GPS is being used by scientists to study the ongoing deformation of the crust in Southern California caused by the San Andreas fault and other faults in the LA Basin. The Southern California Integrated GPS Network (SCIGN) measures the millimeter-scale movements of the crust between earthquakes, and also records displacement of stations during earthquakes, but it does not measure the actual shaking of the ground caused by an earthquake.

As you learned in previous sections, earthquakes can be measured in a variety of ways. Traditionally, earthquake size has been determined by various seismologic methods, which examine the amount of shaking, which directly relates to the energy released in an earthquake.

GPS measures the size of an earthquake by examining the final amount that a station has been displaced in an event. This is done by examining the total distance that a station has moved in an earthquake by comparing its position prior to the event with its position following the event.

Scientists have found that there is a relationship between the amount of displacement caused by an earthquake and its magnitude. It is by using this relationship between slip and magnitude that scientists can measure the relative size of an earthquake using GPS.

GPS is not used to measure the actual shaking of the ground because of the way in which the actual data are collected. Data are sampled at a certain rate, called a sample rate, which means that the receiver records the information being sent to it from the satellites at a certain interval of time all day long.

For example, data can be sampled at a 30-second interval, which means that the receiver records information from the satellite every 30 seconds. That means that if the shaking from the earthquake lasts any less than 30 seconds, it will be missed by the receiver.

Because of this, data are processed and a daily solution is determined, which means that the change in position of the receiver is calculated for one day at a time by combining the data collected througout the day. The data can also be processed at another solution interval. For example, data could be sampled at a 1-second rate and processed, but the solutions would be far less accurate than the daily solutions.

This is the reason why GPS is not used to directly measure the ground shaking during an earthquake. Seismometers are much better equipped to accurately record that sort of high-freqency motion than GPS. So, earthquake size is determined instead by measuring the final displacement of the stations and using the slip versus magnitude relationship.