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The magnitude scale is logarithmic because earthquake energy varies over an enormous range. Each whole-number increase represents about 31.6 times more energy released. A linear scale would be impractical because a M9.0 earthquake releases roughly 1 billion times more energy than a M3.0 earthquake.

Magnitude measures the total energy released at an earthquake's source — each earthquake has one magnitude. Intensity (Modified Mercalli scale) measures the effects at a specific location — the same earthquake can have different intensities at different distances. A M7.0 earthquake might be MMI IX near the epicenter but only MMI III 200 km away.

Theoretically, maximum magnitude is limited by the length of faults that can rupture simultaneously. The longest subduction zones could produce earthquakes up to about M10, but no fault system on Earth is long enough to exceed this. The largest recorded earthquake was M9.5 (Chile, 1960).

A magnitude 7.0 earthquake releases approximately 1,000 times more energy than a magnitude 5.0 earthquake. Each whole-number step represents 31.6 times more energy, so two steps (5 to 7) is 31.6 x 31.6 = approximately 1,000 times. In terms of amplitude, each step is 10 times larger.

A foreshock is a smaller earthquake that precedes a larger mainshock in the same area — though it can only be identified as such after the mainshock occurs. An aftershock follows the mainshock and is caused by stress adjustments along and near the ruptured fault. Aftershock sequences can last days to years depending on mainshock magnitude.