Borehole observations of continuous strain and fluid pressure: Chapter 9

Strain is expansion, contraction, or distortion of the volcanic edifice and surrounding crust. As a result of magma movement, volcanoes may undergo enormous strain prior to and during eruption. Global Positioning System (GPS) observations can in principle be used to determine strain by taking the difference between two nearby observations and dividing by the distance between them. Two GPS stations 1 km apart, each providing displacement information accurate to the nearest millimeter, could detect strain as small as 2 mm km^-1, or 2 × 10^-6. It is possible, however, to measure strains at least three orders of magnitude smaller using borehole strainmeters. In fact, it is even possible to measure strains as small as 10^-8 using observations of groundwater levels in boreholes.