Earthquake Magnitude Explained: Richter, Moment, and More
Understand earthquake magnitude scales — Richter, moment magnitude (Mw), and what each level actually feels like. Learn why scientists prefer Mw and how apps display magnitude.
What Is Earthquake Magnitude?
Earthquake magnitude is a numerical measure of the energy released by a seismic event at its source. Unlike intensity — which describes how shaking is felt at a specific location — magnitude is a single value that describes the intrinsic size of the earthquake itself, regardless of where you are standing.
The concept was introduced by Charles Richter in 1935, but the seismological community has since developed more accurate and universally applicable scales. Today, the moment magnitude scale (Mw) is the global standard used by USGS, GFZ, EMSC, and all major seismological agencies.
The Richter Scale: History and Limitations
Charles Richter developed his local magnitude scale (ML) specifically for earthquakes in Southern California, using a particular seismograph model (Wood-Anderson torsion seismometer). The scale is logarithmic: each whole number increase represents a tenfold increase in the amplitude of ground motion as recorded by the seismograph.
The Richter scale works well for small-to-moderate earthquakes (approximately M2–6.5) near the recording station. However, it breaks down for very large earthquakes (above M7), earthquakes far from California, and events recorded on modern digital instruments. This is why Richter "saturation" occurs — very large earthquakes can appear identical on the scale even though they release dramatically different amounts of energy.
Despite popular usage, professional seismologists have largely abandoned the Richter scale. When you hear "5.4 on the Richter scale" in news reports, the journalist almost certainly means moment magnitude or a closely related scale.
Moment Magnitude (Mw): The Modern Standard
Moment magnitude (Mw) was introduced in 1979 by Tom Hanks and Hiroo Kanamori as a replacement for the Richter scale. It is based on seismic moment — a measure of total energy release derived from the fault area, the amount of slip, and the rigidity of the rock.
Unlike Richter, Mw works accurately for all earthquake sizes and locations. The 1960 Chile earthquake (Mw 9.5), the 2004 Indian Ocean earthquake (Mw 9.1), and the 2011 Tōhoku earthquake (Mw 9.0) are all precisely characterized using this scale. USGS, JMA, and all modern seismological agencies report Mw as their primary magnitude measurement.
Magnitude Scale: What Each Level Feels Like
| Magnitude | Category | Typical Effects | Frequency |
|---|---|---|---|
| < 2.0 | Micro | Not felt. Detected only by instruments. | ~1,000,000 / year |
| 2.0–3.9 | Minor | Often felt, rarely damaging. Slight shaking. | ~130,000 / year |
| 4.0–4.9 | Light | Felt by many, indoor objects shake. Minor structural damage. | ~13,000 / year |
| 5.0–5.9 | Moderate | Strong shaking. Significant damage to weak structures. | ~1,300 / year |
| 6.0–6.9 | Strong | Destructive in areas up to 160 km. Many buildings damaged. | ~134 / year |
| 7.0–7.9 | Major | Widespread destruction. Felt across large areas. | ~17 / year |
| 8.0–8.9 | Great | Catastrophic damage over thousands of km. | ~1 / year |
| 9.0+ | Mega | Total destruction near epicenter. Tsunamis likely. | Every 10–50 years |
The Logarithmic Scale: Why M8 Is Not "Twice" M4
The magnitude scale is logarithmic, which means equal steps represent multiplicative — not additive — differences. Each full magnitude step represents:
- 10× increase in amplitude of ground motion recorded at a standard distance
- ~31.6× more energy released (because energy scales with amplitude to the power 1.5)
This means a M7.0 releases about 31.6 times more energy than M6.0, and about 1,000 times more than M5.0. The 2004 Indian Ocean earthquake (M9.1) released more energy than all earthquakes combined in the previous decade.
How Earthquake Globe Displays Magnitude
In the Earthquake Globe app, every event on the real-time 3D globe is color-coded and sized by magnitude:
- Green: M1.0–2.9 (micro and minor events)
- Amber: M3.0–4.9 (light — typically felt)
- Orange: M5.0–6.9 (moderate to strong)
- Red: M7.0+ (major and great earthquakes)
You can filter by magnitude threshold in the app to focus on events of a specific size. Notification alerts can also be configured to only trigger above a chosen magnitude threshold.
Related Reading
Frequently Asked Questions
- What is the difference between the Richter scale and moment magnitude?
- The Richter scale (ML) was designed for local California earthquakes using a specific seismometer type. Moment magnitude (Mw) is the modern scientific standard, accurate for all earthquake sizes and locations worldwide. For most earthquakes, the two scales give similar readings, but Mw is more precise for large events.
- What is the largest possible earthquake magnitude?
- There is no theoretical upper limit. The largest recorded earthquake was the 1960 Valdivia, Chile earthquake at M9.5. A M10 earthquake is theoretically possible but would require a fault rupture length on the order of thousands of kilometers.
- How much more energy does a M7.0 release than a M6.0?
- Each full magnitude step represents approximately 31.6 times more energy release. A M7.0 releases about 31.6 times more energy than a M6.0, and about 1,000 times more energy than a M5.0.
- How does Earthquake Globe display magnitude?
- Earthquake Globe color-codes earthquake pins by magnitude: green for M1–3, amber for M3–5, orange for M5–7, and red for M7+. The pin size also scales with magnitude, making large events immediately visible on the globe.
See Magnitude in Action on a Live Globe
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