SHIPS - Tacoma Fault and Basin

Hiett, B., 2001, Structure of the Tacoma Basin, M.S. thesis, Univ. Texas El Paso. PDF

Within the highly populated Puget Lowland of western Washington, local faulting and the subduction of the Juan de Fuca plate pose a seismic threat that is compounded by the potential for amplification of ground shaking within basins that underlie the region. Studies of other basins on the west coast of the United States (e.g., in San Francisco and Los Angeles) show that they have the propensity to amplify seismic energy up to 15 times. Although the Pacific Northwest has been relatively aseismic compared to other subduction zones, studies have found that this region has M7.0+ earthquake potential on more than one seismogenic structure. The objective of the Seismic Hazards Investigation in Puget Sound (SHIPS) in March 1998 was to produce a 3-D velocity and structural model of the Puget Lowland to help identify and resolve structures not previously studied. Over 300 data acquisition systems were deployed in the Puget Lowland and recorded airgun shots every 16 seconds for an 18-day period. This study presents the results of tomographic inversion of SHIPS data in and around the Tacoma basin. Inversion of over 140,000 arrivals from 61 recorders produces a velocity model that shows that the Tacoma basin is made up of 3 subbasins that trend northwest-southeast. Assuming velocities of 5.5 km/s as a proxy for the top of the Crescent Formation, depths of the subbasins vary from 4.5 to 6 km. Previous studies had estimated the depth of the Tacoma basin to be 3 to 4.5 km depth. The Tacoma basin may have formed as the result of relief created by thrusting up the Seattle uplift. The subbasins may have originated as pull-apart basins in an Eocene from north-south strike-slip regime.

Johnson, S.Y., R.J. Blakely, W.J. Stephenson, S.V. Dadisman, and M.A. Fisher, 2004, Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland: Tectonics, v. 23, TC1011, doi:10.1029/2003TC001507, 27 pages. PDF

Margin-parallel shortening of the Cascadia forearc is a consequence of oblique subduction of the Juan de Fuca plate beneath North America. Strikeslip, thrust, and oblique crustal faults beneath the densely populated Puget Lowland accommodate much of this north-south compression, resulting in large crustal earthquakes. To better understand this forearc deformation and improve earthquake hazard assessment, we here use seismic reflection surveys, coastal exposures of Pleistocene strata, potential-field data, and airborne laser swath mapping to document and interpret a significant structural boundary near the City of Tacoma. This boundary is a complex structural zone characterized by two distinct segments. The northwest trending, eastern segment, extending from Tacoma to Carr Inlet, is formed by the broad (11.5 km), southwest dipping (11–20 degree) Rosedale monocline. This monocline raises Crescent Formation basement about 2.5 km, resulting in a moderate gravity gradient. We interpret the Rosedale monocline as a fault-bend fold, forming above a deep thrust fault. Within the Rosedale monocline, inferred Quaternary strata thin northward and form a growth triangle that is 4.1 to 6.6 km wide at its base, suggesting 2–3 mm/yr of slip on the underlying thrust. The western section of the >40-km-long, north dipping Tacoma fault, extending from Hood Canal to Carr Inlet, forms the western segment of the Tacoma basin margin. Structural relief on this portion of the basin margin may be several kilometers, resulting in steep gravity and aeromagnetic anomalies. Quaternary structural relief along the Tacoma fault is as much as 350–400 m, indicating a minimum slip rate of about 0.2 mm/yr. The inferred eastern section of the Tacoma fault (east of Carr Inlet) crosses the southern part of the Seattle uplift, has variable geometry along strike, and diminished structural relief. The Tacoma fault is regarded as a north dipping backthrust to the Seattle fault, so that slip on a master thrust fault at depth could result in movement on the Seattle fault, the Tacoma fault, or both.

Sherrod, B.L., A.R. Nelson, H.M. Kelsey, T.M. Brocher, R.J. Blakely, C.S. Weaver, N.K. Rountree, S.B. Rhea, and B.S. Jackson, 2003, The Catfish Lake scarp, Allyn, Washington: Preliminary field data and implications for earthquake hazards posed by the Tacoma fault, U.S. Geol. Surv. Open-File Rept. 03-455, 11 pp., 1 sheet. http://pubs.usgs.gov/of/2003/of03-455.

The Tacoma fault bounds gravity and aeromagnetic anomalies for 50 km across central Puget lowland from Tacoma to western Kitsap County. Tomography implies at least 6 km of post-Eocene uplift to the north of the fault relative to basinal sedimentary rocks to the south.

Coastlines north of the Tacoma fault rose about 1100 years ago during a large earthquake. Abrupt uplift up to several meters caused tidal flats at Lynch Cove, North Bay, and Burley Lagoon to turn into forested wetlands and freshwater marshes. South of the fault at Wollochet Bay, Douglas-fir forests sank into the intertidal zone and changed into saltmarsh. Liquefaction features found beneath the marsh at Burley Lagoon point to strong ground shaking at the time of uplift.

Recent lidar maps of the area southwest of Allyn, Washington revealed a 4 km long scarp, or two closely spaced en-echelon scarps, which correspond closely to the Tacoma fault gravity and aeromagnetic anomalies. The scarp, named the Catfish Lake scarp, is north-side-up, trends east-west, and clearly displace striae left by a Vashon-age glacier. A trench across the scarp exposed evidence for postglacial folding and reverse slip. No organic material for radiocarbon dating was recovered from the trench. However, relationships in the trench suggest that the folding and faulting is postglacial in age.

Sherrod, B.L., T.M. Brocher, C.S. Weaver, R.C. Bucknam, R.J. Blakely, H.M. Kelsey, A.R. Nelson, and R. Haugerud, 2004, Holocene fault scarps near Tacoma, Washington, Geology, 32, 9-12. PDF

Airborne laser mapping confirms that Holocene active faults traverse the Puget Sound metropolitan area, northwestern continental United States. The mapping, which detects forest-floor relief of as little as 15 cm, reveals scarps along geophysical lineaments that separate areas of Holocene uplift and subsidence. Along one such line of scarps, we found that 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 of the Seattle uplift. The northern flank of the Seattle uplift is bounded by a reverse fault beneath Seattle that broke in A.D. 900–930. Observations of tectonic scarps along the Tacoma fault demonstrate that active faulting with associated surface rupture and ground motions pose a significant hazard in the Puget Sound region.

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