by Erik Sturkell
As Hekla is one of the most active volcanoes of Iceland
today, crustal deformation of the volcano has been studied by various means,
including optical levelling, tilt, electronic distance measurements, Global
Positional System geodetic measurements, borehole strainmeters, and satellite
Few people live in the direct surroundings of the volcano, and no inhabited areas are threatened by lava flows from the current eruption. By the same token, very few roads exist close to the volcano, which causes logistic problems in the crustal deformations studies of Hekla.
Continuous monitoring of the crustal deformation of Hekla is today performed by five continuously recording borehole strainmeters, operated by the Icelandic Meteorological Office (http://www.vedur.is/). The permanent strainmeters are located 15-45 km from the volcano.
Eysteinn Tryggvason installed three water-tube tilt stations to the west and the north of the Hekla summit in 1968. By 1970 it had become clear that optical levelling gave the same precision as water-tube tilt meters but the measurements were much easier to perform so the water tube-tilt meters were abandoned and optical tilt stations were established (Tryggvason, 1994).
In 1970 three optical levelling stations had been installed for tilt measurements. They consist of an array of ten or eleven permanent benchmarks. Today six optical levelling stations are present. The closest tilt station is located below Litla Hekla 4 km north of the Hekla summit. This station gave a tilt of 2918 microrad in the period Aug. 1968 to Aug. 1970. This time span includes the eruption 1970 (Tryggvason, 1994).
The optical levelling tilt measurements showed similar inflation rate in the month prior to the eruption 1991 as were observed in the ten-year period between the last two eruptions. This indicates that no significant pre-eruptive changes occurred in the deformation rate (Sigmundsson and Einarsson, 1996). In 1999 only the tilt station at Næfurholt, located 11.6 km from the summit, was measured at 20th Oct, showing minor amounts of tilt since 1998, and giving no indication of the coming eruption.
After the 1980-1981 eruption, electronic distance measurements (EDM) were carried out on a net with nine benchmarks on 17 May 1998 and were repeated 19 March 1982. Uplift and expansion of the volcano was detected by these distance measurements. Kjartansson and Gronvold (1983) interpreted these results as magma inflow into a reservoir at a depth of 7 - 8.5 km, approximately 2 km south of the summit. This network has not been re-measured on a systematic basis since 1985.
Measurements with GPS have been performed repeatedly around Hekla since 1989 with an increasingly denser network around the volcano. Sigmundsson et al. (1992) re-measured the 1989 network in 1991 after the eruption that year. This survey indicated that surface deflation centred at 63.995 degrees N, 19.96 degrees W was associated with the eruption. Due to the large distance of the nearest of the re-measured GPS control points from the volcano (13 km from the summit) the depth of the point source at 9 km is poorly constrained. The network was re-measured and made denser in 1993 with a point only 4 km from the summit. These points were then re-surveyed 1996. The solution for the period 1993-1996 showed only minor movements at Hekla, within the uncertainty limits of the measurements.
Crustal deformation observations of the Hekla volcano with InSAR image show subsidence in recent lava fields in the period 1992 - 1998 (Sigmundsson et al. 1999).
The magma reservoir feeding Hekla is thought to be deep
seated. Crustal deformation at Hekla has been modelled with a point source
ranging in depth from 5 - 9 km (Tryggvason, 1994; Sigmundsson et al., 1992).
Linde et al. (1993) propose a magma reservoir with centroid depth of 6.5
km with its top at 4 km depth, interpreted from data collected by the five
continuously monitoring borehole strainmeters during to the 1991 eruption
of Hekla. A deep-seated point source gives a wide deformation field, therefore
stations at some distance may give a good tilt signal generated by inflation/deflation
of magma reservoir.
Kjartansson, E., and K. Gronvold, Location of a magma reservoir beneath Hekla volcano, Iceland, Nature, 301, 139-141, 1983