THE OLYMPIA STRUCTURE: RAMP OR DISCONTINUITY? NEW GRAVITY DATA PROVIDE MORE INFORMATION

Detailed gravity surveys were conducted over a large NW trending gravity and aeromagnetic anomaly in the southern Puget Sound region. The geophysical anomaly, called the Olympia structure, represents the faulting or folding of Eocene bedrock. It is similar and parallel to other geophysical anomalies in the region suspected or identified as faults, and is perpendicular to a convergent boundary. Evidence in the soil record of coseismic subsidence and ground shaking in an area roughly coincident with the Olympia structure suggests a large earthquake 1100 yr B.P. (Sherrod, GSA Bull., 13(10), 2001). This supports a Holocene fault model for the Olympia Structure. Another model suggests the structure is the expression of uplift caused by thrust faulting south of the Black Hills (Pratt, et al., JGR, 102(B12), 1997).

Gravity data were collected on three traverses orthogonal to strike of the Olympia structure. Two traverses coincide with ground magnetic data we previously collected. The traverses are 10, 14, and 17km long, and are spread 35km along the structure. Station spacing is 200m. The data were terrain corrected, and a complete Bouguer anomaly map was produced by combining these with preexisting regional data. A 90+ mGal anomaly extends from the Black Hills into the Tacoma Basin (~35km). The new data provide detail about the slope of the anomaly, especially near its junction with a geophysical anomaly paralleling Hood Canal. The data show the Olympia structure is symmetrical along strike.

Gravity and magnetic data were modeled simultaneously using commercial 2-3/4D modeling software. Gridded regional gravity and aeromagnetic data were first modeled along extended profiles, incorporating depth to basement values determined from seismic reflection data (Pratt, et al., 1997). The detailed gravity and ground magnetic survey data were then incorporated into the models, using the regional models as starting points. The gravity data alone do not capture apparent complexities in the basement revealed in the magnetic data. The magnetic data provide some constraints for modeling, but multiple geologic scenarios can still be modeled which may or may not include faulting. Further data is required to determine the nature of the Olympia structure and any implications the structure may pose for seismic risk.