doi:10.1130/G19914.1 2004;32;9-12 Geology M. Kelsey, Alan R. Nelson and Ralph Haugerud Brian L. Sherrod, Thomas M. Brocher, Craig S. Weaver, Robert C. Bucknam, Richard J. Blakely, Harvey Holocene fault scarps near Tacoma, Washington, USA Geology on 18 April 2009 geology.gsapubs.orgDownloaded from E-mail alerting services cite this article to receive free e-mail alerts when new articleswww.gsapubs.org/cgi/alertsclick Subscribe to subscribe to Geologywww.gsapubs.org/subscriptions/index.ac.dtlclick Permission request to contact GSAhttp://www.geosociety.org/pubs/copyrt.htm#gsaclick viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or politicalarticle's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to thescience. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education andto use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA,employment. Individual scientists are hereby granted permission, without fees or further requests to Copyright not claimed on content prepared wholly by U.S. government employees within scope of their Notes © 2004 Geological Society of Americaq 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]; January 2004; v. 32; no. 1; p. 9–12; DOI 10.1130/G19914.1; 4 figures. 9Holocene fault scarps near Tacoma, Washington, USABrian L. Sherrod* U.S. Geological Survey, Department of Earth and Space Sciences, University of Washington, Box 351310,Seattle, Washington 98195, USAThomas M. Brocher U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, California 94025, USACraig S. Weaver U.S. Geological Survey, Department of Earth and Space Sciences, University of Washington, Box 351310,Seattle, Washington 98195, USARobert C. Bucknam U.S. Geological Survey, MS 966, Denver Federal Center, Lakewood, Colorado 80225, USARichard J. Blakely U.S. Geological Survey, MS 989, 345 Middlefield Road, Menlo Park, California 94025, USAHarvey M. Kelsey Department of Geology, Humboldt State University, Arcata, California 95521, USAAlan R. Nelson U.S. Geological Survey, MS 966, Denver Federal Center, Lakewood, Colorado 80225, USARalph Haugerud U.S. Geological Survey, Department of Earth and Space Sciences, University of Washington, Box 351310,Seattle, Washington 98195, USAABSTRACTAirborne laser mapping confirms that Holocene active faults traverse the Puget Soundmetropolitan area, northwestern continental United States. The mapping, which detectsforest-floor relief of as little as 15 cm, reveals scarps along geophysical lineaments thatseparate areas of Holocene uplift and subsidence. Along one such line of scarps, we foundthat a fault warped the ground surface between A.D. 770 and 1160. This reverse fault,which projects through Tacoma, Washington, bounds the southern and western sides ofthe Seattle uplift. The northern flank of the Seattle uplift is bounded by a reverse faultbeneath Seattle that broke in A.D. 900–930. Observations of tectonic scarps along theTacoma fault demonstrate that active faulting with associated surface rupture and groundmotions pose a significant hazard in the Puget Sound region.Keywords: Holocene, scarp, Puget Sound, paleoseismology, fault.Figure 1. Tectonic setting of Cascadia sub-duction zone. Western Washington region(brown), between fixed North America andOregon Coast Range, is undergoing trans-pression, which creates folds and reversefaults across Puget Sound. Bold arrows in-dicate motions of tectonic blocks inferredfrom geologic and geodetic data. Modifiedfrom Wells et al. (1998) and Wang et al.(2003).INTRODUCTIONA tectonic scarp along a fault, as a geo-morphic signature of surface rupture, demon-strates the fault’s capacity to produce earth-quakes. In addition, by providing clues to thefrequency and size of past earthquakes, thescarp may help define the hazard that the faultposes. Until 1998, scarps played little role indefining seismic hazards around Puget Sound,northwestern continental United States. Thissubduction-zone region, home to three millionpeople, faces earthquakes from a multitude ofsources, the nearest of these being faults with-in the overriding North American plate (Fig.1). Geologists used to despair over findingscarps from such faults because the landscapearound Puget Sound is covered with 16 ka gla-cial deposits, forests, and houses. Recent, de-tailed topographic maps based on airborne la-ser surveys show that the region has no fewerthan seven scarps in two urban fault zones.Four of these scarps have histories of postgla-cial earthquakes that have been confirmed bytrenching and radiocarbon dating (Wilson etal., 1979; Nelson et al., 2002) (Figs. 2B, 2C).Here, as an example, we show that near-surface rupture produced Holocene scarpsalong a geophysical lineament that bounds anarea of shoreline uplift and subsidence andprojects through the city of Tacoma.*E-mail: [email protected] faults in the Puget Sound re-gion accommodate contraction of westernWashington between the Coast Ranges ofOregon and British Columbia (Wells et al.,1998) (Fig. 1). The contraction has producedeast-trending uplifts, basins, and associated re-verse faults that traverse Puget Sound (Fig.2B). The faults, which separate areas of coast-al uplift and subsidence (Fig. 2C), were firstidentified from geophysical lineaments (Daneset al., 1965). New geophysical data (Brocheret al., 2001) confirm that one fault zone passesbeneath Seattle and another beneath Tacoma(Figs. 2D–2F). The Seattle fault zone marksthe northern edge of an area of crustal uplift,the Seattle uplift, whereas the Tacoma faultzone bounds this uplift on the south (Pratt etal., 1997; Johnson et al., 1999; Brocher et al.,2001; Blakely et al., 2002). The two zonesprobably have similar structural relief—asmuch as 10 km since the Eocene for the Seat-tle fault—because they coincide with geo-physical anomalies of similar amplitude.TACOMA FAULT ZONE GEOPHYSICSThe Tacoma fault zone consists of severalgeophysical lineaments