On the moment magnitude scale, a great earthquake has a magnitude of 8.0 or higher. The threshold is written as Mw 8.0, where Mw indicates moment magnitude, a measure designed to represent an earthquake’s physical size and energy release.
Moment magnitude is calculated from seismic moment, which incorporates the area of the fault that ruptured, the average amount of slip, and the stiffness of the rocks involved. Because the scale is logarithmic, a one-unit increase represents about 10 times greater wave amplitude and roughly 32 times more energy release.
The term “great” is a magnitude classification, not a guarantee of a particular level of destruction. Damage also depends on depth, distance, local geology, construction quality, and population density. A deep earthquake may cause less surface damage than a smaller, shallow event close to a city.
The numbers in the options are easy to confuse. Magnitude 7.0 is commonly classified as a major earthquake, while 8.0 begins the great category. Magnitude 9.0 is not the threshold; it describes an exceptionally large great earthquake, such as the 2011 Tōhoku event.