Magnitude: the source
Magnitude describes the earthquake’s overall size. A larger magnitude represents a much larger event. The magnitude type identifies the method used; not every reported value is a Richter magnitude.
USGS: magnitude and intensityTHE EARTHQUAKE LAB
The size of a quake.
The feel of the shake.
One earthquake has one magnitude and many local intensities. Explore the difference, from its source deep underground to the objects in a room.
THE REPORT YOU CAME FROM
Back to the report M1.6 earthquake · 74 km W of Nanwalek, Alaska
· reported depth 84.7 km
Explore magnitude and local shaking
TWO SCALES. ONE CLEARER PICTURE.CONCEPTUAL MODEL
Loads on your signal. Motion starts only when you choose.
Illustrative Earth layers, source and wavefronts. Not a map of an actual earthquake.
Explore the controls without starting motion.
M−1M9.5
0.0398×recorded-wave
amplitude
amplitude
0.00794×approximate
energy
energy
Under comparable conditions. Ratios are not local shaking or damage multipliers.
I · Not feltX+ · Extreme
Light
Many indoors notice shaking; dishes or windows may rattle.
Changing magnitude does not set this intensity. The room is an illustrative example, never a prediction for the report.
Explore depth & distance
Moving the source or observer changes their separation. Neither control calculates local intensity.
Magnitude describes source size. Felt shaking depends on the location. Available source values provide the starting magnitude and depth; missing values use M4.0 and 12 km examples. MMI IV remains a separate example.
Geometry, wave speed, object movement and visual amplitudes are simplified and compressed for clarity. This is not a ShakeMap, seismic replay, arrival-time estimate or structural simulation. Depth uses a compressed display scale (1–700 km); reported values outside the slider ranges remain unchanged in the original report.
START WITH THE DISTINCTION
“How big?” and “What did you feel?”
are different questions.
Intensity: a place
Modified Mercalli intensity describes local shaking and its effects. The same earthquake can be barely noticed in one place and strongly felt in another. Magnitude alone cannot tell you the intensity at your home.
Explore the felt-intensity scaleSMALL NUMBER. BIG DIFFERENCE.
The scale grows faster
than the number.
For a magnitude difference ΔM, the recorded-wave amplitude ratio is 10ΔM under comparable conditions. The typical energy ratio is approximately 101.5ΔM. These ratios do not measure damage or convert magnitude into local intensity.
| Magnitude | Amplitude ratio | Approx. energy ratio |
|---|---|---|
| M2 | 0.1× | 0.032× |
| M3 | 1.0× | 1.0× |
| M4 | 10.0× | 31.6× |
| M5 | 100.0× | 1,000.0× |
| M6 | 1,000.0× | 31,622.8× |
| M7 | 10,000.0× | 1,000,000.0× |
| M8 | 100,000.0× | 31,622,776.6× |
THE GROUND MAKES A DIFFERENCE
The same earthquake.
A different experience.
Soft ground can amplify some shaking. Firm rock responds differently. Distance, depth, the rupture and the building itself also shape the experience. The illustration shows materials; it does not assign an intensity to either home.
Explore the USGS explanationTHE MODIFIED MERCALLI SCALE
What shaking can feel like.
These representative effects describe a place, not an earthquake’s magnitude. Building response varies with construction, condition and local ground. The room illustration offers visual cues; it does not reproduce a ground-motion record or assess a structure.
Not felt
Usually unnoticed.
Weak
A few resting people notice it, particularly upstairs.
Weak
Noticeable indoors; can resemble a passing vehicle.
Light
Many indoors notice shaking; dishes or windows may rattle.
Moderate
Nearly everyone notices; unstable objects may topple.
Strong
Widely felt; furniture can move and minor damage may occur.
Very strong
Some ordinary buildings are damaged; vulnerable construction fares worse.
Severe
Considerable damage in ordinary structures; partial collapse is possible.
Violent
Severe damage can affect substantial buildings; some shift from foundations.
Extreme
Extreme shaking with very heavy damage; many ordinary structures may be destroyed.
Representative effects adapted from the USGS Modified Mercalli intensity scale. X+ groups the highest intensities in this explorer. These are descriptions, not safety instructions or a diagnosis of any building.
PUT THE NUMBERS IN CONTEXT
Read the whole report.
- 01
Check the time and source.
Separate when the event occurred from when the report was revised. Open the original USGS report for later updates. A preliminary magnitude can change.
- 02
Find the epicenter and depth.
The epicenter is on the surface above the source. Shaking generally decreases farther from the rupture; a deeper source often reduces nearby surface shaking for a similar event. Soil and rupture direction also matter. USGS explains depth and distance.
- 03
Look for local intensity information.
A ShakeMap summary is not the intensity at your address. Open the source’s geographic intensity map, then consider its uncertainty. Felt-report counts describe submissions, not the number of affected people.
- 04
Keep uncertainty visible.
A missing value means unknown. Nearby events are not automatically aftershocks. A USGS tsunami-information flag does not establish that a warning is active; check official tsunami messages.
Apply this to your report
The saved report lists magnitude 1.6, with no ShakeMap intensity available. These are source values, not a prediction for your location. The interactive room’s selected MMI is an independent example.
Return to the earthquake reportFeel shaking? Protect yourself first.
Drop, cover and hold on. Follow local emergency guidance before checking a screen. Read USGS safety guidance.
Explore recent earthquake reports · The earthquake field guide