Holocene earthquakes and right-lateral slip on the left-lateral Darrington– Devils Mountain fault zone, northern Puget Sound, Washington Stephen F. Personius1,*, Richard W. Briggs1,*, Alan R. Nelson1,*, Elizabeth R. Schermer2,*, J. Zebulon Maharrey1,3,*, Brian L. Sherrod4,*, Sarah A. Spaulding5,*, and Lee-Ann Bradley1,* 1Geologic Hazards Science Center, U.S. Geological Survey, MS 966, P.O. Box 25046, Denver, Colorado 80225, USA 2Geology Department, Western Washington University, 516 High Street, Bellingham, Washington 98225-9080, USA 3Department of Geology and Geophysics, University of Alaska, Fairbanks, 900 Yukon Drive, P.O. Box 755780, Fairbanks, Alaska 99775, USA 4U.S. Geological Survey at Department of Geological Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA 5U.S. Geological Survey at INSTAAR, 1560 30th Street, Boulder, Colorado 80309, USA ABSTRACT nel margin, and 45–70 cm of north-side-up British Columbia (Fig. 1A; Wells et al., 1998, vertical separation across the fault zone. 2014; Wells and Simpson, 2001; McCaffrey Sources of seismic hazard in the Puget These offsets indicate a net slip vector of 2.3 ± et al., 2007). From south to north, paleo seismic Sound region of northwestern Washington 1.1 m, plunging 14° west on a 286°-strik- and geologic mapping studies have identi- include deep earthquakes associated with ing, 90°-dipping fault plane. The dominant fi ed surface deformation indicating Holocene the Cascadia subduction zone, and shal- right-lateral sense of slip is supported by the activity on the Olympia and Tacoma faults in low earthquakes associated with some of presence of numerous Riedel R shears pre- the Tacoma Basin (Bucknam et al., 1992; Sher- the numerous crustal (upper-plate) faults served in two of our trenches, and probable rod, 2001; Sherrod et al., 2004a, 2004b; Nelson that crisscross the region. Our paleoseismic right-lateral offset of a distinctive bedrock et al., 2008; Barnett et al., 2010; Sherrod and investigations on one of the more prominent fault zone in a third trench. Holocene north- Gomberg, 2014), the Seattle fault in the Seattle crustal faults, the Darrington–Devils Moun- side-up, right-lateral oblique slip is opposite Basin (Bucknam et al., 1999; Johnson et al., tain fault zone, included trenching of fault the south-side-up, left-lateral oblique sense 1999; Sherrod et al., 2000; Nelson et al., 2003a, scarps developed on latest Pleistocene glacial of slip inferred from geologic mapping of 2003b, 2014; Kelsey et al., 2008), the South- sediments and analysis of cores from an adja- Eocene and older rocks along the fault zone. ern Whidbey Island, Utsalady Point, and Dar- cent wetland near Lake Creek, 14 km south- The cause of this slip reversal is unknown but rington–Devils Mountain faults in the Everett east of Mount Vernon, Washington. Trench may be related to clockwise rotation of the Basin (Johnson et al., 1996, 2001, 2004; Drago- excavations revealed evidence of a single Darrington–Devils Mountain fault zone into vich and DeOme, 2006; Sherrod et al., 2008), earthquake, radiocarbon dated to ca. 2 ka, a position more favorable to right-lateral slip and the Boulder Creek, Birch Bay, Sandy Point, but extensive burrowing and root mixing of in the modern N-S compressional stress fi eld. and Drayton Harbor faults in the Bellingham sediments within 50–100 cm of the ground Basin (Kelsey et al., 2012; Sherrod et al., 2013; surface may have destroyed evidence of other INTRODUCTION Sherrod and Gomberg, 2014). Other Holocene earthquakes. Cores in a small wetland adja- faults that lie outside of the margins of the Ter- cent to our trench site provided stratigraphic The Cascadia subduction zone is the domi- tiary basins include the Lake Creek–Boundary evidence (formation of a laterally extensive, nant source of seismic hazard in the Puget Creek (Little River) fault and the Saddle Moun- prograding wedge of hillslope colluvium) of Sound region of northwestern Washington (e.g., tains fault zone, which form parts of the north an earthquake ca. 2 ka, which we interpret Petersen et al., 2008), but additional seismic and east fl anks, respectively, of the uplifted to be the same earthquake documented in sources related to crustal (upper-plate) faults in Olympic Mountains (Wilson et al., 1979; Nel- the trenches. A similar colluvial wedge lower the forearc of the subduction zone have been son et al., 2007; Walsh and Logan, 2007; Witter in the wetland section provides possible evi- identifi ed in the last decade (Fig. 1) through et al., 2008; Blakely et al., 2009; Sherrod and dence for a second earthquake dated to ca. the acquisition and analysis of high-resolution Gomberg, 2014). Shallow seismicity is asso- 8 ka. Three-dimensional trenching tech- light detection and ranging (LiDAR) data (e.g., ciated with some of these structures but not niques revealed evidence for 2.2 ± 1.1 m of Harding and Berghoff, 2000; Haugerud et al., others (Fig. 1B). right-lateral offset of a glacial outwash chan- 2003). The E-W to NW-SE strikes of most of We focused our investigation on one of these these faults and the coincidence of some of crustal faults, the Darrington–Devils Mountain *E-mails: Personius— personius@ usgs .gov; these faults with the margins of similarly trend- fault zone, because despite prior work indicat- Briggs— rbriggs@ usgs .gov; Nelson— anelson@ ing Tertiary basins and uplifts are thought to ing the likelihood of Holocene displacement, usgs .gov; Schermer— schermer@geol .wwu .edu; Maharrey— jzmaharrey@ alaska .edu; Sherrod— refl ect the N-S compression caused by north- the late Quaternary paleoseismic history of this bsherrod@ usgs .gov; Spaulding— sspaulding@ usgs ward transport of the Oregon block against prominent structure was heretofore unknown. .gov; Bradley— bradley@ usgs .gov. a backstop of quasi-stable crust in southern As currently mapped, the Darrington–Devils Geosphere; December 2014; v. 10; no. 6; p. 1482–1500; doi:10.1130/GES01067.1; 12 fi gures; 1 table; 10 supplemental fi les. Received 28 April 2014 ♦ Revision received 19 August 2014 ♦ Accepted 28 August 2014 ♦ Published online 12 November 2014 1482 For permission to copy, contact [email protected] © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1482/3341611/1482.pdf by guest on 01 October 2021 Darrington–Devils Mountain fault zone Figure 1. Tectonic setting of Puget Sound region and Darrington–Devils Mountain fault zone (DDMFZ). (A) Regional tectonic 125.0°W 123.0°W 121.0°W setting of Pacifi c Northwest modifi ed from 50.0°N block rotation model of Wells et al. (1998). Abbreviations: JDF—Juan de Fuca plate, B NAM—North American plate, and PAC— Pacific plate; B&R—Basin and Range, NCS—Northern Cascadia, ORC—Oregon Coast Range, SNV—Sierra Nevada, VOL— VVancouverancouver volcanic extensional, and YAK—Yakima fold belt blocks; SAF—San Andreas fault; CSZ—Cascadia subduction zone. Gray VancouverV Island BBBB anc arrows show direction of modeled block ouv CCanadaanada er CF Isl BBCF rotations from McCaffrey et al. (2007). and BBBF DDHFHF U.S.U.S. BF White arrows are plate motion directions SSPFP and velocities (in mm/yr) relative to North F BBellinghamellingham America (Wells et al., 1998; McCaffrey et al., 2007). Location of “backstop” but- PPacific Ocean MMountount a VVictoriaictoria WWhidbeyhidbey VVernonernon c i tress and location of northern margin of f i IIslandsland c FFigs.igs. 2,2, 3 Northern Cascadia forearc block are from DDDMFZ DM FZ O DDarringtonarrington Kelsey et al. (2012). (B) Locations of Holo- UUPFP c F e cene-active (red) and other (black) Qua- a n EBEB LLCFCF SSWFW ternary faults, margins of Tertiary forearc F EEverettverett basins, and shallow (<25 km) seismicity in 48.0°N CCascade Range a s Puget Lowland of northwest Washington. c OOlympiclympic a KAKA d Fault traces are modifi ed from Barnett et al. MountainsMountains e (2010), Kelsey et al. (2012), and Quaternary R a SSFF SBSB n Fault and Fold Database of the United States ““backstop”backstop” g e (U.S. Geological Survey, 2013). Outlines A aarearea SSeattleeattle of Tertiary basins (heavy pink lines) are ofof B SUSU modifi ed from Brocher et al. (2001), Kelsey NNAMAM TTFF 4433 TTacomaacoma et al. (2012), and Mace and Keranen (2012). NCSNCS OFOF Abbreviations: BB—Bellingham, EB— CCSZ 446°N6°N S TBTB Everett, SB—Seattle, and TB—Tacoma Z YAKYAK JDFJ basins; KA—Kingston Arch, SU—Seattle D OOlympialympia F F M uplift; BBF—Birch Bay, BCF—Boulder SSMF Creek, DHF—Drayton Harbor, LCF— ORCORC VOLVOL Lake Creek, OF—Olympia, SF—Seattle, 3366 BB&R&R SMF—Saddle Mountain, SPF—Sandy 442°N2°N WashingtonWashington Point, SWF—South Whidbey Island, TF— Tacoma, and UPF—Utsalady Point faults; SNVSNV 4488 DDMFZ—Darrington–Devils Mountain SSAF 0 2 20000 A kkmm fault zone. Seismicity (orange circles) from PACPAC F 1124°W24°W 1120°W20°W 0 km 50 ANSS (Advanced National Seismic System) OregonOregon catalog (M) ≥3.0, depth ≤25 km, 1900–2013 46.0°N (http:// earthquake .usgs .gov /earthquakes /search/, accessed 07 September 2013). Mountain fault zone extends >125 km roughly and Easton metamorphic suite) to the north crops of Eocene intrusive and extrusive igneous E-W across the northern Puget Sound region, (Fig. 2; Whetten et al., 1988; Tabor, 1994; John- rocks are localized along the Darrington–Devils from near Victoria, British Columbia, east- son et al., 1996; Tabor et al., 2002; Dragovich Mountain fault zone and other structures in the ward to south of Darrington, Washington, in and DeOme, 2006). In our area of investigation, area (Dragovich and DeOme, 2006). The Dar- the foothills of the North Cascades (Fig. 1B). the Darrington–Devils Mountain fault zone is rington–Devils Mountain fault zone has had a The Darrington–Devils Mountain fault zone coincident with the northern margin of the Ter- long (~50 m.y.) and complicated slip history, is thought to have formed in the early Eocene, tiary Everett Basin, and it juxtaposes a thick which may have included an early period of and it juxtaposes Mesozoic accreted terranes section of Eocene to Oligocene continental right-lateral oblique slip (Tabor, 1994).