The instrument used to measure earthquake intensity is called a seismometer or seismograph. These devices can detect and record the vibrations caused by seismic waves during an earthquake. Seismometers consist of a mass attached to a fixed base, which allows the mass to move independently when the earth shakes. The movement of the mass produces a trace on a recording medium, illustrating the amplitude and frequency of the seismic waves. This information is crucial for understanding the earthquake's magnitude and intensity, enabling scientists to assess the earthquake's impact and provide data for future preparedness.