Which scale measures the intensity of an earthquake based on its observed effects?

The Modified Mercalli Intensity scale is the correct choice because it is specifically designed to assess the intensity of an earthquake through its observable effects on people, buildings, and the Earth's surface. This scale uses qualitative observations rather than quantitative measurements, focusing on how the shaken population experiences the earthquake. It ranges from I (not felt) to XII (total destruction), providing a way to categorize the severity of the earthquake's impact on various locations.

In contrast, the Richter scale and the moment magnitude scale measure the energy released at the source of an earthquake, which is based on instrument data rather than personal accounts or visible damage. The seismic scale is not a commonly recognized scale in seismology, thus making it less relevant for this question. The emphasis on observable effects sets the Modified Mercalli Intensity scale apart as the chosen method for evaluating the impact of an earthquake.