Mormonism is most concentrated in the counties surrounding salt lake city, utah, and the population of mormons as a percentage of total county population begins to decline the further away from salt lake city. this is an example

distance decay

Explanation:

Based on the information provided within the question it can be said that this is an example of distance decay. This term refers to the effects that distance can have on cultural interactions, mostly focusing on the decline in a variable as distance increases. Which in this scenario the variable is the population of Mormons outside of Salt Lake City.