What instrument is used to measure seismic activity?

The instrument that measures seismic activity is a seismograph. A seismograph is specifically designed to detect and record the vibrations and movements of the ground caused by seismic waves generated by earthquakes or other geological activities. It operates by using a mass suspended on a spring, which remains stationary as the ground moves during an earthquake, allowing it to record the motion of the Earth.

The other instruments listed have different functions. A barometer measures atmospheric pressure, a thermometer measures temperature, and an altimeter measures altitude or elevation above a specific reference point, usually sea level. Each of these instruments is valuable in its own field—meteorology for the barometer, environmental science for the thermometer, and aviation or mountaineering for the altimeter—but none are capable of detecting or measuring seismic activity. This targeted design of the seismograph is what makes it the correct answer for monitoring and understanding seismic events.