New Views of the Seattle and Tacoma Faults

Tom Brocher and Brian SherrodU.S. Geological Survey

Menlo Park, California and Seattle, Washington

Cascadia eq types

{Not anEarthquake!

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

Typical fold type

Future fault

Fault

The Prevailing Model

Faults move at depth

Two Problems

1)Surface faults are up to the north not to the south

2) Tacoma fault not yet recognized

Seismic Hazard Investigations in Puget Sound (SHIPS)

March 1998

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

By folding and faulting!!

SHIPS Tomography

A more complex fold type

Wedge tip

One sided fold

(monocline)

Folding along Seattle fault

A New View

You areHere!!

18 24

Distance (miles)

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

Seattle fault zone

Downtown Seattle

Seattlefaultzone

Seattle fault zone

Wedge tip

Seattle fault and folds

Seismic hazard issues

Faulting towards SeattleBasin effectsShallow faulting Shallow foldingUpliftLocal tsunamisLandslides

Wedge tip

Seattle fault zone - AD 900

Blake Island

M6.4

M7.2

Shallow splay faults

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

Other wedges (Pakistan)

Flatirons of Boulder, Colorado

Widespread movement 900 AD

Olympia

Seattle

Tacoma

Vancouver

Seattle

Tacoma

Phase

II

Phase II results:

Delineated the Seattle basin

Recorded 1999 Chi-Chi EQ

Identified basin amplification atlow frequencies andattenuation at higher frequencies of weakground motions

Vancouver

Seattle

Tacoma

Phase

III

Phase

III

Frankel