Figure 1. Pacoima Dam. The GPS antennae are the circled white cones, DAM1 on the left abutment, and DAM2 near the center of the dam arch. A third station, CMP9, located several kilometers away on stable ground, serves as a nearby reference point for monitoring dam displacements. The dam is a 113 meter tall concrete arch structure, completed in 1928, that experienced severe shaking in both the 1971 San Fernando and the 1994 Northridge earthquakes.