SHIPS - Tacoma 2002

[Tacoma SHIPS has been indefinetely postponed due to lack of funding.]

What is Tacoma SHIPS?

Tacoma SHIPS (Seismic Hazard Investigations in Puget Sound) will map the geological structures that can potentially cause damaging earthquakes at the southern end of Puget Lowland, and provide information on the shaking that can be expected during future earthquakes. Tacoma SHIPS will locate the Olympia and Tacoma faults in the subsurface and determine whether they these geological structures are capable of producing large (M7) earthquakes. Both faults may be responsible for abrupt uplift and/or subsidence of marsh tidal flat deposits in southern Puget Lowland about 1100 years ago.

Graphic showing locations of seismic recorders relative to Seattle and Tacoma basins. Also shown is the 2001 Nisqually earthquake in the middle of the Tacoma basin.

Tacoma SHIPS will also map out the subsurface geometry of the basin beneath Tacoma. This basin is a deep depression in the bedrock filled with sedimentary rocks and sediments that are more prone to strong shaking during earthquakes than the surrounding bedrock.

Finally, Tacoma SHIPS will determine the location of the oceanic crust beneath the southern end of the Puget Lowland. The damaging 1949, 1965, and 2001 earthquakes were caused by deformation in this thin slab of old oceanic crust.

Methodology

Tacoma SHIPS will be a detailed seismic refraction and tomography study of the southern Puget Lowland. Approximately 1500 recorders spaced at 200 m (200 yard) intervals will be installed along several lines crossing the Olympia and Tacoma faults and Tacoma basin. Approximately 100 shots will be detonated in boreholes spaced at 2 to 6 mile intervals along each of the recorder lines. The small seismic waves these explosions generate will be used to make a detailed three-dimensional model of the rocks beneath the southern Puget Lowland.

Logistics

This work requires a large (100-person) field crew to survey and site the locations of the instruments, to install the recorders, and detonate the shots. Partners outside of the USGS are essential for the planning, acquisition, and processing of the data to be acquired during Tacoma SHIPS. During the summer of 2002, several temporary positions will be available for surveyors and drill watchers.

Contact: Tom Brocher, (650) 329-4737; U.S. Geological Survey, MS 977, Menlo Park, CA 94025 or

Craig Weaver, (206) 553-0627; U.S. Geological Survey, Geophysics, Box 351650, Univ. of Washington, Seattle, WA 98195