Geophysical Investigation of a Geologic Unconformity at Seward Park - Seattle, WA

Abstract

This report presents the results of field and geophysical analyses of a geologic unconformity located within Seattleís Seward Park. The unconformity consists of an apparently abrupt contact between Quaternary Vashon till in the north and Tertiary bedrock to the south. Seward Park is located in the middle of the Seattle Fault Zone (SFZ). The SFZ is an active crustal fault capable of producing Mw7+ earthquakes, triggering tsunamis in Puget Sound, and causing ground deformation. Previous interpretations of regional fault zone imaging indicate the possible presence of a fault strand in Seward Park. Methods of geophysical analysis used in my study include seismic refraction, multichannel analysis of surface waves (MASW), and ground-penetrating radar (GPR). Data results include a compressional velocity (Vp) model, shear velocity (Vs) model, and a radargram. These results were used to interpret the nature of the unconformity at depth. This information was then coupled with surficial geologic observations in order to develop suitable interpretations. The data results depict an abrupt transition in seismic velocities within the measured transects. Three potential geologic mechanisms have been proposed to explain this phenomenon. One scenario, consistent with interpretations from previous studies, is a fault-generated unconformity caused by a south-side-up thrust fault dipping (20-30o) to the south. A second possibility: the unconformity was formed via one or multiple erosional processes. Third, the bedrock was deformed before Quaternary deposition and the contact represents a buttress unconformity. Additional work is required in order to confirm or refute these geologic scenarios. Recommendations for future work include utilizing seismic reflection in order to image this location at greater depth, running longer geophysical arrays, measuring another transect in the middle of the Bailey Peninsula, and conducting borehole analyses or trenching. Limitations of this study include sparse surficial data, depth of geophysical penetration, topographic constraints, and limited survey extent due to time factors.