Why everyone still says Richter
Charles Richter introduced his magnitude scale in 1935 as a way to compare the size of earthquakes recorded on a specific type of seismograph in Southern California. The concept was elegant: a logarithmic scale where each whole number step represented a tenfold increase in measured wave amplitude. It stuck in public consciousness and never left.
Media, emergency managers, and the general public still commonly refer to earthquake magnitude as being on the Richter scale. In practice, however, seismologists stopped using the original Richter local magnitude for anything except small, nearby events decades ago. The scale that appears in USGS reports and PlanetSentry feeds is almost always moment magnitude.
What the original Richter scale actually measured
Richter's original scale, formally called ML or local magnitude, measured the maximum amplitude of seismic waves on a Wood-Anderson torsion seismograph at a standardized distance from the epicenter. It worked well for moderate earthquakes in Southern California — the exact context it was designed for.
The scale's limitations became clear as seismology expanded. ML saturates around magnitude 6.5 to 7, meaning it underestimates the size of truly large earthquakes. It also depends on the specific instrument type and distance calibration, making it inconsistent across different networks and geographies.
Moment magnitude: measuring the actual energy
Moment magnitude (Mw), developed by Hiroo Kanamori and Tom Hanks in 1979, solves these problems by measuring seismic moment — a physical quantity directly related to the fault area that ruptured, the average displacement on the fault, and the rigidity of the surrounding rock.
Unlike ML, moment magnitude does not saturate at high magnitudes. It accurately captures the size of magnitude 7, 8, and 9+ events because it measures the total energy release rather than a single wave amplitude. The 2011 Tohoku earthquake (Mw 9.1) and the 2004 Indian Ocean earthquake (Mw 9.1) would have been significantly underestimated on the original Richter scale.
The logarithmic nature: what each number means
Both scales are logarithmic, which causes persistent public confusion. A magnitude 6 earthquake is not twice as strong as a magnitude 3 — it releases roughly 31,600 times more energy. Each whole magnitude step represents approximately 31.6 times more energy released.
In terms of wave amplitude, each step is a factor of 10. So a magnitude 7 earthquake produces ground motion amplitudes 10 times larger than a magnitude 6, and 100 times larger than a magnitude 5. But because energy scales as amplitude to the 1.5 power, the energy difference is even more dramatic.
- M4 to M5: 31.6× more energy, 10× more amplitude
- M5 to M7: ~1,000× more energy, 100× more amplitude
- M5 to M9: ~63,000,000× more energy, 10,000× more amplitude
Other magnitude scales still in use
Seismologists use several magnitude scales depending on context. Body wave magnitude (mb) measures P-wave amplitude and is useful for deep earthquakes. Surface wave magnitude (Ms) measures Rayleigh wave amplitude and works well for shallow events at teleseismic distances. Duration magnitude (Md) estimates size from how long the seismograph records the event.
USGS and most monitoring agencies now report Mw as the authoritative magnitude for events above approximately magnitude 3.5. For smaller events, local or duration magnitude may appear first because they can be calculated faster from nearby stations.
Why the difference matters for public understanding
When a news report says a magnitude 7.1 earthquake struck, most people have no intuitive sense of what that means beyond 'big.' The logarithmic scale compresses an enormous range of destructiveness into a narrow number line, which makes all earthquakes between 4 and 7 sound vaguely similar even though they differ by factors of thousands in energy.
Understanding the exponential nature of the scale helps explain why emergency responses differ so dramatically. A magnitude 5.5 might crack plaster in older buildings. A magnitude 7.5 can level entire districts. A magnitude 9.0 reshapes coastlines and triggers transoceanic tsunamis. These are not small increments — they are separate categories of physical phenomenon.
Magnitude versus intensity: another common confusion
Magnitude describes the earthquake at the source. Intensity describes the shaking at a specific location. The Modified Mercalli Intensity scale (MMI) ranges from I (not felt) to XII (total destruction) and varies based on distance, soil conditions, building types, and depth.
A single earthquake has one magnitude but many intensities depending on where you are. A magnitude 6.0 shallow earthquake directly beneath a city produces higher intensity shaking than a magnitude 7.0 earthquake 300 kilometers offshore. This is why magnitude alone does not predict damage — distance and local geology matter enormously.
How PlanetSentry displays magnitude data
PlanetSentry pulls magnitude data from USGS and EONET feeds, which report the authoritative magnitude type for each event. For most events above magnitude 3.5, this is Mw. The globe visualization scales marker size and color intensity based on the reported magnitude to give an immediate visual sense of relative event significance.
When you click into an event detail, the magnitude type is displayed alongside the value so you can see whether the reported number is Mw, mb, ML, or another variant. This matters for technical users who want to understand the measurement basis, not just the headline number.