How Earthquakes Are Measured: Magnitude Versus Intensity

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"A 7.0 earthquake" and "very strong shaking here" are two different measurements answering two different questions. Here is the logarithmic maths of magnitude, why the Richter scale was replaced, and what intensity really tells you.

Two Numbers People Confuse

Every big earthquake gets reported with more than one figure, and the most common public confusion is treating them as the same thing. There are really two measurements:

• Magnitude — how much energy the quake released at its source. One number, for the whole event.

• Intensity — how hard the ground shook at a particular place. Many numbers, one per location, fading with distance.

A shallow magnitude-6 quake right under a city can be far more destructive at that city than a magnitude-8 quake a thousand kilometres away. Magnitude is about the earthquake; intensity is about you.

Magnitude Is a Logarithm

Magnitude scales are logarithmic, which is why people say an 8.0 is "ten times a 7.0." Strictly: each whole step is about a 10× larger ground-amplitude reading on a seismograph, but the energy released grows about 32× per step. So a magnitude 8.0 releases roughly 32 times the energy of a 7.0, and about 1,000 times the energy of a 6.0. That steep multiplication is why a small-looking increase in the number means a very large increase in real power.

Why "Richter" Is Rarely Used Now

The original Richter scale (1935) was a local magnitude (M_L) built for one region, on one type of seismograph, for shallow California quakes. It has two flaws: it saturates — very large quakes all read about the same because the instrument's measure of shaking amplitude hits a ceiling — and it doesn't relate cleanly to the physical size of the fault. Seismologists therefore moved to the moment magnitude (M_w), which is computed from the actual physics of the rupture: the area of fault that slipped, how far it slipped, and the strength of the rock. Moment magnitude does not saturate, so it is the standard for the biggest events — when an 8.8 or 9.1 megathrust quake is measured, it is a moment magnitude.

The reason a large quake's magnitude sometimes gets revised upward after the fact is exactly this: early estimates use the first fast-arriving waves, while the full rupture — which can run hundreds of kilometres over minutes — is only captured once more data is in.

Epicentre, Hypocentre, Depth

Three more numbers shape the damage. The hypocentre (or focus) is the point underground where the rock first breaks; the epicentre is the point on the surface directly above it. Depth matters enormously: a shallow quake, even modest in magnitude, shakes the surface hard because the energy has barely spread out before it reaches you, while a deep one dumps most of its force in the mantle above your head. Subduction zones produce both shallow and very deep quakes; the deadliest are usually shallow and close to populated ground.

Intensity Scales: How It Felt

Intensity is graded on scales like the Modified Mercalli Intensity (MMI), which runs from I (not felt) to XII (total destruction) using human effects, not instruments: at low numbers, only hanging objects sway; mid-scale, cracks appear and heavy furniture shifts; high numbers mean buildings collapse and the ground itself cracks and liquefies. Because intensity fades with distance from the epicentre and depends on local ground, a single event produces a whole isoseismal map of intensity zones — and the worst damage is not always at the epicentre, since soft-fill ground can amplify shaking far more than bedrock a short way off.

What To Watch For

For public safety the practical reading is simple: magnitude tells you the size of the release, but depth, distance and local soil tell you what your building will actually feel. Modern networks publish both an initial magnitude and a shaking map within minutes, and revise them as more stations report.

You can follow live seismic events — newest first, with magnitude, depth, place and nearest town — on our earthquakes page, understand the physics of faulting and waves in the earthquakes explained guide, and see how quakes sit alongside other active natural events on the disasters tracker.

See also

• Why You Get Jet Lag: The Body Clock Explained
• Why We Have Seasons — It Is Not Because Earth Gets Closer to the Sun
• Why the Sky Is Blue and Sunsets Are Red: One Effect Explains Both
• Why the Sea Rises and Falls: How Tides Work
• Why Is the Sea Salty? (And Is It Getting Saltier?)
• What Your Blood Pressure Numbers Actually Mean