Final answer:
Geologists use GPS and VLBI to measure movement along faults, such as the transform faults. By analyzing the rate of plate motion and historical data on earthquakes, they can calculate the total distance moved, like an 8-meter slippage along the San Andreas Fault from 1857 to 2017.
Step-by-step explanation:
Geologists measure the distance the Earth has moved along a fault, such as movement in a transform fault, by using various high-precision techniques. Among these methods, global positioning satellites (GPS) and very-long baseline interferometry (VLBI) are two commonly used techniques to measure the rates of motion between two tectonic plates. For instance, considering the Pacific plate moves at a rate of about 5 cm/year relative to the North American plate, the amount of slippage required to relieve the accumulated strain since the last major earthquake can be calculated. For example, if the last significant slip along the San Andreas Fault was in 1857, and the next one occurs in 2017, with a movement of 5 cm/year, the slippage over 160 years (1857 to 2017) would be 800 cm or 8 meters.
Moreover, by placing sensitive instruments within fault zones, such as those within the Los Angeles basin, additional data on the distortion and contraction of the Earth's crust can be obtained, providing further information on the movements along fault lines.