Scientists measure earthquakes in several ways, most often with the Richter Scale and the Modified Mercalli Scale. The Richter Scale estimates an earthquake's magnitude through a precise instrument called a seismograph, which measures and records the seismic waves, or vibrations, created by the sudden release of energy when segments of the Earth's crust move. Its scale ranges from 1 to 10 and is logarithmic, so a recording of 7 shows movement of the earth 10 times greater than a recording of 6. Earthquakes with a Richter value of 6 or more are considered major in magnitude. The 1954 earthquake that exposed the fault in front of the viewer measured 7.3 on the Richter Scale, while the San Francisco earthquake of 1906 had a value of 8.3. The Modified Mercalli Scale expresses how intense an earthquake's effects are in a given location, ranging from I, not felt except by a very few favorably situated, to XII, damage total, lines of sight and level disturbed, and objects thrown into the air. A value on this scale is assigned only after eyewitness reports and results of field investigations are studied and interpreted. The maximum intensity of the 1954 earthquake was X, the same as the 1964 quake in Alaska, while the 1906 San Francisco earthquake measured XI.