Slides for Ben

Study Area

500 kmN

Great Earthquakes, Strongly-Coupled Arc

Pacific plate motion

1938, 8.3

1946, MS7.4 tsunami earthquake

1957, 9.1

1964, 9.2

1965, 8.9

1986, 8.01996, 7.9-8.0

Geometry of Subducting Slab

What Do We Expect to See?

Alaska Peninsula

Alaska Peninsula Velocities

Semidi Profile

Semidi Profile

Fletcher et al., 2001

Semidi Profile Model

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

The Creeping Section (Shumagins to Unimak)

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

Sanak Profile

Sanak to Unimak Data

Sanak Profile Model• Best-fit is no locked zone• How wide can locked zone be without violating data?

95%

99%

at tr

ench

30 k

m fr

om t

renc

h

Freymueller and Beavan, 1999

35 km53 km

How Far Does Creeping Extend?

Westdahl

Fisher

Shishaldin

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.

1964 Rupture Zone

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

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

Zweck et al. (2002)

Regional Plate CouplingSlip deficit/Vplate

Non-linear Deformation

1998.5

Three Time Periods

1998-2001Velocities measurably different over area >100x200 km2

Before and After

Data and Model

Comparison of Slip Models

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

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

Slip Model

Interpreted

1938 asperity

Kodiak asperity

PWS asperity

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)

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?

Alaska and the Lower 48

Aleutian Velocities

GPS Uplift Rates

GPS Horizontal Velocities