Seismic instruments are designed to measure vibrations in the Earth. Earthquakes are just one of many things that cause these vibrations: Cars on the road.
Wind blowing through trees. Water flowing in rivers. These all cause the ground to vibrate a small amount.
Trained data analysts can distinguish larger sources of vibrations from others based on their distinct signals.
Watch the video to the right to learn how seismometers work.
Glacial Calving Event