2. Slides for Ben

3. Study Area 500 km N

4. Great Earthquakes, Strongly-Coupled Arc Pacific plate motion 1938, 8.3 1946, M S 7.4 tsunami earthquake 1957, 9.1 1964, 9.2 1965, 8.9 1986, 8.0 1996, 7.9-8.0

5. Geometry of Subducting Slab

6. What Do We Expect to See?

7. Alaska Peninsula

8. Alaska Peninsula Velocities Semidi Profile

9. Fletcher et al., 2001

10. Semidi Profile Model

11. Semidi Profile Results Locked zone is ~180 km wide Estimated slip deficit is ~80% of plate motion rate –> Wide, strongly-coupled seismogenic zone Residual trench-parallel component of several mm/yr

12. The Creeping Section (Shumagins to Unimak)

13. Previous Work in the Shumagins Velocities relative to centroid of network Estimated 3.2±2.3 mm/yr contraction across islands Contrasts with 16±3 mm/yr contraction across Semidi islands in same position (28±3 Chirikof to Pac. coast) Minimal data collected since 1993 Larson and Lisowski, 1994

14. Sanak Profile

15. Sanak to Unimak Data

16. Sanak Profile Model Best-fit is no locked zone How wide can locked zone be without violating data? 95% 99% at trench 30 km from trench Freymueller and Beavan, 1999 35 km53 km

17. How Far Does Creeping Extend? Westdahl Fisher Shishaldin

18. Conclusions: Alaska Peninsula Wide locked zone corresponds to 1938 M W 8.3 rupture zone Narrow or nonexistent locked zone from Shumagin “gap” west to end of Unimak Along-strike boundary between these two segments is sharp -- within a few 10s of km. –Correlates spatially with change in magnetic lineations on seafloor, but no big age change. Unlocked segment includes 1946 “tsunami earthquake” zone –No strain seen in 1946 segment –> unlikely to be a giant asperity as required if tsunami generated solely by earthquake.

19. 1964 Rupture Zone

20. Kenai Combination of –locked subduction zone (NNW) –postseismic deformation (SSE) Up to 55 mm/yr relative to NOAM Up to ~75 mm/yr relative motions Along-strike changes in seismogenic zone

21. Kenai Detail Obvious transition between western and eastern Peninsula Look at sites same distance from trench Edge of plate coupling toward western edge of Peninsula –Edge of PWS asperity

22. Zweck et al. (2002) Regional Plate Coupling Slip deficit/V plate

23. Non-linear Deformation 1998.5

24. Three Time Periods 1998-2001 Velocities measurably different over area >100x200 km 2

25. Before and After

26. Data and Model

27. Comparison of Slip Models

28. 1964 Rupture Zone Results Two large asperities with distinct gap –Corresponds to 1964 coseismic slip Strong Postseismic Deformation continues –Both afterslip and viscoelastic mechanisms Slip event from 1998-2000 –Downdip of seismogenic zone –Equivalent to M W ~7-7.1 earthquake over 2.5 years Can still identify asperities

29. Latest Results Zweck et al. used data through 1999, update uses data through 2002 Averages over time, so includes slip event in Anchorage area Adds important new data from two Alaska Peninsula sites Kodiak asperity remains poorly resolved Working on separation of viscoelastic and afterslip mechanisms

30. Slip Model

31. Interpreted 1938 asperity Kodiak asperity PWS asperity

32. Conclusions There are large along-strike variations in behavior of seismogenic zone –Width of zone from 100s of km to <50 km or zero –Shallow interface: fully locked to fully creeping –Locked == asperities of last great earthquakes Along-strike length scales for transition from locked to creeping are a few 10s of km or less –Slowly-varying properties cannot control seismogenic zone Convergence rate Sediment thickness Oceanic plate age or dip angle (except for sharp changes)

33. Conclusions Dynamics of plate boundary downdip of seismogenic zone (and near base) are complex Slowly-varying properties cannot control seismogenic zone –Convergence rate –Sediment thickness –Oceanic plate age or dip angle Perhaps these parameters define a “potentially seismogenic zone” Fault frictional properties vary over short distances? –Why? –Why do parts of the San Andreas fault system creep?

34. Alaska and the Lower 48

35. Aleutian Velocities

36. GPS Uplift Rates

38. GPS Horizontal Velocities