Question

4. How do geologists determine the age of a fault line

Answer 1

To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This is called the Rule of Superposition. ... With absolute age dating, you get a real age in actual years.

Answer 2

hey friend :-)

here is your answer

A fault cuts other rocks. The other rocks had to exist before the fault or they wouldn't be there to be faulted. That gives a maximum age (the age of the rocks being cut). Any younger rocks which cross the fault zone and are not offset (not cut) place a minimum young age to the fault movement. 

Usually, there are rocks being added while the fault is active, and some rocks will show more movement than others and demonstrate syn-formation movement of the fault (syn means same time as). 

So, the age of fault activity is generally fairly easy to place within a window of time. The dates marking the limits correspond to the dates relating to the rocks surrounding the faults. Those dates come from one or several of the many methods for rock dating.

hoping it helps

thanks :D