Moment magnitude scale facts for kids
The moment magnitude scale (often written as Mw or just M) is the main way scientists measure how powerful an earthquake is. The bigger the number on this scale, the stronger the earthquake was. It measures the total energy released by an earthquake when it happens. Like the older Richter scale, it uses a special kind of measurement called a logarithm, which means each step up on the scale shows a much bigger increase in energy.
The moment magnitude scale (Mw) is now the most trusted way to rank earthquakes by their size. It's better than other scales because it directly relates to the earthquake's energy. It also doesn't "saturate," meaning it doesn't underestimate the size of very large earthquakes, which other scales sometimes do. Because of this, groups like the U.S. Geological Survey use it for reporting big earthquakes (usually those stronger than M 4). It has replaced older scales like the local magnitude (ML) and surface wave magnitude (Ms) scales.
| Scale Number | Earthquake Effect |
|---|---|
| less than 3.5 | This is a very weak earthquake. People usually don't feel it, but geologists can record it with special tools. |
| 3.5-5.4 | People generally feel these quakes, but they rarely cause damage. |
| 5.4-6.0 | These won't damage strong, well-built buildings. However, they can damage or destroy small or poorly-built structures. |
| 6.1-6.9 | These can cause damage in areas up to about 100 kilometers (60 miles) across where people live. |
| 7.0-7.9 | These are called "major earthquakes" and cause a lot of damage. |
| 8 or greater | These are very large and destructive earthquakes that can destroy big cities. |
Contents
How We Measure Earthquakes
The First Earthquake Scale: The Richter Scale
At the start of the 1900s, scientists didn't know much about how earthquakes happened. They also didn't fully understand how earthquake waves traveled through the Earth. The first ways to measure earthquake size were based on observations.
In 1931, a Japanese scientist named Kiyoo Wadati found that the biggest shake from an earthquake's waves got smaller the further away you were. Then, in 1935, Charles Francis Richter used this idea to create a scale. He figured out how to adjust for distance and other things. This allowed him to use the size of the wiggles on a seismograph (an earthquake recording machine) to measure an earthquake's "magnitude." He made it so each step up on the scale meant the shaking was ten times bigger. This was called the "local magnitude scale" or the Richter scale (ML).
The Richter scale worked well for shallow, medium-sized earthquakes that were not too far away. But it had a problem: it didn't work well for very deep, very far, or very large earthquakes. For these, it would "saturate," meaning it would underestimate their true size. For example, the 1960 Chilean Earthquake and the 1964 Alaska earthquake were huge, but the Richter scale only gave them magnitudes around 8. These quakes were much more powerful than other M 8 quakes. Scientists needed a better way to measure them.
Understanding Earthquake Energy: Seismic Moment
Scientists realized they needed a way to measure the actual physical size of an earthquake, not just how much the ground shook. This led to the idea of "seismic moment" (M0).
Seismic moment measures how much a fault slips and how large the area of that slip is during an earthquake. Think of it as the total "push" or "twist" that causes the Earth's crust to move permanently. It's measured in units like Newton meters (N·m).
In 1966, a scientist named Keiiti Aki was the first to calculate an earthquake's seismic moment from its waves. He studied the 1964 Niigata earthquake. He found a way to connect the earthquake's physical features (like how much the fault slipped and how big the slipped area was) to the seismic moment measured from the waves. This was a big step forward!
Creating the Moment Magnitude Scale
Older earthquake scales had trouble showing the true energy of very large earthquakes. For example, the 1960 Chilean earthquake, which was a magnitude 9.5, was only measured as an 8.2 on the surface-wave magnitude scale. This was because the shaking from these giant quakes lasted longer than the measurement tools could handle.
Caltech scientist Hiroo Kanamori saw this problem. He created a new magnitude scale called Mw, where "w" stood for "work" (meaning energy). He found a way to estimate the total energy released by an earthquake.
Later, in 1979, USGS scientist Thomas C. Hanks and Kanamori worked together. They used the idea of seismic moment to define the new moment magnitude scale. This scale is calculated using the seismic moment (M0) of an earthquake. The formula they developed is:
In this formula, M0 is the seismic moment measured in Newton meters (N·m). This new scale made it much easier and more accurate to measure the true size of all earthquakes, especially the very large ones.
Current Use of the Scale
Today, the moment magnitude scale is the most common way to measure medium to large earthquakes. However, for very small earthquakes (less than M 3.5), scientists don't usually calculate the seismic moment. For these tiny quakes, other simpler scales are still used.
When you hear about a big earthquake in the news (usually larger than M 4), the magnitude reported is almost always the moment magnitude (Mw), not the older Richter scale (ML). This is because the moment magnitude scale gives a more accurate picture of the earthquake's true power and size.
Related pages
See also
In Spanish: Escala sismológica de magnitud de momento para niños
