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Q&A February 2009 Q How do you know you are measuring an earthquake and not something else? A 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.  A fiber optic seismometer developed by a Scripps research team. 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 coastlines. 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.  A waveform image from a magnitude 5.4 earthquake that occurred in Mexicali, Calif. on Feb. 8, 2008. The Scripps Anza network recorded the arrival of the earthquakes waves. 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 prepare for earthquakes, read our story |
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Scripps Scientists take on questions from students curious about ocean and earth sciences 
"If more than one seismometer measures the same thing over a short amount of time, we can normally conclude it is a natural phenomenon." |
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