In situ test of a borehole extensometer

Summary Stresses in the Earth's crust are known to represent a key element of geodynamical processes at various spatial and temporal scales. Static stresses within lithospheric plates can be estimated on the basis of direct in situ measurements in boreholes or near the surface. Earthquake focal mechanisms yield the same type of information by sampling at greater depths. The quantification of geological deformations happens to be the principal tool to determine possible changes in time of the tectonic stress pattern. These changes can however be recorded only qualitatively and on the geological time scale.In seismic regions it is clear that the regional stress field varies on a time scale defined by the recurrence of earthquakes, say several tens of years. Our goal was to design a robust and cheap instrument which could continuously monitor stress changes at depths large enough to reduce meteorological influences.