THE EXPLAINER
What magnitude means
Magnitude is a single number for the size of an earthquake at its source — how much energy the fault released. It is not a measure of how hard any particular place shook. The same earthquake has one magnitude and a whole map of different shaking.
The words this site uses
| Word | Magnitude | What it usually means in Alaska |
|---|---|---|
| Micro | under 2.5 | Recorded by instruments; almost never felt. |
| Minor | 2.5 – 3.9 | Felt by some people close by, especially indoors and upstairs. |
| Light | 4.0 – 4.9 | Felt across a town or a valley; rattles dishes; rarely any damage. |
| Moderate | 5.0 – 5.9 | Felt widely; can crack plaster and topple shelves near the epicenter. |
| Strong | 6.0 – 6.9 | Can damage buildings near the epicenter; felt across a region. |
| Major | 7.0 – 7.9 | Serious damage over a wide area; felt hundreds of miles away. |
| Great | 8.0 and up | The largest class there is. Alaska's 1964 M9.2 is the largest ever recorded in North America. |
Each whole step is about 32 times the energy
The magnitude scale is logarithmic. A step of 0.1 is roughly 1.4 times the energy; a full step — M5 to M6 — is about 32 times the energy, and two steps about 1,000 times. That is why the difference between M4.9 and M5.1 is small but the difference between M6 and M8 is enormous.
Depth matters as much as size
A moderate earthquake 10 km down can shake a town harder than a strong one 150 km down in the subducting slab. Every listing on this site carries the depth, and the deepest Alaska events — inside the sinking Pacific slab — are often felt as a long, gentle roll over a huge area rather than a sharp jolt anywhere.
Felt is not damage
How strongly people feel an earthquake depends on distance, depth, local soil, and the building they are in — soft lowland soils can amplify shaking several-fold. A quake can be felt by thousands and damage nothing, and a damaging quake in a remote area can be felt by almost nobody. Felt reports measure people's experience, not destruction.
Why numbers change after the fact
USGS postings take a few minutes to appear, and magnitudes are sometimes revised as more instruments report — a listing here re-reads the feed each visit, so it shows the current posted value. Historical entries are stable at the scale this site uses them, and the catalog is labeled "USGS catalog, as recorded."