The instrument used to measure earthquakes is called a "seismometer" or "seismograph." This device detects and records the vibrations produced by seismic waves as they travel through the Earth. When an earthquake occurs, the seismometer captures the motion of the ground, allowing scientists to analyze the magnitude, duration, and location of the quake. Seismometers are essential for understanding earthquake patterns, predicting future seismic activity, and improving safety measures in earthquake-prone regions. They contribute to the field of seismology, providing crucial data for both scientific research and disaster preparedness.