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Complete Report for Southern Whidbey Island fault zone (Class A) No. 572

Brief Report ||Partial Report

citation for this record: Johnson, S.Y., Blakely, R.J., Brocher, T.M., Sherrod, B.L., Kelsey, H.M., and Lidke, D.J., compilers, 2004, Fault number 572, Southern Whidbey Island fault zone, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 04/24/2014 04:44 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 as wide as 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) , WASHINGTON
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). 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. Approximate locations of fault strands are by S.Y. Johnson and based on available geologic and geophysical data, mainly from Johnson and others(1996 #4751; 2000 #4750).

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.

Comments:

The map shows the minimum fault length (~65 km). At its northwest end, seismic-reflection data indicate the southern Whidbey Island fault zone does not continue west of 123?. The southern Whidbey Island fault zone could continue more than 20 km farther to the southeast along the flanks of a moderate amplitude aeromagnetic anomaly (Blakely and others, 1999 #4747).


Average strike N51°W
Sense of movement Thrust

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°

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. Paleoseismologic investigations of shoreline deposits have been conducted at two sites on southern Whidbey Island that are briefly described below.

Crockett Marsh (572-1) and Hancock Marsh (572-2) sites. Kelsey and Sherrod (2001 #4757) conducted paleoseismologic investigations at these two sites along the northeast strand of the fault zone. The Crockett Marsh site (572-1) is located directly north of the northeastern fault strand of the southern Whidbey Island fault and the Hancock Marsh site (572-2) is located south of that fault strand. Kelsey and Sherrod (2001 #4757) tentatively concluded that an earthquake on the northeastern strand of the fault zone, about 2,900 to 3,400 years ago, caused the salt marsh on the north side of this fault strand to rise about 2.5 m relative to the marsh on the south side.

Geomorphic expression At present, there are no known tectonic landforms associated with the southern Whidbey Island fault zone. Washington's Puget Lowland was occupied at least five times during the Pleistocene by lobes of the continental ice sheet, with the most recent ice retreat occurring about 16 ka (Porter and Swanson, 1998 #6237). Most of the present landscape (including Whidbey Island) reflects this dynamic glacial history (Booth, 1994 #4719) and, as a result, tectonic landforms are generally buried or otherwise obscured.

Age of faulted surficial deposits Onland on Whidbey Island, interglacial deposits of the ~80-125 ka Whidbey Formation near Lagoon Point (Easterbrook, 1968 #4752) are apparently the youngest strata that are clearly cut by faults within the southern Whidbey Island fault zone (Gower and others, 1985 #4725). On offshore seismic-reflection data, inferred Quaternary strata are both folded and faulted by strands of the southern Whidbey Island fault zone (Johnson and others, 1996 #4751; 2000 #4755).
Historic earthquake
Most recent prehistoric deformation Latest Quaternary (<15 ka)

Comments: Based on paleoseismologic investigations, Kelsey and Sherrod (2001 #4757) tentatively concluded that there was an earthquake on the northeastern fault of the southern Whidbey Island fault zone about 2,900 to 3,400 years ago. The earthquake caused the salt marsh on the north side of the fault to abruptly rise ~ 2.5 m relative to the salt marsh on the south side of the fault.

Recurrence interval

Comments: There is insufficient information to infer a recurrence interval for earthquakes on the southern Whidbey Island fault zone.
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) 2004
Samuel Y. Johnson, U.S. Geological Survey
Richard J. Blakely, U.S. Geological Survey
Thomas M. Brocher, U.S. Geological Survey
Brian L. Sherrod, U.S. Geological Survey
David J. Lidke, U.S. Geological Survey
Harvey M. Kelsey, Humboldt State University
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.

#4757 Kelsey, H.M., and Sherrod, B.L., 2001, Investigation of late Holocene fault displacement on the southern Whidbey Island fault zone in the northern Puget lowland: Collaborative Research between Humboldt State University and the U.S. Geological Survey, Earthquake Loss Reduction Products, Cascadia: NEHRP Annual Report, External Grant award number 00HQGR0067, 6 p.

#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.

#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.