Scientific Investigations Map 3060
1U.S. Geological Survey, Box 25046, MS 966, Denver, CO 80225
2U.S. Geological Survey at Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195
3LACO Associates, 21 West 4th Street, Eureka, CA 95501
4Department of Geology, Humboldt State University, Arcata, CA 95521
5CGISS/Boise State University, 1910 University Drive, Boise, ID 83725
6Oregon Department of Geology, Coastal Field Office, Box 1033, Newport, OR 97365
7Center for Neotectonic Studies, University of Nevada, Reno, NV 89557
8Western Washington University, Department of Geology, Bellingham, WA 98225
9Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195
As part of the effort to assess seismic hazard in the Puget Sound region, we map fault scarps on Airborne Laser Swath Mapping (ALSM, an application of LiDAR) imagery (with 2.5-m elevation contours on 1:4,000-scale maps) and show field and laboratory data from backhoe trenches across the scarps that are being used to develop a latest Pleistocene and Holocene history of large earthquakes on the Tacoma fault. We supplement previous Tacoma fault paleoseismic studies with data from five trenches on the hanging wall of the fault. In a new trench across the Catfish Lake scarp, broad folding of more tightly folded glacial sediment does not predate 4.3 ka because detrital charcoal of this age was found in stream-channel sand in the trench beneath the crest of the scarp. A post-4.3-ka age for scarp folding is consistent with previously identified uplift across the fault during AD 770–1160. In the trench across the younger of the two Stansberry Lake scarps, six maximum 14C ages on detrital charcoal in pre-faulting B and C soil horizons and three minimum ages on a tree root in post-faulting colluvium, limit a single oblique-slip (right-lateral) surface faulting event to AD 410–990. Stratigraphy and sedimentary structures in the trench across the older scarp at the same site show eroded glacial sediments, probably cut by a meltwater channel, with no evidence of post-glacial deformation. At the northeast end of the Sunset Beach scarps, charcoal ages in two trenches across graben-forming scarps give a close maximum age of 1.3 ka for graben formation. The ages that best limit the time of faulting and folding in each of the trenches are consistent with the time of the large regional earthquake in southern Puget Sound about AD 900–930.
First posted April 20, 2009
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Nelson, A.R., Personius, S.F., Sherrod, B.L., Buck, Jason, Bradley, L-A, Henley II, Gary, Liberty, L.M., Kelsey, H.M., Witter, R.C., Koehler, R.D., Schermer, E.R., Nemser, E.S., and Cladouhos, T.T., 2008, Field and laboratory data from an earthquake history study of scarps in the hanging wall of the Tacoma fault, Mason and Pierce Counties, Washington: U.S. Geological Survey Scientific Investigations Map 3060, 3 sheets.