Geologists use a seismograph to detect and measure the strength of an earthquake. The modern-day seismograph was invented by John Milne in 1880 while he was studying earthquakes in Japan.
A seismograph records the seismic waves that are created during an earthquake. The location and power of an earthquake can be determined by the information recorded on a seismograph.
Developed in 1935 by Charles F. Richter, the Richter scale measures the intensity of an earthquake. A ground-shaking event registering less than a 2.0 is considered minor, while one above 4.5 is considered a major event. The largest earthquake ever recorded was in 1977 in Chile, and it registered a devastating 9.5.