Aregion based on common properties such as population is called a(n)?

A region based on common properties such as population is called 
a formal region

Formal region.

Explanation:

A formal region is an area inhabited by the people who have one or more characters in common and some have a distinct boundary which makes them easy to identify such as states as that of the U.S, Canada and the Africa, etc. Theses proprieties can be a language and the behavior or the way of perceiving things. Expressed by a community at large and is thus a group or collection of the traits and the standard values of the place.

Geologists use  seismic waves to  locate an earthquake's epicenter..