The Richter scale and moment magnitude are both logarithmic ways to describe earthquake source size, but they use different measurements. “Richter” most accurately refers to local magnitude, ML. Mw derives from seismic moment and is preferred for comparing large earthquakes because it does not saturate in the same way as several amplitude-based scales.
Quick answer
| Question | Local magnitude, ML | Moment magnitude, Mw |
|---|---|---|
| Main basis | Calibrated wave amplitude | Seismic moment |
| Original scope | Nearby southern California earthquakes | Physical source size across broad event ranges |
| Strength | Fast and useful for calibrated local networks | Represents large ruptures consistently |
| Limitation | Instrument, distance and saturation range | Can require more waveform and source analysis |
Both are magnitude, not local shaking intensity.
What ML and Mw mean
Charles Richter developed local magnitude in the 1930s with Beno Gutenberg. The method compares the logarithm of recorded amplitude with a distance correction for a specified instrument and regional calibration. The “L” in ML means local.
Moment magnitude starts with seismic moment, M0: rock rigidity multiplied by the fault area that slipped and the average slip. Mw converts M0 into a logarithmic magnitude compatible with older scales over their shared useful range. This makes it a physical source measure rather than a label read from one peak amplitude.
How the two scales are calculated
ML uses amplitudes at stations within the method’s calibrated distance range. Networks correct for distance and regional wave attenuation. Modern instruments are not limited to Richter’s original hardware, but the result remains a local-magnitude calculation.
Mw uses waveforms, moment tensors or related source analyses to estimate seismic moment. The USGS relation for M0 in newton-metres is Mw = 2/3 × (log10(M0) − 9.1). Large events can take time to model, so an early catalog may first show mb, ML or another rapidly available type and later replace it with Mw.
How to interpret catalog labels
Read the letters as well as the number. ML 4.2 identifies a local-magnitude estimate. Mw 7.1 identifies moment magnitude. A plain M 5.0 means the display has not supplied or emphasized a more specific type.
Values from different methods need not match exactly. They use different wave periods, station geometry and calibration. A change from ML 5.8 to Mw 6.0 can be a change of method and evidence rather than an error. The magnitude classification guide explains the logarithmic differences between numbers.
Limits and common mistakes
Calling every modern magnitude “Richter” hides method information. Saying Richter is completely obsolete is also inaccurate: ML remains useful for many small and regional events. Mw is not automatically the first or only correct value for every earthquake.
Neither scale reports damage, felt area or depth. Those require separate fields and observations. A magnitude may be revised even after human review as better data and models become available.
Common questions
Is Mw larger than Richter by definition?
No. The scales were calibrated to be broadly comparable where their useful ranges overlap. Differences arise from data and method, not a fixed offset.
Why does ML saturate?
The measured waves and periods used by an amplitude scale stop increasing proportionally for very large ruptures. Seismic moment continues to represent fault area and slip.
Which value does Earthquake24 show?
Earthquake24 preserves the source’s reported magnitude and type. It does not relabel every event as Richter or independently calculate Mw.