The device used to measure earthquakes is called a seismometer or seismograph. These instruments detect and record the vibrations caused by seismic waves as they travel through the Earth. Seismometers typically consist of a mass that remains stationary while the ground moves during an earthquake, allowing the device to record the amplitude and frequency of the seismic waves on a graph. This information is crucial for understanding earthquake characteristics, predicting their potential impacts, and improving safety measures in earthquake-prone regions.