What is the difference between Richter magnitude and Mercalli intensity?

Magnitude measures the energy released at the earthquake's source and is a single number per event, calculated from seismograph recordings. Mercalli intensity measures the shaking and damage observed at a particular place, so one quake has many intensities that fall off with distance.

Is the Richter scale still used?

The original Richter (local magnitude) scale saturates for large quakes, so seismologists now usually report moment magnitude (Mw), which is accurate at all sizes. Both share roughly the same number scale, which is why the public still calls any value the Richter magnitude.

How much more energy does each magnitude step release?

Magnitude is logarithmic: each whole step is about 31.6 times more energy released (10 raised to 1.5). So a magnitude 7 quake releases roughly 1,000 times the energy of a magnitude 5, even though the ground motion amplitude only rises tenfold per step.

What is the highest possible Mercalli intensity?

The Modified Mercalli Intensity scale runs from I (not felt) to XII (extreme, total destruction). Unlike magnitude it has no instrument; it is assigned from felt reports and observed damage, so it stops at XII where damage is already total.

How is earthquake energy converted to TNT?

The Gutenberg-Richter relation estimates radiated energy in joules from magnitude, and dividing by 4.184 billion joules per ton gives a TNT equivalent. A magnitude 6.0 quake is roughly comparable to tens of kilotons of TNT, though this is an order-of-magnitude estimate only.

Richter & Mercalli Scale Reference

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Two scales for one earthquake

This reference pairs the two systems used to describe earthquakes. Magnitude (Richter or the modern moment magnitude, Mw) measures the energy released at the source on a logarithmic scale. The Modified Mercalli Intensity (MMI) scale rates the shaking and damage felt at a location on twelve descriptive levels. A built-in estimator converts any magnitude into an approximate energy release.

How it works

Magnitude is logarithmic. Each whole-number increase represents about ten times larger ground-motion amplitude and roughly 31.6 times more energy released, since energy scales with 10^(1.5 M). The energy estimate uses the Gutenberg-Richter relation:

energy_joules = 10 ^ (1.5 * M + 4.8)
tons_TNT      = energy_joules / 4.184e9

Mercalli works the opposite way: it is observational. Trained assessors assign a roman-numeral level from I to XII based on felt reports and structural damage, so a single quake produces a patchwork of intensities that fade with distance from the epicentre.

Tips and notes

A magnitude 6.0 releases roughly 1,000 times the energy of a magnitude 4.0.
Intensity depends on local geology: soft soil amplifies shaking, so two towns at the same distance can report very different MMI values.
“Great” quakes (Mw 8.0 and above) happen about once a year worldwide; Mw 9.0+ events occur only every one to several decades.
The energy figure is an order-of-magnitude guide, not a precise yield.