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This presentation explores the complex seismic behavior of the Alaska Peninsula, focusing on the interplay between locked and creeping subduction zones, historical seismic events like the 1964 Alaskan earthquake, and contemporary data trends. It details the estimated slip rates, locked zone widths, and the implications of GPS data collected across various profiles in the region. The analysis reveals critical insights into seismic risk and plate motion, contributing to the understanding of the dynamics governing major earthquakes and their potential impact on communities throughout the area.

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Slides for Ben

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  1. Slides for Ben

  2. Study Area N 500 km

  3. Great Earthquakes, Strongly-Coupled Arc 1964, 9.2 1938, 8.3 1946, MS7.4 tsunami earthquake Pacific plate motion 1957, 9.1 1986, 8.0 1996, 7.9-8.0 1965, 8.9

  4. Geometry of Subducting Slab

  5. What Do We Expect to See?

  6. Alaska Peninsula

  7. Alaska Peninsula Velocities Semidi Profile

  8. Semidi Profile Fletcher et al., 2001

  9. Semidi Profile Model

  10. 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

  11. The Creeping Section (Shumagins to Unimak)

  12. 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

  13. Sanak Profile

  14. Sanak to Unimak Data

  15. 53 km 35 km Sanak Profile Model • Best-fit is no locked zone • How wide can locked zone be without violating data? at trench 30 km from trench 99% 95% Freymueller and Beavan, 1999

  16. Shishaldin Fisher Westdahl How Far Does Creeping Extend?

  17. Conclusions: Alaska Peninsula • Wide locked zone corresponds to 1938 MW 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.

  18. 1964 Rupture Zone

  19. 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

  20. 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

  21. Regional Plate Coupling Slip deficit/Vplate Zweck et al. (2002)

  22. Non-linear Deformation 1998.5

  23. Three Time Periods 1998-2001 Velocities measurably different over area >100x200 km2

  24. Before and After

  25. Data and Model

  26. Comparison of Slip Models

  27. 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 MW~7-7.1 earthquake over 2.5 years • Can still identify asperities

  28. 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

  29. Slip Model

  30. Interpreted PWS asperity Kodiak asperity 1938 asperity

  31. 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)

  32. 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?

  33. Alaska and the Lower 48

  34. Aleutian Velocities

  35. GPS Uplift Rates

  36. GPS Horizontal Velocities

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