Plate Boundary Observatory borehole strainmeters measure very small changes in the dimension of a borehole at depths ranging from 100 m to 250 m. This is accomplished by measuring the change in diameter or volume of a strainmeter installed in the borehole.
The Plate Boundary Observatory uses a instrument developed and constructed by GTSM Technologies which measure the change in borehole diameter along three azimuths separated by 120 degrees perpendicular to the borehole. Strainmeters can detect changes in the diameter of a borehole on the order of four picometers, about one ten millionth of the width of a human hair and smaller than the width of a Hydrogen atom. A change of four picometers along the ten cm width of the strainmeters equates to approximately 0.05 nanostrain while signals of geophysical interest range from several nanostrain to many hundreds of nanostrain. For example the strains induced by earth tides, the effect of the Sun and Moon on the Earth's crust, are ten's of nanostrain.
Seventy-Five borehole strainmeters were installed as part of the Plate Boundary Observatory. They are located along the West Coast of the United States and Canada ranging from Vancouver Island to Souther California, with a focused array of five strainmeters located around the Yellowstone Caldera. The strainmeters are grouped into arrays of two to nine instruments each targeted at a particular geophysical process ranging from volcanism to subduction zones and major strike slip faulting.
A typical installation starts with a borehole that is 15 cm in diameter and approximately 200 m deep, the actual depth depends on the location of desirable rock. A strainmeter is installed at the bottom of the borehole with specialized expansive grout. A seismometer is then installed from three to ten meters above the strainmeter. The upper portion of the borehole is used for auxiliary instruments such as: pore pressure transducers, tiltmeters and continuos thermal measurements. In total the borehole network contains nearly 300 instruments and produces close to 1800 channels of data.
From a science perspective, the PBO component of EarthScope is designed to study the three-dimensional strain field in the western United States (see more) that results from deformation across the active boundary zone between the Pacific and North American plates.