Magnitude 4.90 — 338 t of TNT

One step is a multiplication

The base-ten log of the energy is 1.5 × magnitude + 4.8. With a slope of 1.5, one point of magnitude means ten to the power 1.5, or 31.6 times the energy. Two points make ten cubed — a thousand times exactly. In other words, thirty-odd magnitude-6 quakes fit inside a single 7.

Shaking ×10, energy ×31.6

One point of magnitude multiplies the amplitude a seismograph traces by ten, but the released energy by 31.6. The two differ because energy depends not only on how far the ground moves but on how long it shakes and how much fault surface tore. When the news says “ten times” it means the shaking; “thirty-two times” means the energy.

Nearby magnitudes

Same first decimal

Find it by magnitude

Magnitude is not intensity

Magnitude describes the energy the earthquake itself released, so there is one number per quake. Intensity describes how hard a particular place shook, so it differs from street to street. A shallow magnitude-5 directly underfoot can shake harder than a deep magnitude-7 far away — depth, distance and ground conditions decide intensity.

How to read this

• log10(energy) = 1.5 × magnitude + 4.8, with energy in joules.
• One point of magnitude is 31.6 times the energy; two points exactly a thousand.
• The amplitude grows tenfold, the energy 31.6-fold.
• Magnitude belongs to the quake; intensity differs from place to place.