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New Views of the Seattle and Tacoma Faults
Tom Brocher and Brian SherrodU.S. Geological Survey
Menlo Park, California and Seattle, Washington
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Cascadia eq types
{Not anEarthquake!
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Puget Lowland is being squeezed from south to north at a rate of about 1/4 inch/year(6 mm/yr)
1/4 in/yr
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Typical fold type
Future fault
Fault
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The Prevailing Model
Faults move at depth
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Two Problems
1)Surface faults are up to the north not to the south
2) Tacoma fault not yet recognized
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Seismic Hazard Investigations in Puget Sound (SHIPS)
March 1998
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2 miles below we can see the Tacoma and Seattle basins, a ridge between them, and evidence for a sharp contrast across the Tacoma fault
Pink symbols show uplift/subsidence at ~AD900
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By folding and faulting!!
SHIPS Tomography
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A more complex fold type
Wedge tip
One sided fold
(monocline)
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Folding along Seattle fault
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A New View
You areHere!!
18 24
Distance (miles)
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Depth in miles to the main faults
Seattle fault
Tacoma fault
Unlike in California,faults here dip gentlyso that the entireregion between Seattleand Tacoma is underlainat shallow depth by anactive fault
Seattle
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Seattle fault zone
Downtown Seattle
Seattlefaultzone
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Seattle fault zone
Wedge tip
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Seattle fault and folds
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Seismic hazard issues
Faulting towards SeattleBasin effectsShallow faulting Shallow foldingUpliftLocal tsunamisLandslides
Wedge tip
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Seattle fault zone - AD 900
Blake Island
M6.4
M7.2
Shallow splay faults
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Shallow splay faults
Shallow splay faults only slip when the master floor thrusts slip - they do not represent independent seismic sources. Paleo slip on them tells us about slip on the floor thrusts.
Shallow splay faults
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Other wedges (Pakistan)
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Flatirons of Boulder, Colorado
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Widespread movement 900 AD
Olympia
Seattle
Tacoma
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Vancouver
Seattle
Tacoma
Phase
II
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Phase II results:
Delineated the Seattle basin
Recorded 1999 Chi-Chi EQ
Identified basin amplification atlow frequencies andattenuation at higher frequencies of weakground motions
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Vancouver
Seattle
Tacoma
Phase
III
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Phase
III
Frankel