Why can a magnitude 2 earthquake shake harder than a magnitude 8?

Magnitude and intensity sound like the same thing, but they answer two different questions. Magnitude asks how big the earthquake was at its source. Intensity asks how hard the ground shook where you are standing.

Magnitude is about the energy released by the quake, and each earthquake has only one magnitude. Intensity is the degree of shaking at a particular spot on the surface. It is strongest at or near the epicentre and fades to zero far away, so one earthquake has many intensities: strong in one town, mild in the next.

The intensity scale most used is the Modified Mercalli scale. Its lower degrees describe how the shaking feels to people, and its higher degrees describe the damage to buildings. It does not need instruments at all, just reports of what people noticed and what broke.

Here is a puzzle that shows why the difference matters. In 1994, a magnitude 8.2 earthquake hit Bolivia, yet its strongest felt intensity was only VI. Back in 1865, a magnitude 2.2 event in Barrow-in-Furness, England, reached an intensity of VIII. The reason is depth. The Bolivian earthquake was 631 km underground, so its energy spread through a huge volume of rock before reaching the surface. The English one was only about 1 km deep, practically beneath the houses.

So when a news report says magnitude 6, it tells you about the earthquake. It does not tell you what happened to your street. That depends on depth, distance, the soil and the buildings.