Gravity and aeromagnetic data of western Washington (Finn, et al., 1991; Blakely, et al., 1999) show large-amplitude regional anomalies in the southern Puget Sound area. The anomalies are indicative of a NW-SE trending geologic structure (hereon called the Olympia structure), which is possibly the faulting or folding of Eocene bedrock. Supporting the fault hypothesis, Sherrod (2001) presents soil record evidence of coseismic subsidence and ground shaking in an area roughly coincident with the Olympia structure, suggesting a large earthquake 1100 yr B.P. as a likely cause of the subsidence.

Because the geology of the area is obscured by deposits of Quaternary age, multiple detailed ground magnetic traverses were conducted across the aeromagnetic anomaly. Very high-amplitude, short-wavelength ground magnetic anomalies are superimposed over the longer wavelength regional anomaly seen in the aeromagnetic data. Initial review of the ground-based data indicate some of these features may represent structures extending some distance over strike of the aeromagnetic anomaly. These data, and the results of a detailed gravity survey in the area, may yield important information regarding the true nature of the Olympia structure.