A strainmeter is an instrument used by geophysicists to measure the deformation of the earth. Linear strainmeters measure the changes in the distance between two points, using either a solid piece of equipment or a laser interferometer. Benioff invented the type using a solid length standard in 1932 using an iron pipe; later instruments used rods of fuses quartz, which is the most abundant mineral in the earth’s continental crust. Modern instruments of this type can make measurements of length change over small distances, and are commonly placed in borehole, which is a narrow shaft drilled in the ground either vertically or horizontally. The borehole instruments detect changes in a volume filled with fluid. The most popular type is the dilatometer invented by Sacks and Evertson in the US; a design that uses specially shaped volumes to measure the strain tensor.
All these types of strainmeters can measure deformation over time from a few hours to periods of days, months, and years. This provides them to measure signals in lower frequencies that can be detected with seismometers. The most strainmeter records shoe signs from the earth’s tides, and seismic waves from earthquakes. At longer time periods, they can also record gradual accumulati9on of stress caused by plate tectonics, the release of this stress in earthquakes, and rapid changes of stress following earthquakes.
The most extensive network of strainmeters is installed in Japan; it includes mostly quartz-bar instruments in tunnels and boreholes strainmeters, with a few laser instruments. Starting in 2003 there has been a major effort to install many more strainmeters along the Pacific/ North America plate boundary in the US. The aim is to install about 100 boreholes, strainmeters, primarily in Washington, Oregon, and California and five laser strainmeters in California. Strain meters help scientists a lot in time waiting.