How Earthquakes Are Measured

Read magnitude, depth and shaking without confusing them with damage.

Follow these steps

1
Open an earthquake record and find the magnitude: it describes the size of the earthquake, not the damage at your address.
2
Read the depth in kilometers. This describes where the earthquake began below the surface.
3
Find the epicenter: the map point above that starting location. It is not the full area that experienced shaking.
4
Look for an intensity or ShakeMap product from the original agency. Intensity describes shaking at different places.
5
Compare issue and update times; preliminary measurements can change as more stations contribute.

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Why this matters

Magnitude is one number for the whole earthquake. Shaking is different at every place, so two towns can feel the same earthquake very differently.

Each whole step on the magnitude scale means roughly 32 times more energy, so an M6 releases about 32 times the energy of an M5.

A shallow earthquake usually shakes the surface harder than a deep one of the same magnitude, because the energy has less distance to travel.

Same magnitude, different shaking

Two fictional records both say M 5.8. Record A has a depth of 8 km. Record B has a depth of 120 km.

Magnitude: the same. Both earthquakes released similar energy.
Depth: A started much closer to the surface, so shaking near its epicenter is likely to be stronger.
Damage: neither record tells you. Look for the agency’s intensity map (ShakeMap) or "Did You Feel It?" reports.

Try it yourself

Read one real earthquake record and separate size, depth and shaking.

Open EarthWatch and choose the most recent earthquake listed. If none is listed, open the USGS earthquake page instead.
Write down the magnitude, the depth in kilometers, the location description, and the time (note whether it is in UTC).
Open the original USGS event page from the source link and look for an intensity map (ShakeMap) or "Did You Feel It?" reports.
Write one sentence that explains the difference between the magnitude and the shaking reported near you or the nearest town.

Expected result: A short note with four values (magnitude, depth, location, time) plus one intensity value or the words "no intensity map yet." Your sentence should not claim damage from magnitude alone.

Check your understanding

You can answer: "If the magnitude is 6, does that tell me how hard my street shook?" (No. Intensity tells you that, and it varies by place.)

If you get stuck

Two websites show different magnitudes.

Agencies use different magnitude types and update as more stations report. Use the original agency’s latest update and note its time.

The time looks hours off.

Many agency records use UTC (Coordinated Universal Time). Convert it to your local time before comparing.

The depth says exactly 10 km.

A depth of 10 km is sometimes a fixed default when the true depth isn’t well measured yet. Check later updates.

There is no ShakeMap.

Intensity maps are not made for every small earthquake. Use "Did You Feel It?" reports or wait for an update.

Official sources

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