The surface of the earth moves under many conditions, not just during an earthquake. Depending on what a seismologist wants to measure, he or she must pick the proper instrument for the job. Normally to measure earthquakes, seismologists use a detection device called a seismometer.
Seismometers measure ground motion relative to a fixed point. They can measure the solid Earth tide, which is the effect of the continents bending and Earth’s gravity changing as the sun and moon pull on the planet. Seismometers can measure microseisms, ground motions caused by ocean waves moving within the ocean and hitting the beaches of Earth’s coastline. They have been used to monitor nuclear explosions and can detect minor nearby movements such as cars driving by, horses running, and even the wind moving trees.
Riding your bike down a hill might affect one seismometer, but not all of them. However, a big earthquake (over magnitude 5) can be detected by almost every seismometer on the planet! Many things cause the earth to move with different frequencies and speeds. However, if more than one seismometer measures the same thing over a short amount of time, researchers normally conclude that it was a natural phenomenon such as an earthquake.
By looking at the frequency of the signals from the seismometers, as well as the signal’s power and timing, seismologists are able to tell earthquakes from other sources of ground movements. In fact, by looking at multiple recordings from many different seismometers, scientists can understand the depth, location, and size of the earthquake.
--Jose Otero, graduate student, Institute of Geophysics and Planetary Physics
To learn more about how scientists are getting ready for earthquakes, read our story “A Step Ahead of the Big One”