Slides for Ben Study Area 500 km N Great Earthquakes, Strongly-Coupled Arc Pacific plate motion...
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Transcript of Slides for Ben Study Area 500 km N Great Earthquakes, Strongly-Coupled Arc Pacific plate motion...
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
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
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 Model• Best-fit is no locked zone• How wide can locked zone be without violating data?
95%
99%
at tr
ench
30 k
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om t
renc
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Freymueller and Beavan, 1999
35 km53 km
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.
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
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
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?