Earthquake Magnitude Explained: What the Numbers Mean
Earthquake magnitude is a single number for the size of an earthquake at its source, calculated from seismic waves recorded on instruments. The U.S. Geological Survey (USGS) prefers moment magnitude (Mw), which works for earthquakes of every size. Each whole number up the scale means about 32 times more energy released and 10 times larger wave amplitude on a seismogram.
Magnitude does not tell you how hard the ground shook where you are. That is intensity, which changes from town to town and is covered in our guide to magnitude vs intensity. To see magnitudes as USGS publishes them, open our live list of USGS earthquakes worldwide.
What does an earthquake magnitude number measure?
Magnitude describes the earthquake itself, not its effects. One earthquake has one preferred magnitude, while it produces many different levels of shaking across a region.
Moment magnitude comes from the seismic moment. USGS describes the seismic moment as the product of the average slip on the fault, the area of the fault that ruptured and the shear modulus, a measure of rock strength in the source region. A longer and wider rupture with more slip gives a larger moment and a larger Mw.
The scale is logarithmic. A magnitude 6.0 is not 20 percent bigger than a 5.0. It releases about 32 times more energy, according to the USGS page on magnitude, energy release and shaking intensity.
How much bigger is each step on the magnitude scale?
USGS states that each whole-number increase in magnitude is a tenfold increase in measured amplitude and about 32 times more energy. The exact energy factor is 10 to the power of 1.5, about 31.6, so some older USGS pages round it to 31.
The table applies that USGS rule to common differences between two earthquakes.
| Magnitude difference | Wave amplitude on a seismogram | Energy released |
|---|---|---|
| 0.2 (for example 6.8 vs 7.0) | about 1.6 times | about 2 times |
| 1.0 (5.0 vs 6.0) | 10 times | about 32 times |
| 2.0 (5.0 vs 7.0) | 100 times | about 1,000 times |
| 3.0 (5.0 vs 8.0) | 1,000 times | about 32,000 times |
Small decimal changes matter more than they look. A revision from 6.8 to 7.0 roughly doubles the estimated energy.
Which magnitude types does USGS report?
USGS uses several magnitude types, and the type appears next to the number on every event page. Each method has a range of sizes and distances where it works well.
| Type | Name | Based on | How USGS uses it |
|---|---|---|---|
| Mww | Moment magnitude (W-phase) | A very long-period wave that arrives at the same time as the P wave | The authoritative USGS magnitude when it can be computed; calculated for M5.0 and larger worldwide, dependable from about M5.5 |
| Mwr | Moment magnitude (regional moment tensor) | Seismic moment from regional recordings | Another way of computing moment magnitude |
| ML | Local magnitude (“Richter”) | Wave amplitude on nearby seismographs | Small earthquakes recorded locally; works for about M2 to M6.5 within about 600 km (about 370 miles) |
| Md | Duration magnitude | How long the ground keeps shaking on the record | Earthquakes of about M4 or smaller |
| mb | Body-wave magnitude | Body waves recorded at distant stations | Global earthquakes without a moment magnitude; tends to saturate at about M6.5 and above |
| mb_Lg | Regional magnitude | Lg waves on short-period instruments | Smaller earthquakes in the central and eastern United States |
“Saturate” means the method stops growing with earthquake size. A very large earthquake can get an mb value that is too low, which is one reason the first number can rise once Mww is ready. Full definitions are on the USGS Magnitude Types page.
Is the Richter scale still used?
Charles F. Richter of the California Institute of Technology introduced the idea of earthquake magnitude in 1935. His original definition applied only to California earthquakes within 600 km of one type of seismograph, the Wood-Anderson torsion instrument.
USGS says the Richter scale (ML) is the magnitude most people have heard of, but in practice it is not commonly used anymore, except for small earthquakes recorded locally. Moment magnitude has replaced it as the standard for large earthquakes worldwide. When a report mentions the Richter scale for a large earthquake, the USGS event page shows which magnitude type was actually used.
Why does the magnitude change after the first report?
The first magnitude is an early estimate. USGS explains that initial values use real-time data from the closest stations, before seismic energy has reached every station. Some waves used in the analysis take more than an hour to travel around the Earth, and data from some contributing networks arrive minutes or even days late.
USGS describes two main update points. The first comes within a few hours, when most real-time data have arrived. The second comes days to weeks later, when the event is reanalyzed for the archival catalog. In California, Alaska and the Pacific Northwest, updates are also possible in the first minutes after an earthquake.
Changes in the first hours often come from a switch of magnitude type, for example from mb to Mww, as well as from more data. Most revisions are small. The USGS Hawaiian Volcano Observatory says a magnitude can go up or down by a few tenths after a seismologist reviews the automatic value.
Revisions also happen after detailed study. USGS raised the 2016 Pawnee, Oklahoma, earthquake to Mw 5.8 from 5.6 after further analysis. You can follow current events in recent earthquakes in Oklahoma, and our guide on how to read a USGS earthquake report shows where the review status appears.
What does each magnitude usually mean for shaking?
USGS publishes a comparison of magnitude with the intensity typically observed near the epicenter. Intensity uses the Modified Mercalli Intensity (MMI) scale, written in Roman numerals.
| Magnitude | Typical maximum intensity (MMI) | USGS shaking label |
|---|---|---|
| 1.0 to 3.0 | I | Not felt |
| 3.0 to 3.9 | II to III | Weak |
| 4.0 to 4.9 | IV to V | Light to moderate |
| 5.0 to 5.9 | VI to VII | Strong to very strong |
| 6.0 to 6.9 | VII to IX | Very strong to violent |
| 7.0 and higher | VIII or higher | Severe or higher |
These are typical values close to the epicenter, not a promise for any one place. USGS notes that destructiveness also depends on focal depth, distance from the epicenter, local geology and building design. Our guide to earthquake depth explains why the same magnitude can feel very different at 10 km and at 100 km below the surface.
How often do large earthquakes happen?
The USGS National Earthquake Information Center locates about 20,000 earthquakes around the world each year, or about 55 per day. Based on records since about 1900, USGS counts an average of about 16 major earthquakes per year: about 15 in the magnitude 7 range and about one of magnitude 8.0 or greater.
The yearly count varies. USGS lists 2010 as the year with the most major earthquakes, 23.
The largest earthquake recorded on instruments was a magnitude 9.5 in Chile on May 22, 1960. USGS says a magnitude 10 or larger cannot happen on any known fault, because a fault long enough to produce one would have to extend around most of the planet. For the biggest events on US soil, see our list of the largest earthquakes in US history.
Bottom line
Read magnitude as a size label for the whole earthquake. One whole step means about 32 times more energy, and two steps mean about 1,000 times more. Check the magnitude type next to the number, expect the first value to move by a few tenths in the first hours, and use intensity maps to learn how strong the shaking was at a given place. The USGS FAQ on magnitude updates explains the timing in more detail.
FAQ
Is a magnitude 6 twice as strong as a magnitude 3?
No. Three whole steps mean 1,000 times larger wave amplitude and about 32,000 times more energy, based on the USGS rule of about 32 times more energy per step.
What is the difference between Mw and ML?
Mw is moment magnitude, based on fault area, slip and rock strength, and it works for all sizes. ML is the local or Richter magnitude, which USGS uses mainly for small earthquakes recorded nearby.
Why do USGS and another agency report different magnitudes?
USGS says magnitude estimates can vary between agencies because of different methods, different available data and the uncertainty in seismic data. The USGS value is the one shown on USGS event pages and in our live lists.
Can there be a magnitude 10 earthquake?
USGS says no known fault is long enough to produce one. The largest earthquake recorded on instruments was the magnitude 9.5 in Chile in 1960.