How Earthquake Magnitude Is Measured, and Why Richter Is Retired

Earthquake magnitude is a single number that describes the size of the quake itself, worked out from seismometer recordings. USGS says moment magnitude (written Mw) is a more accurate measure than the Richter scale for all but small, locally recorded earthquakes. Each whole-number step up means a tenfold larger wave on a seismogram and about 32 times more energy released.

1. Magnitude is one number per earthquake

USGS puts it plainly: magnitude is the size of the earthquake, and an earthquake has a single magnitude. The shaking people feel is different. It has many values, varying from place to place with distance, the type of ground, and other factors. That second measure is called intensity, and it is covered below.

Seismometers record ground vibrations that travel through the Earth. Because sensitive instruments magnify the motion so much, a strong quake can be detected from a source anywhere in the world. From the recordings, seismologists work out the time, location and magnitude.

2. What the Richter scale was

Charles F. Richter, a seismologist at the California Institute of Technology, introduced the concept of earthquake magnitude in 1935. His original definition held only for California earthquakes within 600 km of a particular type of seismograph. The idea was simple: if you know the distance from the seismograph to the quake and the largest signal amplitude it recorded, you can rank the quake's size. The result comes from the logarithm of the recorded wave amplitude, with adjustments for distance.

That scale is called ML, where the L stands for local. USGS says it is not commonly used anymore, except for small earthquakes recorded locally.

3. Why moment magnitude took over

As seismograph stations spread around the world, it became clear that Richter's method was strictly valid only for certain frequency and distance ranges. Other versions, body wave magnitude (Mb) and surface wave magnitude (Ms), extended the idea. Each works for a particular type of signal. For very large earthquakes, some of these scales overestimate the true size and some underestimate it.

Moment magnitude avoids that by using physical properties of the fault, not just wave height. USGS gives the recipe:

• Seismic moment = rigidity x area x slip. Rigidity is the strength of the rock along the fault, area is the part of the fault that slipped, and slip is how far it moved.
• Mw = 2/3 x (log10 of the moment minus 9.1). The conversion is designed so the result roughly matches the Richter scale where the two overlap.

A stronger rock, a larger slipping area, or more movement all produce a larger magnitude. USGS says moment magnitude gives the most reliable estimate for very large earthquakes and is valid over the complete range of sizes.

4. Why one step is 32 times the energy

The scale is logarithmic. Going up one whole number multiplies the measured amplitude by 10. Energy grows faster than amplitude, and USGS states that each whole number is 32 times more energy.

Here is what that looks like when you compare a magnitude 5 quake with larger ones:

The energy column is 32 multiplied once, twice, and three times (32, 1,024, 32,768), rounded. Two magnitude steps is therefore roughly a thousandfold jump in energy, which is why a magnitude 7 is a far bigger event than a 5 even though the numbers look close.

5. Magnitude is not the same as shaking

Intensity measures shaking at a specific place. In the United States it is reported on the Modified Mercalli Intensity Scale, in Roman numerals such as VI or X. Traditionally it came from people's reports of what they felt and what was damaged, collected by mail and now by a web form. Instrument data can also estimate it.

USGS gives a clear example of why the two differ. The magnitude 6.7 Northridge, California earthquake shook harder and over a wider area than the slightly larger magnitude 6.8 Nisqually, Washington earthquake. Northridge's source was 3 to 11 miles deep, while Nisqually's was 30 to 36 miles deep. A shallow quake puts the source closer to buildings. Local ground matters too: in the 1989 magnitude 6.9 Loma Prieta quake, bedrock shook the least and soft mud the most at stations about the same distance away.

6. Key takeaways

• Magnitude is the size of the earthquake. Intensity is the shaking at one location.
• Moment magnitude (Mw) is based on rock strength, fault area and slip, and is more accurate than Richter for all but small local earthquakes.
• The Richter scale (ML) dates from 1935 and is now mainly used for small, local quakes.
• Each whole number is a tenfold larger wave and about 32 times more energy.
• Depth and local ground change how strong the shaking feels, so equal magnitudes can feel very different.