southern Whidbey Island fault zone (Class A) No. 572
Last Review Date: 2016-11-29
citation for this record: Johnson, S.Y., Blakely, R.J., Brocher, T.M., Haller, K.M., Barnett, E.A., Sherrod, B.L., Kelsey, H.M., and Lidke, D.J., compilers, 2016, Fault number 572, southern Whidbey Island fault zone, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 05/19/2024 03:48 PM.
| Synopsis |
This northwest-trending fault zone extends more than 65 km across Possession Sound, southern Whidbey Island, Admiralty Inlet into the eastern Strait of Juan de Fuca. The fault zone is up to 5–7 km, correlates with gravity and magnetic anomalies (Finn and others, 1991 #4753; Blakely and others, 1999 #4747), and has been interpreted as a complex zone of transpressional deformation (Johnson and others, 1996 #4751). |
| Name comments |
Gower (1980 #6229) showed and named the "southern Whidbey Island fault," and Gower and others (1985 #4725) showed this fault on their seismotectonic map of the Puget Sound region and briefly outlined its geologic relationships. Wagner and Wiley (1983 #6230) and Wagner and Tomson (1987 #6249) mapped and briefly discussed offshore parts of this fault zone and also used the name "southern Whidbey Island fault." Johnson and others (1996 #4751) described multiple sub-parallel strands and referred to the overall structure as the "southern Whidbey Island fault zone," and this name is also used herein for this zone of faults that crosses the southern part of Whidbey Island. |
| County(s) and State(s) |
ISLAND COUNTY, WASHINGTON JEFFERSON COUNTY, WASHINGTON SNOHOMISH COUNTY, WASHINGTON |
| Physiographic province(s) |
PACIFIC BORDER |
| Reliability of location |
Good Compiled at 1:100,000 scale. Comments: The offshore location of the southern Whidbey Island fault zone is relatively well constrained, based on interpretation of a dense network of industry and high-resolution seismic-reflection profiles (Johnson and others, 1996 #4751; Dadisman and others, 2000 #4748; Johnson and others, 2000 #4750; 2000 #4755; Liberty and Pape, 2007 #7654; Sherrod and others, 2008 #7652). Onshore, strands of the southern Whidbey Island fault zone are generally concealed beneath a cover of dense vegetation and thick Pleistocene glacial and interglacial deposits. Location of fault is from GER_Seismogenic_WGS84 (http://www.dnr.wa.gov/publications/ger_portal_seismogenic_features.zip, downloaded 05/23/2016) attributed to Wagner and others (1987 #6249), Johnson and others (2000 #4750), Polenz and others (2006 #), and Sherrod and others (2008 #7652).
|
| Geologic setting |
The northwest-trending southern Whidbey Island fault zone occurs along a significant terrane boundary between basement blocks underlain by Eocene marine basalts of the Coast Range province to the southwest and pre-Tertiary metamorphic rocks of the Cascades province to the northeast. However, seismic tomography studies (Brocher and others, 2001 #4718) reveal that only the northwestern end of the fault zone in the southeastern Strait of Juan de Fuca is associated with a strong velocity contrast. The southeastern and central parts of the southern Whidbey Island fault zone form the southwest margin of the Everett basin and northeast boundary of the Seattle basin. The northwestern part of the fault zone forms the northeastern limit of the Port Townsend basin (Brocher and others, 2001 #4718).
|
| Length (km) | 64 km. |
| Average strike | N51°W |
| Sense of movement |
Reverse, Left lateral
Comments: Sense of slip deduced largely from geometry of faults imaged on seismic-reflection profiles (Johnson and others, 1996 #4751). These profiles reveal a complex zone of faulting, including a positive flower structure bounded by reverse faults and thrust faults, which probably also had left- and (or) right-lateral components of offset at various times in the past. Faulting appears to have locally uplifted a block(s) within the fault zone. The northeastern fault in the zone is locally a northeast-dipping thrust fault.
|
| Dip |
45–80° NE
Comments: Dip angles deduced largely from geometry of faults imaged on seismic-reflection profiles (Johnson and others, 1996 #4751).
|
| Paleoseismology studies |
Johnson and others (1996 #4751) described the structure and stratigraphy of the southern Whidbey Island fault zone. Excavations across several LiDAR scarps show evidence for multiple post-glacial folding and faulting event on faults with reverse oblique sense of slip. Kelsey and others (2004 #7651) compared sea-level histories at two salt marshes that straddle a northeast strand of the southern Whidbey Island fault zone: Crockett Marsh (site 572-1) located north of the northeastern fault strand is 8 km north of Hancock Marsh (site 572-2), south of the fault strand. Stratigraphy and diatom assemblages of the marsh cores suggest Crockett Marsh underwent a 1–2 m of abrupt uplift relative to sea level at a time that relative sea level remained the same at Hancock Marsh. Radiocarbon ages of macrofossils constrain uplift timing to 2.8–3.2 ka. Kelsey and others (2004 #7651) suggest that the earthquake resulted in 2.5 m uplift of the salt marsh on the north side of the fault strand relative to the marsh on the south side; no fault scarp has been identified between the marshes. Keaton and Perry (2006 #7653) excavated two trenches on the south end of the Brightwater treatment plant (KP1 site 572-3, and KP2 site 572-4). The trenches exposed glacial deposits disrupted by faults and liquefaction features. They conclude the observed features have a glaciotectonic origin and are not seimotectonic. Trenches by Sherrod and others, 2008 #7652) are less than 1 km south of KP1 and KP2 and Sherrod and others (2008 #7652) conclude that deformation in the area is due to surface-rupturing earthquakes. Sherrod and others, 2008 #7652) report results from four trenches located near Crystal Lake: Flying Squirrel trench (572-5), Mountain Beaver trench (572-6), Beef Barley trench (572-7), and French Onion trench (572-8). The Flying Squirrel and Mountain Beaver trenches cross the Cottage Lake lineament. Stratigraphy in the Flying Squirrel trench showed gentle warping of late glacial and post-glacial sediments; no faults were exposed. A low-angle fault, which is not conclusively earthquake related, separates a diamicton from the overlying recessional outwash deposit. Faulting produced warping at the site of greater than 2 m. Radiocarbon ages provide a maximum age for the folding event of 12,090–11,670 cal yr BP. Beef Barley and French Onion trenches cross the Little Bear Creek lineament. Deformed recessional outwash deposits and Holocene deposits were exposed; three unconformities separated the units. One or possibly two of the unconformities are interpreted as event horizons. If folding on the Little Bear Creek lineament resulted in one or two of unconformities, the poorly constrained timing of the earthquakes is younger than 12,000 yr BP and older than about 2,850 cal yr BP. The French Onion trench exposed glaciolacustrine claystones, till, colluvial deposits, and Holocene soils. The trench did expose faults, but it was not possible to conclusively demonstrate offset of Holocene units.
|
| Geomorphic expression |
Early mapping of the three subparallel, northwest trending strands of the southern Whidbey Island fault zone was constrained by borehole data, potential field anomalies, marine seismic reflection surveys. Subtle scarps and topographic lineaments on Pleistocene surfaces are visible on high-resolution LiDAR topography at a number of locations (Sherrod and others, 2008 #7652); the northeast-side-up scarps exhibit 1–5 m of vertical relief
|
| Age of faulted surficial deposits | late glacial and post-glacial sediments (Sherrod and others, 2008 #7652) |
| Historic earthquake | |
| Most recent prehistoric deformation |
latest Quaternary (<15 ka)
Comments: Based on paleoseismologic investigations of Kelsey and Sherrod (2001 #4757) and Sherrod and others (2008 #7652), the southern Whidbey Island fault had at least four earthquakes since deglaciation (about 16.4 k.y. ago) with the most recent younger than 2,700 yr ago.
|
| Recurrence interval |
0.4–9.2 k.y. (<16.4 ka)
Comments: Recurrence intervals for earthquakes on the southern Whidbey Island fault vary; the longest is 9.2–8.8 k.y. and the shortest is 470 yr (Sherrod and others, 2008 #7652). |
| Slip-rate category |
Between 0.2 and 1.0 mm/yr
Comments: Preferred rate is 0.6 mm/yr. There is 150 m of structural relief on an anticlinal fold north of Lagoon Point on Whidbey Island, which exposes 250-ka Double Bluff drift at its core (Easterbrook, 1968 #4752; fig. 13 in Johnson and others, 1996 #4751). The fold overlies a positive flower structure imaged on seismic-reflection data just offshore (fig. 12 in Johnson and others, 1996 #4751). Assuming that fold growth reflects offset on an underlying steep fault, the minimum slip rate for faults in this zone is 0.6 mm/yr. |
| Date and Compiler(s) |
2016
Samuel Y. Johnson, U.S. Geological Survey Richard J. Blakely, U.S. Geological Survey, Emeritus Thomas M. Brocher, U.S. Geological Survey Kathleen M. Haller, U.S. Geological Survey Elizabeth A. Barnett, Shannon & Wilson, Inc. Brian L. Sherrod, U.S. Geological Survey Harvey M. Kelsey, Humboldt State University David J. Lidke, U.S. Geological Survey |
| References |
#4747 Blakely, R.J., Wells, R.E., and Weaver, C.S., 1999, Puget Sound aeromagnetic maps and data: U.S. Geological Survey Open-File Report 99-514. #4719 Booth, D.B., 1994, Glaciofluvial infilling and scour of the Puget Lowland, Washington, during ice-sheet glaciation: Geology, v. 22, p. 695-698. #4718 Brocher, T.M., Parsons, T., Blakely, R.J., Christensen, N.I., Fisher, M.A., Wells, R.E., and SHIPS Working Group, 2001, Upper crustal structure in Puget Lowland, Washington—Results from the 1998 seismic hazards investigation in Puget Sound: Journal of Geophysical Research, v. 106, p. 13,541–13,564. #4748 Dadisman, S.V., Childs, J.R., Johnson, S.Y., and Rhea, S.V., 2000, Data Report for Cruise G#-95-PS, June, 1995, in Mosher, D.C., Johnson, S.Y., Rathwell, G.J., Kung, R.B., and Rhea, S.B., eds., Neotectonics of the eastern Strait of Juan de Fuca; a digital geological and geophysical atlas: Geological Survey of Canada Open File Report 3931, (CD digital product), 1 sheet. #4752 Easterbrook, D.J., 1968, Pleistocene stratigraphy of Island County: Washington Division of Water Resources, Water Supply Bulletin 25, Part I, 34 p. #4753 Finn, C., Phillips, W.M., and Williams, D.L., 1991, Gravity anomaly and terrain maps of Washington: U.S. Geological Survey Geophysical Investigations Map GP-988, scale 1:500,000 and 1:1,000,000. #6229 Gower, H.D., 1980, Bedrock geologic and Quaternary tectonic map of the Port Townsend area, Washington: U.S. Geological Survey Open-File Report 80-1174, 19 p., 1 sheet, scale 1:100,000. #4725 Gower, H.D., Yount, J.C., and Crosson, R.S., 1985, Seismotectonic map of the Puget Sound region, Washington: U.S. Geological Survey Miscellaneous Investigations Map I-1613, scale 1:250,000. #4750 Johnson, S.Y., Mosher, D.C., Dadisman, S.V., Childs, J.R., and Rhea, S., 2000, Tertiary and Quaternary structures of the eastern Strait of Juan de Fuca—Interpreted map, in Mosher, D.C., Johnson, S.Y., Rathwell, G.J., Kung, R.B., and Rhea, S.B., eds., Neotectonics of the eastern Strait of Juan de Fuca; a digital geological and geophysical atlas: Geological Survey of Canada Open File Report 3931 (CD digital product), 1 sheet. #4751 Johnson, S.Y., Potter, C.J., Armentrout, J.M., Miller, J.J., Finn, C., and Weaver, C.S., 1996, The southern Whidbey Island fault—An active structure in the Puget Lowland, Washington: Geological Society of America Bulletin, v. 108, p. 334-354 and oversize insert. #4755 Johnson, S.Y., Rhea, S., and Dadisman, S.V., 2000, Industry seismic-reflection tracklines, in Mosher, D.C., Johnson, S.Y., Rathwell, G.J., Kung, R.B., and Rhea, S.B., eds., Neotectonics of the eastern Strait of Juan de Fuca; a digital geological and geophysical atlas: Geological Survey of Canada Open File Report D3931, (CD digital product), 1 sheet. #7653 Keaton, J.R., and Perry, D.L., 2006, Chemical building seismic trenches, Brightwater wastewater treatment plant site, Snohomish County, Washington: Technical report to the King County Department of Natural Resources and Parks, MACTEC Project 4953-05-3181, 32 p. #7651 Kelsey, H.M., Sherrod, B., Johnson, S.Y., and Dadisman, S.V., 2004, Land-level changes from a late Holocene earthquake in the northern Puget Lowland, Washington: Geology, v. 32, no. 6, p. 469–472, doi:10.1130/G20361.1. #7654 Liberty, L.M., and Pape, K.M., 2007, Seismic characterization of the Seattle and southern Whidbey Island fault zones in the Snoqualmie River Valley, Washington: Technical report to the U.S. Geological Survey under contract number 06HQGR0111, 17 p., http://earthquake.usgs.gov/research/external/reports/06HQGR0111.pdf #7617 Polenz, M., Schasse, H.W., and Petersen, B.B., 2006, Geologic map of the Freeland and northern part of the Hansville 7.5-minute quadrangle, Island County, Washington: Washington Division of Geology and Earth Resources, Geologic Map GM-64, scale 1:24,000. #6237 Porter, S.C., and Swanson, T.W., 1998, Radiocarbon age constraints on rates of advance and retreat of the Puget lobe of the Cordilleran ice sheet during the last glaciation: Quaternary Research, v. 50, p. 205-213. #7652 Sherrod, B.L., Blakely, R.J., Weaver, C.S. Kelsey, H.M. Barnett, E.,Liberty, L. Meagher, K.L., and Pape, K., 2008, Finding concealed active faults—Extending the southern Whidbey Island fault across the Puget Lowland, Washington: Journal of Geophysical Research, v. 113, no. B5, doi:0.1029/2007JB005060. #6249 Wagner, H.C., and Tomson, J.H., 1987, Offshore geology of the Strait of Juan de Fuca, State of Washington and British Columbia, Canada: Washington Division of Geology and Earth Resources Open-File Report 87-1, 16 p. #6230 Wagner, H.C., and Wiley, M.C., 1983, Offshore Quaternary geology of the northern Puget Sound-eastern Strait of Juan de Fuca region, Washington, in Yount, J.C., and Crosson, R.S., eds., Proceedings of workshop XIV, Earthquake hazards of the Puget Sound region, Washington: U.S. Geological Survey Open-File Report 83-19, p. 178-267. |