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 way to measure how strong an earthquake is. M_w was created in 1979 by Thomas C. Hanks and Hiroo Kanamori. It helps people know how powerful an earthquake is.

Like the local magnitude/Richter scale (M_L ) created by Charles Francis Richter in 1935, it uses a logarithmic scale; this means small earthquakes look similar on both scales. Even though they are different, people often say "Richter scale" when they talk about the moment magnitude scale.

Moment magnitude (M_w ) is the main way to rank earthquakes by size. It shows the energy of an earthquake better than other scales. It is used by groups like the United States Geological Survey for reporting big earthquakes (usually M > 4), instead of the local magnitude (M_L ) and surface-wave magnitude (M_s ) scales. Subtypes of the moment magnitude scale (M_ww , etc.) are ways to estimate the earthquake's strength differently.

History

Richter scale: the original measure of earthquake magnitude

Main article: Richter magnitude scale

In the early 1900s, scientists knew little about earthquakes. In 1931, a Japanese scientist found that seismic waves got smaller with distance. Then, in 1935, Charles Richter made a way to measure earthquake strength called the local magnitude scale, or Richter scale. This helped people 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 energy an earthquake releases to measure its strength better. It works for both small and large earthquakes.

Current use

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

When people read about big earthquakes in the news, they will often see the moment magnitude listed. This is preferred over the older Richter scale for 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 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 learn about the energy stored in the Earth's crust before an earthquake happens. This stored energy comes in two forms: elastic energy from built-up stress and gravitational energy. During an earthquake, some of this energy becomes heat, some creates cracks in rocks, and some causes the shaking we feel.

Scientists can connect the stored energy to the seismic moment of the earthquake. However, this relationship can be complicated. Two earthquakes with the same seismic moment might release different amounts of total energy because of differences in the rocks or how the earthquake occurs. The amount of shaking energy released can also vary 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 looks directly at 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.

Subtypes of M_w

Scientists use different ways to measure how strong an earthquake is with the moment magnitude scale. Each way has a special name.

Some of these ways use different kinds of seismic waves or measurements from far away. For example, Mwb uses long-period body-waves, and Mwr looks at complete waveforms from far away. Other types like Mwc and Mww study the center of the earthquake's energy. There are also faster ways, such as Mwp, to guess how big an earthquake might be using early wave data.