How is the strength of an earthquake measured?
Magnitude measures the strength of an earthquake in terms of the energy it releases, allowing different earthquakes to be compared with each other.
Intensity quantifies the effects an earthquake has at the Earth's surface level, including its impacts on buildings and infrastructure.
Magnitude
Magnitude is a dimensionless quantity specific to each earthquake, unlike intensity, which, for the same earthquake, varies depending on the measured location. An earthquake occurs when a rupture occurs between two tectonic plates: the two blocks slide past each other, moving along the fault line. The energy thus released spreads in the form of seismic waves, which can travel across hundreds of kilometers, moving at speeds of several kilometers per second before eventually fading away.
In 1935, the American seismologist Charles Richter (1900-1985), who studied earthquakes in California, developed an equation based on the amplitude of seismograph recordings and using a logarithmic scale to quantify the scale of earthquakes. This became the well-known Richter scale, which has not been in use since the 1960s.
"Because his approach was rather imprecise, seismologists gradually defined a magnitude scale based on the physical quantity of energy released by an earthquake, called the 'seismic moment,'" said Jean-Paul Montagner, emeritus professor of seismology at the Paris-Cité University and the Institute of Earth Physics of Paris (IPGP). The seismic moment is "the product of the area that ruptured, the displacement between the two blocks, and a final parameter that depends on the physical properties" of the location. The seismic moment, noted as M 0, is expressed in newton-meters (Nm).
To ensure consistency between the old Richter scale and the new method, which is based on measurable physical quantities, "the seismic moment was calibrated to match the Richter magnitude so that, for typical earthquakes, there is no difference between the moment magnitude and the Richter magnitude."
In practice, there is a formula that converts the seismic moment (M 0) to the "moment magnitude" (M w, which is a dimensionless quantity). "For example, a magnitude 7 earthquake corresponds to a rupture of 50 kilometers, with a displacement of about 1 meter, over a duration of roughly 15 seconds," said Montagner. This correspondence works well up to magnitude 7, but "not at all for the large earthquakes."
On this logarithmic scale, the amount of energy released increases by a factor of 30 with each step up the scale, for example, from 6 to 7.
Intensity: A measure of the earthquake's effects
Unlike magnitude, which is a physical quantity that characterizes an earthquake and can be measured instantly, an earthquake's intensity corresponds to the effects it produces at the surface level in a given location: the same earthquake can, therefore, have several different intensities depending on the area measured. Intensity is, notably, measured on the Mercalli scale, which records an earthquake's effects on buildings and infrastructure.
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This scale spans from levels I to XII, with each step describing the earthquake's perceived and material consequences in a given region. The first level corresponds to tremors that are not felt, or barely felt, while level XII, the highest, means that almost all of the infrastructure in an area is damaged or destroyed.
For example, the 1960 Agadir earthquake was a magnitude 5.9 event, weaker than the earthquake that struck Morocco overnight on September 8, 2023 (magnitude 6.8), but it caused considerable damage: 12,000 deaths, 25,000 injuries and the destruction of the entire city. This discrepancy is explained by the fact that the Agadir earthquake's epicenter was located directly beneath the city, at a depth of just 10 kilometers, whereas the more recent earthquake's epicenter was located farther from densely populated areas (70 kilometers away from Marrakech) and at a greater depth (18.5 kilometers underground). The intensity scale, therefore, not only depends on the earthquake's magnitude and the depth of its epicenter, but also on the local geological structure.
According to the United States Geological Survey (USGS), Tuesday's earthquake reached an intensity of VIII at its epicenter, which was located off the coast of Russia's Kamchatka Peninsula. Waves rising between 1 and 3 meters above tide level are likely to sweep across the entire Pacific region, from Japan to Costa Rica, and they could strike many archipelagos, such as French Polynesia, Guam, or Hawaii.
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