Moment magnitude is a number that indicates the strength of an earthquake. It is related to the energy released during the earthquake. A Japanese seismologist (scientist who studies earthquakes) named Hiroo Kanamori designed the moment magnitude scale of numbers in 1977. This scale is an improvement of the Richter magnitude scale, which American seismologist Charles F. Richter invented in 1935. In both scales, the stronger the earthquake is, the higher is the magnitude. The Richter and moment magnitudes are nearly identical for most earthquakes with a magnitude less than 7. However, moment magnitude measures the strongest earthquakes more accurately.
Scientists can determine moment magnitude in two ways. In the first method, they measure the length and depth of the earthquake fault and the movement of rock at the fault. A fault is a fracture in the earth’s rocky outer shell where sections of rock repeatedly slide past each other. In the second method, scientists use data from a seismograph, a device that records ground movements. Richter magnitude is based on seismographic data.
The main difference between the two scales lies in the frequencies of the movements that are recorded. Frequency is a measure of how often the ground moves back and forth in one second. During an earthquake, the ground shakes at several frequencies at the same time. The movements travel together through the ground, just as radio broadcast waves of different frequencies travel through the air. Like a radio receiver, older seismographs were “tuned” to a certain range of frequencies.
Richter magnitude depends on a single measurement–the greatest distance the ground moves in a certain band of frequencies recorded by one type of older seismograph. This band does not include the lowest frequencies at which the ground moves. But as earthquakes become larger, their movement grows more at low frequencies. Thus, the Richter scale may underestimate the strength of an extremely large earthquake.
In contrast, moment magnitude depends on the greatest distance the ground moves at the lowest recorded frequencies. Seismographs can record many frequencies at once, so moment magnitude can be measured accurately for both big and small earthquakes.
The biggest earthquake ever recorded occurred in the Pacific Ocean near Chile in 1960. In this quake, a fault 500 miles (800 kilometers) long moved 70 feet (21 meters). The quake’s moment magnitude was 9.5, but its Richter magnitude was only 8.3.
