Final answer:
Geoscientists estimate long-term geologic slip rates across faults by measuring the movement of tectonic plates, studying intervals between major earthquakes, and analyzing the history of past earthquakes.
Step-by-step explanation:
The long-term geologic slip rates across faults are estimated by geoscientists using various techniques such as measuring the movement of tectonic plates and studying the intervals between major earthquakes.
One method is by measuring the annual rate at which tectonic plates move using techniques like global positioning satellites (GPS) and very-long baseline interferometry (VLBI). Scientists also study the history of past earthquakes to estimate the average interval between major events and the average motion along the fault.