What is the name of the scale used to measure an earthquake's observed effects?

The scale used to measure an earthquake’s observed effects is the Mercalli scale.

More precisely, modern descriptions usually refer to the Modified Mercalli intensity scale. It measures how strongly an earthquake is experienced at a particular location, using observations such as whether people felt the shaking, objects moved, buildings were damaged, or ground failures occurred.

The scale developed from work by Italian volcanologist Giuseppe Mercalli, whose 1902 version had ten degrees. Later researchers expanded and revised it into a twelve-level system. The Modified Mercalli scale ranges from I, meaning not felt, to XII, meaning total destruction and extreme ground effects.

The Mercalli scale is often confused with the Richter scale or the moment magnitude scale. Those are magnitude measures: they estimate the earthquake’s overall size or energy release. One earthquake has one magnitude, but it can produce different Mercalli intensities in different places because distance, depth, soil, geology, and building quality vary. That is why a distant large earthquake may feel mild in one city but severe near its epicenter.