Magnitude: a single number for the whole earthquake
Magnitude quantifies the energy released at the earthquake's source. What the public still calls the Richter scale was developed in 1935 by Charles Richter for measuring local, shallow earthquakes in Southern California from seismograph amplitude.
The key property of magnitude is that one earthquake has exactly one magnitude value, no matter where you are standing when you measure it.
Intensity: how strongly the shaking was felt, place by place
Intensity describes the observed effects of shaking at a specific location: how people felt it, what moved, what cracked, what collapsed.
Because intensity depends on distance from the source, depth, and local ground conditions, a single earthquake produces many different intensity values.
The 12 degrees of the Modified Mercalli scale
Each degree of Mercalli describes what people and structures experience, not an instrument reading: I (Not felt) through XII (Extreme).
This scale is based on eyewitness accounts and observed damage, not on seismometer recordings.
How magnitude and intensity typically relate
There is no exact formula linking the two, because intensity always depends on local conditions, but the USGS publishes an approximate correspondence for a location near the epicentre of a shallow quake.
A magnitude 3 quake typically produces Mercalli intensity I–II, while a magnitude 7 produces VII–IX.
Why the same magnitude can feel completely different
The 2011 Lorca earthquake in Spain (magnitude 5.1, depth 1 km) killed nine people because it was so shallow. The 2013 Sea of Okhotsk earthquake (magnitude 8.3, depth 609 km) caused no significant damage despite being 400 times more energetic.
Depth, distance, and local geology matter far more than magnitude alone.
Which one matters more?
Neither scale replaces the other: they answer different questions.
For earthquake hazard planning, both are essential.