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2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 11
Presentation Time: 10:55 AM

EVIDENCE FOR A LATE HOLOCENE EARTHQUAKE ON THE TACOMA FAULT, PUGET SOUND, WASHINGTON


SHERROD, Brian L.1, BROCHER, Thomas M.2, WEAVER, Craig S.1, BUCKNAM, Robert C.3, BLAKELY, Richard J.4, KELSEY, Harvey M., III5, NELSON, Alan R.3 and HAUGERUD, Ralph A.1, (1)U.S. Geological Survey at Dept. of Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195, (2)U.S. Geological Survey, 345 Middlefield Road, Box 977, Menlo Park, CA 94025, (3)U.S. Geological Survey, P.O. Box 25046, Lakewood, CO 80225, (4)U.S. Geol Survey, 345 Middlefield Road, MS 989, Menlo Park, CA 94025, (5)Department of Geology, Humboldt State Univ, Arcata, CA 95521, bsherrod@ess.washington.edu

Recent LiDAR mapping reveals a 4-km-long series of scarps cutting late Pleistocene glacial deposits between arms of southern Puget Sound. The scarps coincide with gravity and aeromagnetic anomalies likely defining the location of a reverse fault; previous interpretations of seismic velocity models placed the Tacoma fault at this location. The Tacoma fault bounds gravity and aeromagnetic anomalies for 50 km across central Puget Sound. Seismic tomography implies at least 6 km of post-Eocene uplift to the north relative to sedimentary rocks in a structural basin to the south.

Paleoseismic studies along the Tacoma fault provide evidence for late Holocene movement. Coastal marsh deposits along the eastward projection of the fault show evidence for land-level changes. Coastlines north of the Tacoma fault rose as much as 4 m about 1100 years ago during an earthquake that caused tidal flats at several sites to turn into forested wetlands and freshwater marshes. Radiocarbon ages on plant material date the uplift to A.D. 800-1200. At one site south of the fault, Douglas-fir forests sank into the intertidal zone and changed into saltmarsh between A.D. 980 and 1190. Liquefaction features found at one uplifted site show strong ground shaking at the time of uplift.

The scarps seen in LiDAR face south, trend roughly east-west, and offset striae left by the last Quaternary ice sheet to advance into Puget Sound. A trench across one scarp exposed evidence for postglacial folding and reverse faulting. No organic material for radiocarbon dating was recovered. However, tectonic deformation of a postglacial soil in the trench suggests that the folding and faulting happened late in the Holocene.

With this evidence for Holocene activity, the Tacoma fault joins the Seattle fault on the list of shallow, recently active faults in the Seattle-Tacoma metropolitan area. If it were to rupture along its entire known length of 50 km, the Tacoma fault would likely generate an earthquake of M 6.5-7.5.

Session No. 24

T143. Earthquake Geology in Reverse-Faulting Terrains
Sunday, 2 November 2003: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 35, No. 6, September 2003, p.98
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