Strainmeter

The type using a solid length standard was invented by Benioff in 1932, using an iron pipe; later instruments used rods made of fused quartz.

Another type of borehole instrument detects changes in a volume filled with fluid (such as silicone oil).

The most common type is the dilatometer invented by Sacks and Evertson in the USA (patent 3,635,076); a design that uses specially shaped volumes to measure the strain tensor has been developed by Sakata in Japan.

All these types of strainmeters can measure deformation over frequencies from a few Hz to periods of days, months, and years.

Most strainmeter records show signals from the earth tides, and seismic waves from earthquakes.