Moment magnitude scale

The moment magnitude scale (MMS; denoted explicitly with M_w or Mwg and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. M_w was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. It helps scientists and people understand how powerful an earthquake is.

Similar to the local magnitude/Richter scale (M_L ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales. Despite the difference, news media often use the term "Richter scale" when referring to the moment magnitude scale.

Moment magnitude (M_w ) is considered the authoritative magnitude scale for ranking earthquakes by size. It is more directly related to the energy of an earthquake than other scales, and does not underestimate magnitudes in certain conditions. It has become the standard scale used by seismological authorities like the United States Geological Survey for reporting large earthquakes (typically M > 4), replacing the local magnitude (M_L ) and surface-wave magnitude (M_s ) scales. Subtypes of the moment magnitude scale (M_ww , etc.) reflect different ways of estimating the seismic moment.

History

Richter scale: the original measure of earthquake magnitude

Main article: Richter magnitude scale

In the early 1900s, scientists knew little about earthquakes and how they created seismic waves. The first way to measure earthquake strength was created in 1931 by a Japanese scientist who noticed that seismic waves got smaller with distance. Then, in 1935, Charles Richter developed a way to measure earthquake strength called the local magnitude scale, or Richter scale. This scale helped compare the size of different earthquakes.

Moment magnitude scale

Later, scientists wanted a better way to measure very large earthquakes. In 1979, Thomas Hanks and Hiroo Kanamori created the moment magnitude scale. This new scale uses the amount of energy released by an earthquake to give a more accurate measure of its strength. It works well for both small and very large earthquakes.

Current use

Moment magnitude is the most common way to measure the size of medium to large earthquakes. However, for smaller earthquakes, scientists often do not use this scale. For example, the United States Geological Survey usually does not use moment magnitude for earthquakes smaller than 3.5, which are most of the quakes that happen.

When people read about big earthquakes in the news, they will usually see the moment magnitude listed. This is preferred over the older Richter scale for these larger events.

Definition

The moment magnitude scale is a way to measure how strong an earthquake is. It is shown as M_w. This scale was created by scientist Hiroo Kanamori in 1979. It helps us understand the size of an earthquake by looking at the energy it releases.

Earthquakes that are very small, like ones that people might not even feel, have lower numbers on this scale. Big earthquakes, like the one that happened in Chile in 1960, have higher numbers. This scale is similar to another scale called the Richter scale, but it works better for very large earthquakes.

Relations between seismic moment, potential energy released and radiated energy

Seismic moment helps us understand how much energy is stored in the Earth's crust before an earthquake happens. This stored energy is in two forms: elastic energy from built-up stress and gravitational energy. During an earthquake, some of this energy turns into heat, some into creating cracks in rocks, and some into the shaking we feel.

Scientists can relate the amount of this stored energy to the seismic moment of the earthquake, but there are many factors that make this relationship tricky. Two earthquakes with the same seismic moment might release different amounts of total energy because of differences in the rocks involved or how the earthquake happens. Similarly, the amount of shaking energy released can vary a lot between earthquakes, even if their seismic moments are the same. This is why scientists sometimes use a different way to measure earthquake size, called "energy magnitude," which focuses directly on how much shaking energy was released.

Comparative energy released by two earthquakes

The moment magnitude scale helps us understand how much energy an earthquake releases. Scientists can compare the energy released by two earthquakes using a simple formula. If one earthquake has a moment magnitude of m₁ and another has m₂, the ratio of their energy release is about 10^1.5 (m₁ − m₂). This means that for every increase of one step on the magnitude scale, the energy released goes up by about 32 times. For example, an earthquake with a magnitude of 7.0 releases 1,000 times more energy than one with a magnitude of 5.0.

Sometimes, people compare this energy to explosions of TNT to help visualize its power. The energy from an earthquake can be converted into the equivalent number of Hiroshima bombs or tons of TNT. This comparison shows just how much energy is released during these powerful natural events. The largest possible earthquake on this scale would have a magnitude of 10.6, which would mean the Earth's crust breaking apart completely.

Subtypes of M_w

Scientists have developed several methods to measure earthquake strength using the moment magnitude scale. Each method has a special name to show how it was calculated.

Some of these methods include using different types of seismic waves or measurements from specific distances. For example, Mwb uses long-period body-waves, while Mwr looks at complete waveforms from far away. Other types like Mwc and Mww focus on the center of the earthquake's energy. There are also quicker ways, such as Mwp, to estimate how big an earthquake might be based on early wave data.