Roger Groom, Mt Tabor Middle School, developed this activity with assistance from Bob Butler, University of Portland, and Shelley Olds and Becca Walker, UNAVCO. This activity allows students to analyze high-precision differential GPS observations of deformation of the North American Plate near its boundary with the oceanic Juan de Fuca Plate. Students analyze the rates of motion of three GPS receiving stations at different distances from the Cascadia subduction zone boundary. Through this activity, students appreciate how GPS monitors the gradual buildup of elastic energy as the continental margin within the Pacific Northwest is compressed by convergence between the North American and Juan de Fuca plates. This slowly accumulating elastic energy will be released in the next Cascadia megathrust earthquake!