamics at the Cascadia subduction zone: What is assumed, why that is lacking, and how to address it.
Scripps Institution of Oceanography
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):
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):
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):
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.).
*onshore vertical (Weldon)
*faults in forearc (Goldfinger)
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?