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Fieldwork

Mapping Southern Puget Sound Delta Fronts After the 2001 Nisqually Earthquake


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(summarized from an article submitted to EOS)

A magnitude 6.8 earthquake struck southern Puget Sound (see map below) on February 28th, 2001, causing an estimated $0.7 to $1.4 billion in damages to the surrounding area. The epicenter was close to the Nisqually delta, one of three major deltas in southern Puget Sound. Although the Nisqually delta is a wildlife refuge, both the Duwamish delta in Seattle and the Puyallup delta in Tacoma have extensive infrastructure, including major port facilities, at the delta edges. Teams of USGS scientists inspected the area immediately after the earthquake and reported damage in the Ports of Seattle and Tacoma, as well as presence of "mud plumes" in the waters of Puget Sound. A joint NOAA/USGS cruise was rapidly assembled in March 2001 to map bathymetry of the delta fronts using the high-resolution multibeam systems of the NOAA ship Rainier. Results showed a variety of submarine failures on the Puyallup and Duwamish delta fronts that may be related to the earthquake.

Mapping Areas: Map of southern Puget Sound showing epicenter of the 2001 Nisqually earthquake (yellow circle) and the deltas mapped in March 2001 (red boxes).
map of southern Puget Sound

The Rainier carries four survey launches equipped with multibeam systems and two launches equipped with single-beam echo sounders. In addition, the Rainier has a hull-mounted multibeam system. The ship and all the launches have 4-axis motion (pitch, roll, yaw, and heave) sensors and were navigated with DGPS (differential global positioning system)-aided inertial navigation. All the data were processed aboard the Rainier in near real time.

The best available pre-earthquake bathymetry for all three deltas comes from a NOAA compilation of hydrographic-quality single-beam echo-sounder data (http://sposerver.nos.noaa.gov/bathy/) with approximately 30-m spatial resolution. The data were collected from 1994 to 1999 (Nisqually delta), 1972 to 1982 (Puyallup delta), and 1978 to 1992 (Duwamish delta). A 30-m digital terrain model (DTM) for each of the three deltas and adjacent basins was generated from these data for comparison with the new data. The new data for each delta area were gridded at 1-m spatial resolution for water depths of 1 to 50 m, 2-m resolution for depths between 50 and 100 m, and 4-m resolution for water depths greater than 100 m to create DTMs. The legacy DTM and the corresponding new DTM were then inspected for various features that might be related to earthquake-induced failures. Curiously, although the Nisqually delta is within a few kilometers of the earthquake epicenter, no failures are found at 1-m spatial resolution along the delta front.

The Puyallup delta is about 20 km northeast of the Nisqually earthquake epicenter. Numerous fresh-looking small slumps and failures are found in the new bathymetric data along the upper edge of the delta front, especially in the southern half. A linear string of expulsion pits (as much as 50 m in diameter and 0.5 m deep), as well as a head scarp of a large incipient failure, are found in the northern part of the delta front ("D" on multibeam images below). Whether the Nisqually earthquake caused the expulsion pits and head scarp is problematic because the resolution of the legacy data is too low to resolve such small features for comparison. However, these types of features are precisely those that severe ground shaking is expected to generate.

oblique shaded-relief view of Puyallup delta
Puyallup delta: Oblique, shaded-relief views of Puyallup delta. Bathymetry is 3-m gridded data; gray-scale land area is 1-m digital orthophoto draped over 10-m USGS DEM (digital elevation model). In the image above, overlapping lobes of sediment creep are numbered from oldest to youngest (1 through 3). "A" is a landslide that destroyed portions of a railroad track and roadway in 1894; "B" is a zone of fresh-looking landslide scars; "C" is a disposal site used from the 1970s to 1990s; "D" is an area of expulsion pits ("e") and head scarp, shown enlarged in the image below.
closer view of expulsion pits and head scarp in Puyallup delta color-coded depth scale for both images, ranging from 0 to 200 meters

The Duwamish delta is about 40 km northeast of the Nisqually earthquake epicenter. Just as with the Puyallup delta, numerous fresh-looking landslides, slumps, and expulsion features were found on the Duwamish delta front, but whether they are related to the Nisqually earthquake is unclear because of poor resolution and the age of the legacy data.

Although we cannot unequivocally determine whether any of the failure features we mapped are genetically related to the 2001 Nisqually earthquake, the expulsion pits, head scarp, and several of the landslides are very fresh in appearance, suggesting their recency. Certainly, with the new high-quality bathymetric data in hand, we will be in a much better position to evaluate earthquake-related failures in these three delta fronts when the next major earthquake strikes the region.


Related Web Sites
Estuarine Bathymetry
National Oceanic and Atmospheric Administration (NOAA)

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