An accurate Invar-wire extensometer

Stevens Type F water-level recorders have been modified to measure changes in the distance between two points, with amplification of up to 10 times. Such recorders are capable of measuring strains of 10^-5 and possibly 10^-6 over 10-m distances when corrections are made for frictional effects and temperature. In two field experiments, one on Kilauea Volcano in Hawaii, and the other on the San Andreas fault in central California, measurements by the recorders agree well with those by several different methods for the same episodes of ground deformation. Equipment for the entire system costs about $300 and requires 1 to 2 man-days for installation.