Strong-motion sensors are accelerometers, and are designed to measure the large amplitude, high-frequency seismic waves typical of large local earthquakes. These seismic waves result in the strong ground motion we feel during a large earthquake. Strong ground motion is often to blame for the structural damage that occurs during an earthquake. The data seismologists record with strong motion sensors is used to improve the design of earthquake resistant buildings and to understand earthquake-induced geologic hazards like liquefaction and landslides. The range of motions of interest for strong-motion applications includes accelerations from 0.001 to 2 g and frequencies from 0 to 100 Hz or more.