The outer layer of the Earth consists of tectonic plates that interact with each other at their boundaries. The movements of these plates can be measured using GPS. While we use GPS in our phones and cars, we are mostly unaware of how it works. GPS uses a system of satellites to triangulate the position of a receiver anywhere on Earth. By using a network of receivers near plate boundaries, scientists can very accurately determine how the plates behave.

GPS stands for Global Positioning System. According to the Incorporated Research Institutions for Seismology, a GPS system consists of a network of 24 satellites and at least one receiver. Each satellite consists of a very accurate atomic clock, a radio transmitter and a computer. Each satellite orbits at about 20,000 kilometers (12,500 miles) above the surface. It constantly broadcasts its position and time. The ground-based receiver needs to "see" at least three satellites to obtain a triangulated position. The more satellites the receiver can use to triangulate, the more accurate the calculation becomes. A handheld GPS receiver has an accuracy of about 10 to 20 meters. With an anchored system, the accuracy can be in millimeters. The most accurate GPS receivers are accurate to within a grain of rice.

Scientists create large networks of GPS receivers mostly near plate boundaries. If you saw one of these receivers, you would probably not think much of it. They generally have a small fence for protection and a solar panel to power them. They are placed on bedrock if at all possible. They can also be wireless, so they would also have a small antenna. The modern GPS receivers used by scientists are almost real time, and movement can be seen in seconds back at the lab.

Plate movements detected by GPS supports plate tectonic theory. Plates move about as fast as your fingernails grow. Plates spread away from each other at oceanic ridges and converge at subduction zones. Plates slide by each other at transform boundaries. Collision, like at the Himalayas, is accurately recorded. At the San Andreas fault, the Pacific tectonic plate creeps in a northwesterly direction along the North American plate. Because of GPS technology, we know the creep rate at the San Andreas fault is approximately 28 to 34 millimeters, or a little over 1 inch, per year, according to the Nature article "Low Strength of Deep San Andreas Fault Gouge From SAFOD Core."

Scientists can more accurately locate and understand earthquakes using GPS data. They may even help create earthquake early warning systems, according to Phys.org. Also, while they do not predict earthquakes, they can help determine which faults are most likely to have earthquakes.