How Are Earthquakes Measured? Richter, Moment Magnitude, and Why the News Lies

Earthquakes are measured primarily by the moment magnitude scale (Mw), not the Richter scale as most news reports claim. Both are logarithmic: each whole number up means 10x larger ground motion and roughly 32x more energy released. A magnitude 7 quake is about 1,000 times more energetic than a magnitude 5 — not 40% larger.

Richter vs moment magnitude

The Richter scale (ML) was invented in 1935 by Charles Richter for Southern California earthquakes recorded on specific seismometers. It works well for small, local quakes but saturates around magnitude 6.5 — meaning bigger quakes all get labeled "about 7" regardless of actual size.

Moment magnitude (Mw) replaced it in the 1970s. Mw is calculated from the seismic moment — fault area times slip times rock rigidity. It doesn't saturate, so it's accurate all the way to magnitude 9+.

When the news says "magnitude 7 earthquake," they almost always mean Mw 7, even if they say "Richter 7." Mw is the scale in modern use.

What the numbers mean

Each whole-number step in magnitude:

• 10x bigger amplitude — seismogram needle swings 10x farther.
• ~32x more energy — because energy scales as amplitude to the 3/2 power.

So:

• Mw 6 vs Mw 5 → 32x more energy.
• Mw 7 vs Mw 5 → ~1,000x more energy.
• Mw 9 vs Mw 5 → ~1,000,000x more energy.

What each magnitude feels like

• Mw 2-3 — detected by instruments, usually not felt by humans.
• Mw 4 — rattles dishes, wakes sleepers. Happens ~13,000 times/year globally.
• Mw 5 — cracks plaster, moves furniture. ~1,500/year.
• Mw 6 — damages poorly-built buildings. ~150/year.
• Mw 7 — major damage near epicenter, felt hundreds of miles away. ~15/year.
• Mw 8 — destroys cities, triggers tsunamis. ~1/year.
• Mw 9 — continent-scale disaster. ~1 per decade on average.

The biggest recorded

Valdivia, Chile, 1960: Mw 9.5. Still the largest earthquake recorded. It triggered a tsunami that killed people as far as Japan (17,000 km away) 22 hours later.

Other benchmark quakes:

• Sumatra 2004 — Mw 9.1, tsunami killed ~230,000.
• Tōhoku 2011 — Mw 9.1, Fukushima nuclear disaster.
• Great San Francisco 1906 — Mw 7.9.

Why depth matters as much as magnitude

A shallow Mw 6.5 does more damage than a deep Mw 7.5. Depth determines how much energy dissipates before reaching the surface. Quakes shallower than 10 km cause the worst local damage.

Deep quakes (300+ km) can be felt over huge areas but rarely destroy buildings. The 2013 Okhotsk earthquake was Mw 8.3 at 609 km depth — felt from Moscow to Alaska but caused no deaths.

Intensity vs magnitude

Magnitude (Mw) measures the quake itself — energy released. Intensity (Modified Mercalli scale) measures the shaking at a specific place. A Mw 7 directly under a city is catastrophic; the same Mw 7 in the middle of the ocean might be barely noticed except by tsunami.

The MMI scale is expressed as Roman numerals I-XII. XII is "total destruction." News reports often blur magnitude and intensity — watch for this.

Early warning systems

Modern systems (like Japan's JMA and California's ShakeAlert) detect the fast-moving P-wave and warn areas before the slower, more destructive S-wave arrives. Warning times range from seconds to tens of seconds.

Seconds matter. In Japan's Tōhoku quake, high-speed trains automatically braked 15-60 seconds before shaking arrived, preventing derailments.

Can we predict earthquakes?

Short answer: not yet. We can forecast probability over decades (California has ~70% chance of Mw 6.7+ earthquake in next 30 years), but not predict specific dates. Current science suggests there's no reliable precursor we can measure yet.

More earth science

Related: how volcanoes erupt, Krakatoa's roar, and what if an asteroid hit Earth.