Scientists use GPS instruments and tiltmeters to track changes of the ground surface.
Global Positioning System (GPS)
GPS data provides a direct measure of ground movements over periods from a fraction of a second to decades. There are many sources of noise in GPS measurements, and most tend to cancel over longer periods (e.g., 24 hours). Using high-precision GPS receivers and antenna, improved satellite orbits, and sophisticated software, the position of a GPS antenna can be measured to a precision of a few millimeters horizontally and a centimeters vertically using a 24-hour span of data.
At Mount Rainier, continuous GPS (CGPS) stations document changes in position through time, and every few years we reprocess all data to incorporate improvements in the way data is measured. Data for the CGPS stations are available on the Mount Rainier webpage (monitoring map).
Many sites are surveyed periodically (e.g., every few years) during campaigns, when a permanent ground marker is used as a predetermined location to take GPS measurements. During the campaigns, portable equipment is set up over the station and data is recorded for a few hours or days. This technique is used to observe ground movement for many closely spaced points. Further information on the campaign GPS network at Mount Rainier is available through the USGS Earthquake Hazards Program.
On Mount Rainier, tiltmeters are installed in 2-3 m deep drill holes to reduce near surface noise from environmental factors (heating and cooling of the ground, rainfall). In general, shallow borehole tiltmeters, such as those installed on Mount Rainier, provide good resolution of ground tilt over short periods (seconds to hours), but they drift unpredictably over periods of days to months.
Tiltmeters are installed at stations STAR and OBSR on Mount Rainier. Noise that is seen in the tilt data measured at station STAR is very small during the winter, because a thick blanket of snow and ice insulates the ground. OBSR, on the other hand, appears most stable in the summer because the ground does not undergo freeze/thaw cycles. The data at station OBSR is recorded every 30 seconds and it represents an average value for that period. Data at STAR is recorded and averaged every 20 seconds. Even though data is averaged over 20 to 30 seconds, these tiltmeters often record some of the low-frequency ground vibration from earthquakes, particularly those from large, distant earthquakes (teleseisms).