Volcano geodesy and magma dynamics in Iceland - Acri-ST

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E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–34 23and Grímsvötn volcanoes (Fig. 1). This eruption fissurewas termed Gjálp (Gudmundsson et al., 1997).The eruption began on a 7 km long N–S fissure after36 h of intense seismic activity at Bárðarbunga. Theseismicity propagated towards the eruption site suggestingthat the eruption was fed from Bárðarbunga(Einarsson et al., 1997). The eruptive products werebasaltic andesites, different from recent eruptive productsof both Bárðarbunga and Grímsvötn. The similarityof the isotope characteristics with those ofGrímsvötn has led to suggestions that the eruptionwas fed by Grímsvötn volcano (Sigmarsson et al.,2000). The eruption lasted for thirteen days. In earlyNovember a jökulhlaup of approximately 3.8 km 3 ofmeltwater was released over Skeiðarársandur, shownin Fig. 1 (Gudmundsson et al., 1997).3.5. HeklaHekla volcano has had 18 summit eruptions andfive in its direct surroundings during historic time,the last 1100 years. Precursory events have beendetected prior to recent eruptions. Generally, eruptionrelated earthquakes begin less than two hours beforethe eruption starts (Soosalu et al., in press). Heklawas the first volcano in Iceland to be studied by alocal GPS-network. Measurements before and afterthe 1991 eruption revealed contraction towards thevolcano (Sigmundsson et al., 1992). The data suggesteda magma reservoir at 9 km depth, but uncertaintieson this estimate were large because ofabsence of GPS stations close to the volcano (theclosest being at about 13 km distance). Measurementsprior to the 2000 eruption were conducted in1996. After the eruption measurements have beenconducted annually at selected points. The 1996–2000 displacement reveal clearly deformation associatedwith the feeder dyke for the 2000 eruption(amounting to several tens of cm). Further modellingof these data are needed, in particular to constraineventual inflation/deflation cycle at the volcano. GPSdata combined with all other geodetic data fromHekla, including InSAR and optical leveling tilt hasthe potential to provide improved constraints on themagma plumbing system under Hekla (Sigmundssonet al., 2001).Continuous monitoring of crustal deformation inthe South Iceland seismic zone (including Hekla) isdone by six continuously recording borehole strainmetersoperated by the Icelandic MeteorologicalOffice. The permanent strainmeters are located 15–45 km from Hekla and gave a clear strain signalprior to the eruptions of 1991 and 2000 (Fig. 10).The closest volumetric strain meter (15 km distance,at Búrfell in BUR in Fig. 10) detected compressiondue to dyke injection 30 min before magma reachedthe surface and the eruption started on February 26,2000 (Fig. 10). However, it had long been suspectedthat large strain excursions at BUR were accompaniedby localized recovery, which may bias estimatesof magma source depth. This was confirmed bystrain records for the Mw 6.5 South Iceland earthquakes(the first occurring on 17 June and the secondon 21 June, 2000). The original interpretation ofthe strain data at BUR associated with the 1991 and2000 eruptions of Hekla gave too shallow a sourcedepth. By considering the localized strain recovery,Jónsson et al. (2003) prefer a deeper location thanthe 6.5 km suggested by Linde et al. (1993) for themagma source. The most recent estimate based onre-interpretation of strain data places the magmachamber at 11 km depth (Kristján Ágústsson pers.com. 2004). Despite these complications, the combinationof seismic and volumetric strain monitoringhas given strong evidence of magma propagationtowards the Earth’s surface and was used to issuea warning prior to the 2000 eruption.Seven short leveling arrays (bdry-tiltQ stations)are surveyed repeatedly around Hekla. The beststation, Næfurholt, is located 11 km west of thesummit, allowing for easy monitoring of E–W tilt.The tilt signal from the Næfurholt station is rathersmall but is still significant (Fig. 11). It suggestsinflation beginning directly after the 1991 and 2000eruptions. Eastward upward tilt generally increaseswith time, as the volcano builds up pressure for thenext eruption, when a co-eruptive pressure decreaseoccurs. The most recent measurement (18 Nov.2004) of the tilt station at Næfurholt shows upwardtilt towards Hekla (Fig. 11). The east componentindicates that a pressure increase under the volcanois of the same scale as prior to the 1991 and 2000eruptions. It should be pointed out that one tiltstation gives only an indication of possible magmaaccumulation under a volcano, and it should spursome additional measurements.
24E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–34200GEL100SAUSTOHELnanostrain (extension )0-100-200BURSKANANOSTRAIN1000800600400STOSAUGELSKABUR200-3000 17 18 19 20 21 22Time (Days)26 27 28 29 1February 2000March 2000Fig. 10. Five days of data spanning Hekla’s 2000 eruption from borehole strainmeters after removal of the effects of atmospheric pressurevariations and earth tides. Expansion is positive. The inset shows the strainmeter record for the 1991 Hekla eruption, which displays an almostidentical record. The squares show the strain changes calculated from modeling the eruption in two stages. Stage one shows the deflation of adeep reservoir together with the dyke formation and stage two with continued deflation of the deeper source. For location of the strainmeters seeFig. 1. (Modified from Agustsson et al., 2000).East tilt (µrad)50Eruption1985 1990 1995 2000 2005YearFig. 11. Data from the Næfurholt tilt station located 11 km directly west of the Hekla volcano. Its east–west tilt shows the radial tilt componentof deformation of the volcano. The observed tilt pattern repeats it self, suggesting a cyclic pattern, with upward tilt in the direction of the Heklavolcano prior to the eruptions, followed by subsidence as magma drains from beneath the volcano. The most recent measurement (18 Nov 2004)indicates that the east–west tilt is close to the critical value.Eruption
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