The Seattle Fault is a zone of multiple shallow east-west thrust faults that cross the Puget Sound Lowland and through Seattle (in the U.S. state of Washington) in the vicinity of Interstate Highway 90. The Seattle Fault was first recognized as a significant seismic hazard in 1992 when a set of reports showed that about 1,100 years ago it was the scene of a major earthquake of about magnitude 7, an event that entered Indian oral legend. Extensive research has since shown the Seattle Fault to be part of a regional system of faults.
First suspected from mapping of gravitational anomalies in 1965 and an uplifted marine terrace at Restoration Point (foreground in picture above), the Seattle Fault's existence and likely hazard was definitely established by a set of five reports published in Science in 1992. These reports looked at the timing of abrupt uplift and subsidence around Restoration Point and Alki Point (distant right side of picture), tsunami deposits on Puget Sound, turbidity in lake paleosediments, rock avalanches, and multiple landslides around Lake Washington, and determined that all these happened about 1100 years ago (between A.D. 900–930), and most likely due to an earthquake of magnitude 7 or greater on the Seattle Fault.
Although the A.D. 900–930 earthquake was over a thousand years ago, local native legends have preserved an association of a powerful supernatural spirit – a'yahos, noted for shaking, rushes of water, and landsliding – with five locales along the trace of the Seattle Fault, including a "spirit boulder" near the Fauntleroy ferry dock in West Seattle.
The Seattle Fault is the structural boundary where 50–60 millions of years old (early Tertiary) basalt of the Crescent Formation on the south has been uplifted – the Seattle Uplift – and is tipping into the Seattle Basin, where the Tertiary bedrock is buried under at least 7 km (4.3 miles) of relatively softer, lighter sedimentary strata of the younger Blakeley and Blakely Harbor formations. This has resulted in a 4 to 7 km (2.5 to 4.3 miles) wide zone of complex faulting, with three or more main south-dipping thrust faults. Most of the faulting is "blind" (not reaching the surface), and generally difficult to locate because of the generally heavy vegetation or development. Three principal strands have been identified, their location determined by high-resolution seismic reflection and aeromagnetic surveys. The northernmost strand lies nearly along Interstate 90 and then under Lake Sammamish. The central section of the fault zone – where it crosses the apparent location of the Olympic-Wallowa Lineament – shows marked variation in the location of the strands and of the underlying structure, but the nature and significance of this is not understood.
The fault extends for approximately 70 km (43 miles) from near Fall City on the east, where it appears to be terminated by the South Whidbey Island Fault, to Hood Canal on the west (not shown on the map). where matters are as yet unclear (see discussion at Puget Sound faults#Question of western termination). It is the northern edge of the Seattle Uplift, of which the Tacoma Fault is the southern edge. One model has the Seattle and Tacoma faults converging at depth to form a wedge, which is being popped up by approximately north–south oriented compression that ultimately derives from plate tectonics. Another model (see diagram) interprets the Seattle Uplift as a sheet of rock that is being forced up a ramp. Subsequent work suggests that the structure of the Seattle Fault may vary from east to west, with both models being applicable in different sections. A later model has part of the north-thrusting sheet forming a wedge between the sedimentary formations of the Seattle Basin and the underlying bedrock.
The Seattle Fault is believed to date from about 40 million years ago (late Eocene). This is about the time that the strike-slip movement on the north-striking Straight Creek Fault to the east ceased, due to the intrusions of plutons. It appears that when the Straight Creek Fault became stuck the north–south compressive force that it had accommodated by strike-slip motion was transferred to the crust of the Puget Lowland, which subsequently folded and faulted, and the various blocks jammed over one another.
Other scarps associated with the Seattle fault have been identified by LIDAR-based mapping; trenching has generally shown the faulting to be more complex than was first realized. Many of the details of the Seattle Fault, including recurrence rate, remain to be resolved. A study of sediments in Lake Washington found evidence of seven large (M > 7) earthquakes in the last 3500 years.
Surface scarps due to faulting are rarely observed in this area (due to topography, vegetation, and urbanization); a rare exception can be seen at Mee Kwa Mooks Park south of Alki Point. The is the site of the West Seattle Fault; the prominent rise there is due to uplift on the north side of the fault.
The Seattle Fault (and the related Tacoma Fault) is not the only source of earthquake hazard in the Puget Lowland. Other faults in the near surface continental crust, such as the South Whidbey Island Fault (near Everett), and the yet to be studied Olympia Fault (near Olympia), though historically quiescent, are suspected of generating earthquakes of around magnitude 7. Others, such as the 2001 Nisqually earthquake, originate about 50 to 60 km (31 to 37 miles) below Puget Sound in the Benioff zone of the subducting Juan de Fuca Plate; being so deep their energy is dissipated. And there are the infrequent but very powerful great subduction events, such as the magnitude 9 1700 Cascadia earthquake, where the entire Cascadia subduction zone, from Cape Mendocino to Vancouver Island, slips.
But the Seattle and Tacoma faults are probably the most serious earthquake threat to the populous Seattle–Tacoma area. A 2002 study of bridge vulnerability estimated that a magnitude 7 earthquake on the Seattle Fault would damage approximately 80 bridges in the Seattle–Tacoma area, whereas a magnitude 9 subduction event would damage only around 87 bridges in all of Western Washington. The same study also found that with failure of just six bridges (the minimum damage for a Benioff M 6.5 event) there could be at least $3 billion lost in business revenue alone. Subsequent retrofitting by the Washington Department of Transportation and the City of Seattle would likely reduce damage to key bridges. But there is concern that such an earthquake on the Seattle Fault would devastate unreinforced masonry (URM) buildings, of which the City of Seattle is estimated to have around a thousand, concentrated in Capitol Hill, Pioneer Square, and the International District.
Other recent work indicates that the Seattle Fault can generate two types of earthquakes; both pose "considerable hazard" to the Seattle metropolitan region. The A.D. 900–930 earthquake is believed to be the only instance in the past 7,000 years of the type that causes a regional uplift. The other type is more localized and shallower (and therefore more damaging); at least four such events are believed to have occurred in the past 3,000 years on the west end of the fault. (The history of the central and eastern segments is not known.)
Calculations based on fault length and paleoseismological studies show that the Seattle Fault can generate a very damaging magnitude 7.0 earthquake. In addition to extensive damage to unreinforced structures and structures built on fill (such as much of Seattle's Pioneer Square area, the industrial area, and the waterfront), computer modeling has shown that such earthquakes could cause a tsunami of about 2 m (6 feet 7 inches) high on Elliott Bay. The modeling shows that such a tsunami would also inundate the industrial areas on Commencement Bay 30 miles south (Tacoma) and low-lying areas on the Puyallup River delta. There is also concern that a severe or prolonged event could cause failure of the Duwamish or Puyallup River deltas, where the main port facilities for Seattle and Tacoma are located (Harbor Island and Commencement Bay).
- Danes & others 1965.
- Bucknam & others 1992.
- Atwater & Moore 1992.
- Karlin & Abella 1992.
- Schuster & others 1992.
- Jacoby & others 1992.
- Atwater 1999.
- Ludwin & others 2005; Buerge 1985.
- Excerpt from DGER Geological Map GM-52
- Pratt & others 1997, p.27,471; Johnson & others 1994, p.72; Nelson & others 2003, p.1389.
- QFFDB Fault 570
- Johnson & others 1999.
- Blakely & others 2002.
- Blakely & others 2002, p.170.
- Blakely & others 2002, p.170; QFFDB Fault 570.
- Dragovich & others 2009, GM-73.
- Blakely & others 2009, p.118; Liberty 2009.
- Brocher & others 2001, p.13,558.
- Pratt & others 1997, §4.3 and figure 2.
- Johnson & others 2004, §72.
- Kelsey & others 2008. See also Nelson & others 2003.
- Pratt & others 1997, p.27,481; Blakely & others 2002, p.169.
- Vance & Miller 1994; Tabor & others 1984, pp.26, 43.
- Haugerud & others 2003.
- Nelson & others 2003, p.1389.
- Karlin & Abella 1992, p.1619; Karlin & others 2004.
- Kelsey & others 2008, p.1588; Troost & Booth 2004.
- Bucknam & others 1992, p. 1611; Fisher & others 2005, p. 8.
- Ballantyne & others 2002, p.9.
- The much greater energy of subduction earthquake is spread out over a larger area, and concentrated mainly on the coast, where there is less development.
- Ballantyne & others 2002, p.11.
- Swanson & Findlay 2007, "Executive Summary", §2.1.3, and figure 2.
- Kelsey & others 2008, p. 1596.
- Bucknam & others 1992, p. 1613.
- Koshimura & Mofjeld 2001, p. 872.
- Walsh & others 2009.
- Gonzalez & others 2003, §3; Kayen & Barnhardt 2007.
- Anderson, M. L.; Dragovich, J. D.; Blakely, R. J.; Wells, R.; Brocher, T. M. (2008), Where Does the Seattle Fault End? Structural Links and Kinematic Implications [Abstract #T23B-2022] (abstract #T23B-2022), American Geophysical Union Fall Meeting 2008
- Atwater, B. F. (1999), "Radiocarbon dating of a Seattle earthquake to A.D. 900–930.", Seismological Research Letters 10: 232
- Atwater, B. F.; Moore, A. L. (4 December 1992), "A Tsunami About 1000 Years Ago in Puget Sound, Washington", Science 258: 1614–1617, Bibcode:1992Sci...258.1614A, doi:10.1126/science.258.5088.1614
- Ballantyne, D.; Pierepiekarz, M.; Chang, S. (2002), Seismic Vulnerability Assessment of the Seattle–Tacoma Highway Corridor using HAZUS
- Blakely, R. J.; Wells, R. E.; Weaver, C. S.; Johnson, S. Y. (February 2002), "Location, structure, and seismicity of the Seattle fault zone, Washington: Evidence from aeromagnetic anomalies, geologic mapping, and seismic-reflection data", Geological Society of America Bulletin 114 (2): 169–177, Bibcode:2002GSAB..114..169B, doi:10.1130/0016-7606(2002)114<0169:LSASOT>2.0.CO;2
- Blakely, R. J.; Sherrod, B. L.; Hughes, J. F.; Anderson, M. L.; Wells, R. E.; Weaver, C. S. (2009), "The Saddle Mountain Fault Deformation Zone, Olympic Peninsula, Washington: Western Boundary of the Seattle Uplift", Geosphere 5 (2): 105–125, doi:10.1130/GES00196.1
- Brocher, T. M.; Parsons, T.; Blakely, R. J.; Christensen, N. I.; Fisher, M. A.; Wells, R. E.; the SHIPS Working Group (10 July 2001), "Upper crustal structure in Puget Lowland, Washington: Results from the 1998 Seismic Hazards Investigation in Puget Sound", Journal of Geophysical Research 106 (B7): 13,541–13,564, Bibcode:2001JGR...10613541B, doi:10.1029/2001JB000154
- Brown, E. H.; Dragovich, J. D. (December 2003), "Tectonic elements and evolution of northwest Washington", Washington Division of Geology and Earth Resources, Geologic Map GM–52, 1 sheet, scale 1:625,000, 12 p. text
- Bucknam, R. C.; Hemphill-Haley, E.; Leopold, E. B. (4 December 1992), "Abrupt Uplift Within the Past 1700 Years at Southern Puget Sound, Washington", Science 258: 1611–1614, Bibcode:1992Sci...258.1611B, doi:10.1126/science.258.5088.1611
- Buerge, D. M. (March 6–13, 1985), "Lost Seattle, our shameful neglect of a rich archeological past", Seattle Weekly
- Daneš, Z. F.; Bonno, M. M.; Brau, E.; Gilham, W. D.; Hoffman, T. T.; Johansen, D.; Jones, M. H.; Malfait, B.; Masten, J.; Teague, G. O. (15 November 1965), "Geophysical investigation of the Southern Puget Sound area, Washington", Journal of Geophysical Research 70 (22): 5573–5580, Bibcode:1965JGR....70.5573D, doi:10.1029/JZ070i022p05573
- Dragovich, J. D.; Walsh, T. J.; Anderson, M. L.; Hartog, R.; DuFrane, S. A.; Vervoot, J.; Williams, S. A.; Cakir, R.; Stanton, K. D.; Wolff, F. E.; Norman, D. K.; Czajkowski, J. L. (2009), "Geologic map of the North Bend 7.5-minute quadrangle, King County, Washington", Washington Division of Geology and Earth Resources, Geological Map GM–73, 1 sheet, scale 1:24,000, 39 p. text
- Fisher, M. A.; Hyndman, R. D.; Johnson, S. Y.; Brocher, T. M.; Crosson, R. S.; Wells, R. A.; Calvert, A. J.; ten Brink, U. S. (2005), "Crustal Structure and Earthquake Hazards of the Subduction Zone in Southwestern British Columbia and Western Washington", U. S. Geological Survey, Professional Paper 1661-C
- González, F. I.; Sherrod, B. L.; Atwater, B. F.; Frankel, A. P.; Palmer, S. P.; Holmes, M. L.; Karlin, R. E.; Jaﬀe, B. E.; Titov, V. V.; Mofjeld, H. O.; Venturato, A. J. (June 2003), "Puget Sound Tsunami Sources – 2002 Workshop Report", NOAA
- Haugerud, R. A.; Harding, D. J.; Johnson, S. Y.; Harless, J. L.; Weaver, C. S.; Sherrod, B. L. (June 2003), "High-Resolution Lidar Topography of the Puget Lowland, Washington – A Bonanza for Earth Science", GSA Today: 4–10
- Jacoby, G. C.; Williams, P. L.; Buckley, B. M. (4 December 1992), "Tree Ring Correlation Between Prehistoric Landslides and Abrupt Tectonic Events in Seattle, Washington", Science 258: 1621–1623, Bibcode:1992Sci...258.1621J, doi:10.1126/science.258.5088.1621, PMID 17742529
- Johnson, S. Y.; Blakely, R. J.; Stephenson, W. J.; Dadisman, S. V.; Fisher, M. A. (2004), "Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland", Tectonics 23 (TC1011): 1–27, Bibcode:2004Tecto..23.1011J, doi:10.1029/2003TC001507
- Johnson, S. Y.; Blakely, R. J.; Brocher, T. M.; Bucknam, R. C.; Haeussler, P. J.; Pratt, T. L.; Nelson, A. R.; Sherrod, B. L.; Wells, R.E.; Lidke, D.J.; Harding, D.J.; Kelsey, H.M. (2004), "Fault number 570, Seattle fault zone", in U.S. Geological Survey, Quaternary fault and fold database for the United States
- Johnson, S. Y.; Dadisman, S. V.; Childs, J. R.; Stanley, W. D. (July 1999), "Active Tectonics of the Seattle Fault and Central Puget Sound, Washington: Implications for earthquake hazards", Geological Society of America Bulletin 111 (7): 1042–1053, Bibcode:1999GSAB..111.1042J, doi:10.1130/0016-7606(1999)111<1042:ATOTSF>2.3.CO;2
- Johnson, S. Y.; Potter, C. J.; Armentrout, J. M. (January 1994), "Origin and evolution of the Seattle Fault and Seattle Basin, Washington", Geology 22 (1): 71–74, Bibcode:1994Geo....22...71J, doi:10.1130/0091-7613(1994)022<0071:OAEOTS>2.3.CO;2
- Karlin, R. E.; Abella, S. E. B. (4 December 1992), "Paleoearthquakes in the Puget Sound Region Recorded in Sediments from Lake Washington, U.S.A", Science 258: 1617–1620, Bibcode:1992Sci...258.1617K, doi:10.1126/science.258.5088.1617
- Karlin, R. E.; Holmes, M.; Abella, S. E. B.; Sylvester, R. (February 2004), "Holocene landslides and a 3500-year record of Pacific Northwest earthquakes from sediments in Lake Washington", Geological Society of America Bulletin 116 (1–2): 94–108, Bibcode:2004GSAB..116...94K, doi:10.1130/B25158.1
- Kayen, R. E.; Barnhardt, W. A. (2007), "Seismic stability of the Duwamish River delta, Seattle, Washington", U. S. Geological Survey, Professional Paper 1661-E
- Kelsey, H. M.; Sherrod, B. L.; Nelson, A. R.; Brocher, T. M. (November–December 2008), "Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone.", Geological Society of America Bulletin 120 (11/12): 1581–1597, Bibcode:2008GSAB..120.1581K, doi:10.1130/B26282.1
- Koshimura, S.; Mofjeld, H. O. (2001), "Inundation modeling of local tsunamis in Puget Sound, Washington, due to potential earthquakes", ITS 2001 Proceedings (Session 7, Number 7–18): 861–873
- Liberty, L. M. (2009), "The western extension of the Seattle fault: new insights from seismic reflection data", U.S. Geological Service, NEHRP, Project Award Number 08HQGR0075
- Ludwin, R. S.; Thrush, C. P.; Buerge, D.; Jonientz-Trisler, C.; Rasmussen, J.; Troost, K.; de los Angeles, A. (July/August 2005), "Serpent Spirit-power Stories along the Seattle Fault", Seismological Research Letters 76 (4): 426–431, doi:10.1785/gssrl.76.4.426
- Nelson, A. R.; Johnson, S. Y.; Kelsey, H. M.; Wells, R. E.; Sherrod, B. L.; Pezzopane, S. K.; Bradley, L.; Koehler, R. D.; Bucknam, R. C. (November 2003), "Late Holocene earthquakes on the Toe Jam Hill fault, Seattle fault zone, Bainbridge Island, Washington", Geological Society of America Bulletin 115 (11): 1368–1403, Bibcode:2003GSAB..115.1388N, doi:10.1130/B25262.1
- Pratt, T. L.; Johnson, S. Y.; Potter, C. J.; Stephenson, W. J. (10 December 1997), "Seismic reflection images beneath Puget Sound, western Washington State: The Puget Lowland thrust sheet hypothesis", Journal of Geophysical Research 102 (B12): 27,469–27,490, Bibcode:1997JGR...10227469P, doi:10.1029/97JB01830
- Schuster, R. L.; Logan, R. L.; Pringle, P. T. (4 December 1992), "Prehistoric Rock Avalanches in the Olympic Mountains, Washington", Science 258: 1620–1621, Bibcode:1992Sci...258.1620S, doi:10.1126/science.258.5088.1620
- Swanson, D. B.; Findlay, A. (December 2007), "City of Seattle Unreinforced Masonry Building Seismic Hazards Study", City of Seattle Department of Planning and Development
- Tabor, R. W.; Frizzell, V. A., Jr.; Vance, J. A.; Naeser, C. W. (January 1984), "Ages and stratigraphy of lower and middle Tertiary sedimentary and volcanic rocks of the central Cascades, Washington: Application to the tectonic history of the Straight Creek fault", Geological Society of America Bulletin 95 (1): 26–44, Bibcode:1984GSAB...95...26T, doi:10.1130/0016-7606(1984)95<26:AASOLA>2.0.CO;2
- Troost, K. G.; Booth, D. B. (May 22, 2004), Geology of Seattle: City of Seattle Field Trip, Seattle-Area Geologic Mapping Project Department of Earth and Space Sciences University of Washington
- Vance, J. A.; Miller, R. B. (1994), "Another look at the Fraser River-Straight Creek Fault (FRSCF) [abstract]", Geological Society of America 1994 Annual Meeting, Abstracts with Programs 24: 88
- Walsh, T. J.; Arcas, D.; Venturato, A. J.; Titov, V. V.; Mofjeld, H. O.; Chamberlin, C. C.; González, F. I. (July 2009), "Tsunami hazard map of Tacoma, Washington—Model results for Seattle fault and Tacoma fault earthquake tsunamis", Washington Division of Geology and Earth Resources, Open File Report 2009-9
- USGS Professional Paper 1560: Assessing Earthquake Hazards and Reducing Risk in the Paciﬁc Northwest Technical, but readable. And has many good pictures.
- USGS: Reducing Earthquake Hazards in the Pacific Northwest
- USGS: Earthquake ground motion movies
- Pinpointing devastation if Seattle Fault ruptures Seattle Times, February 20, 2005.
- Scenario for a Magnitude 6.7 Earthquake on the Seattle Fault Vivid and comprehensive.
- Washington State's Bridge Seismic Retrofit Program
- Earthquake Study: Four Vashon-Specific Scenarios Considerations for all islands.
- Puget Sound Tsunami Inundation Modeling (NOAA)
- Elliott Bay inundation map (DNR)
- Tsunami Hazard Map of the Elliott Bay Area (NOAA)
- Additional inundation maps (DNR Publications)
- Puget Sound Lidar Consortium Use of Lidar for locating faults.