Hi Petra. What you’re looking at on the map for the Parkfield area indicates where the GPS are located. The arrows indicate the direction of travel. Most of the GPS data at Parkfield has not been updated to present time. To see updated data you may want to look at the Long Valley data. That data is current.
GPS satellites continuously broadcast messages on 2 radio frequencies. These messages contain a very accurate time signal, a rough estimate of the satellite's position in space, and a set of coded information that a GPS receiver can decipher.
They want to know the latitude, longitude, and elevation. The receiver uses its internal clock and the coded information from each GPS satellite to determine the time it took the signals to reach the receiver. Since the signals travel at the speed of light, the receiver can calculate the distance to each satellite.
Once the receiver knows the distances to at least 4 satellites, and their positions, it can determine its clock correction and position on the Earth. They want to know how stations near active faults move relative to each other. When they occupy several stations at the same time, and all stations observe the same satellites, the relative positions of all the stations can be determined very precisely. Often they are able to determine the distances between stations, even over distances up to several 100 miles, to better than 5 millimeters (about a 1/4 of an inch).
Months or years later they will occupy the same stations again. By determining how the stations have moved they can calculate how much strain is accumulating and which faults are slipping.
Keep in mind that all of this is occurring much slower then it takes fingernails to grow. When looking at a plot it looks like there has been a lot of movement, but in reality it can be less then 1mm per year at some stations and more at others.
When looking at the GPS data for Long Valley you can see there has been considerable uplifting of the resurgent dome. The resurgent dome is also the area where ground uplift has been greatest over the past 18 years. It has risen more than two feet since 1980, and the scientists say that rising magma explains the uplift.
There has also been a change in the magnetics for the same area. This change has been brought about by the introduction of new magma. When you combine all of the data for that area you can see it is an eruption waiting to happen. When all of the data is brought together it’s not likely the eruption will catch anyone sleeping. Take Care…Don in creepy town.