US Geological Survey Studies the Ground Beneath Our Feet

Both gravitational and magnetic data is used to describe the underground geology. This illustration is of gravitational readings locating structures in the South Sound. Click the image for a larger view.

A few years ago, the Steamboat Peninsula was visited by a research team from the Geologic Hazards Science Center of the U.S. Geological Survey. These researchers were using equipment to view cross-sections of geologic structures far beneath the ground. This last July, the results of this research, a paper entitled, Shallow geophysical imaging of the Olympia anomaly: An enigmatic structure in the southern Puget Lowland, Washington State, was published.

A significant benefit of this kind of research is to identify areas where stress might build and quickly release in the form of an earthquake. The Puget Sound occupies a seismically active area, located along a line where the Juan de Fuca plate is squeezed under the North America plate.

The convergence of the Juan de Fuca plate, at a rate of ~50 mm/yr (Atwater, 1970; DeMets et al., 1994), has historically produced great (magnitude, M8–9) earthquakes on the Cascadia subduction zone (e.g., Nelson et al., 2006) that pose a primary seismic hazard for the region (Petersen et al., 2002).

But what’s the story, closer to our home here on the Steamboat Peninsula?

Curse you, Mud Bay

The paper describes the difficulty in getting a good look at what’s going on, underground, in our area.

Although numerous marine seismic reflection profiles have been acquired near the surface location of the Olympia structure as defined by potential field anomalies, its tectonic character remains enigmatic, in part because inlets of southern Puget Sound are too shallow for the collection of deep-penetration marine seismic profiles across the geophysical anomalies.

But there are clues in the research to geologic details here in our area. To begin, the paper describes the significant influences on local geology. “The Black Hills are one of the largest uplifted exposures of basement rocks in the area surrounding the Puget Lowland.”

Using Reflection Seismology Along Steamboat Island Road

At it happens, our area marks a boundary between something called the “Olympia structure” and another, the “Tacoma basin.”

The least studied of the known major structures beneath the Puget Lowland is the Olympia structure, the surface projection of which along the northeast flank of the Black Hills is located solely on the basis of potential field interpretations.

Letters A–D represent reference points along aeromagnetic value graph and Steamboat Island Rd. seismic profile. White dashed line represents the surface projected location of the Olympia structure based on regional potential field interpretation.

The Olympia plate isn’t easy to see. But some of its boundary, near us, is marked by a subsidence that may have occurred as a result of an earthquake some 1100 years ago. Researchers who worked along Steamboat Island Road sought to acquire more data about the Olympia structure.

To supplement existing shallow-marine data near the structure, we acquired 14.6 km of land-based seismic reflection data along a profile that extends from Crescent Formation exposed in the Black Hills northward across the projected surface location of the Olympia structure.

There may exist a boundary between the Olympia plate and the Tacoma basin, actually running across our peninsula. The paper includes illustrations and descriptions of this boundary crossing from somewhat south of Elizan Beach, on the Totten side, to somewhat north of Young Cove, on the Eld Inlet side.

Is this boundary an actual fault line? “Overall, seismic and potential-field profiles presented in this study identify only minor shallow faulting within the projected surface location of the Olympia structure.” Previous investigations have not identified a specific fault line, across our peninsula. However, this paper concludes the authors cannot discount that possibility. “We suggest that until there are additional geophysical surveys and geologic mapping across the Olympia structure, this enigmatic feature should be considered a geophysically defined structure rather than a fault.”

The research paper is an interesting glimpse into not only the geology of our area, but also the methods used to study and describe the ground beneath our feet.

All quotes are from the paper by Jack K. Odum and William J. Stephenson, of the Geologic Hazards Science Center, U.S. Geological Survey; and Thomas L. Pratt and Richard J. Blakely, of the U.S. Geological Survey. Click here to download a copy of the research paper.

Posted in earthquake, Nature Notes from the Steamboat Peninsula.