The Rochester and Adna 7.5 minute quadrangles in the Washington forearc of the Cascadia subduction zone encompass the Doty fault, a large forearc fault crossing the I-5 corridor south of Centralia. We have begun a cooperative geological and geophysical study of the area to assess the seismic hazard to a water retention facility that has been proposed to mitigate flooding along the Chehalis River and the I-5 corridor. This region between Olympia and Portland is undergoing north-south compression, clockwise rotation, and regional uplift associated with both subduction processes and the northward migration of the forearc block. Past studies identified multiple faults that strike NW-SE and E-W in the northern and southern parts of the study area, respectively. The Kopiah, Scammon Creek, Salzer Creek and Doty faults all interact within our study area, in ways that are poorly understood. An integrated geophysical investigation will assist the State-Federal cooperative mapping program called STATEMAP efforts to produce detailed 1:24,000 scale geologic maps of the area. Geophysical field work in the summer of 2018 includes a roughly 15 x 32 km gravity grid with ~2 km station spacing. Station spacing along known geologic structures is ~1 km to provide greater resolution. Results from our coarse gravity grid will provide targets for additional high resolution profiles. A high resolution ground magnetic grid also extends across both quadrangles, and preliminary results demonstrate its efficacy at elucidating structure. Seismic profiles acquired by the USGS across the Doty fault will constrain our geophysical modeling, which will combine the high resolution gravity and magnetic profiles in a geologic model of the subsurface to support the mapping efforts of the STATEMAP program. The data and models will provide insight about total offset across these faults, precisely identify locations of faults that are not exposed at the surface, and allow us to better understand the structure of these faults. These interpretations will allow us to more accurately understand the potential seismic risk these faults pose to nearby population centers and infrastructure.