What is the difference between Richter and Mercalli scales?

The Richter Scale, the more widely known of the two, is a measure of the earthquake's size at its source, determined objectively by instruments. It tells you how big the earthquake was. It uses a logarithmic scale to measure. It was developed in 1935 by Charles F. Richter and Beno Gutenberg at Caltech to measure the magnitude of shallow, local earthquakes in Southern California based on the maximum amplitude recorded by a specific type of seismometer. It is now pretty obsolete, having been replaced by the Moment Magnitude Scale, though this is erroneously quoted in its measurements as "Richter".

The Mercalli Scale, on the other hand, is a measure of the earthquake's effect at a specific location, determined by observations. It tells you how strong the shaking was and how much damage it caused there. It's rated using Roman numerals (I to XII). It originated in the late 19th century with Italian volcanologist Giuseppe Mercalli, who created a simple ten-degree intensity scale based purely on observations of ground shaking effects. This scale was later expanded, notably by Harry O. Wood and Frank Neumann in the United States, to become the twelve-level Modified Mercalli Intensity (MMI) scale.

https://en.wikipedia.org/wiki/Modified_Mercalli_intensity_scale

https://en.wikipedia.org/wiki/Richter_scale