Distance Measurements across the Heimaey Eruptive Fissure

IN describing the latest Icelandic eruption, Kirkjufell, Thorarinsson et al.1 showed how it began as an erupting fissure about 1,600 m long and how activity ceased on all but the central three vents after 3 d. We have made precise distance measure ments between four fixed stations located around the southern end of the fissure (Fig. 1) on two visits in February and April. The measurements, made with the National Physical Laboratory Mekometer III prototype2,3, corrected for atmospheric refractive index and modulating frequency, are shown plotted with their expected errors against time in days after January 23 on Fig. 3. Also shown on Fig. 3 are the curves of the form d=d0(1–exp(aT)) where d is the strain at time T, d0 is the strain at time zero and a is the inverse of a time constant, which best fit the observed results. Each best fit has the same time constant, 7.5 d, and is in good agreement with the observations, excepting the April result of line 3–1A.