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 19µrad1000900 20 Dec. 758007006005004003002001000 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 198926 Sep. 7631 Oct. 7620 Jan. 7727 Apr. 778 Sep. 777 Jan. 7810 Jul. 7810 Nov. 7813 May 7916 Mar. 8010 July 8018 Oct. 8030 Jan. 8118 Nov. 81Fig. 4. Record of the one component water-tube tiltmeter at the Krafla power plant from August 1976 to January 1990. Data before August 1976originate from leveling measurements of the power plant foundation. The water-tube tiltmeter was installed along the longer wall of the powerplant building that is orientated N138 E. The figure is modified after Tryggvason (1995). The stars mark eruptions.4 Sep. 84velocity within the Krafla caldera (Einarsson, 1978;Brandsdóttir et al., 1997). Several lines of evidencesuggest a deeper magma chamber at Krafla in additionto the one at 3 km depth. The continuous flow ofmagma towards the shallow chamber for over a decadeduring the Krafla fires suggest magma was transportedfrom a localized reservoir or magma storage area at adeeper level in the crust or mantle. The location of it isuncertain, but geodetic data shows that deformation onthe surface cannot fully be explained by the 3 km deepchamber. The deformation field is wider thanexpected. Tryggvason (1986) interpreted recordsfrom tiltmeters during the September 1984 eruptionin terms of three stacked magma chambers, but locationof the two deeper ones was not well resolved. Therole of two deeper sources, rather than one, in the 1984eruption, is also not certain. Árnadóttir et al. (1998)interpreted EDM and leveling data from the 1984eruption and confirmed that magma was drainedfrom a deeper source in addition to the 3 km deepsource, to form a 1 m wide, 9 km long dyke, extendingfrom the surface to about 7 km depth, and lava flowson the surface. The location of the deeper source wasconstrained to be deeper than 5 km, but narrow apertureof the geodetic network did not allow furtherresolution of its depth. The deformation associatedwith dyke intrusion events during the Krafla fires isfor most events not well constrained. Leveling profilescrossing the fissure swarm in two places (Sigurdsson,1980; Kanngiesser, 1983) show how the flanks of thefissure swarm were uplifted while a central blockabove the dikes was downfaulted.The caldera region inflated after the last eruption(1984) until the pre-eruption level was approached.Then the inflation became intermittent. The last inflationperiod occurred in the summer of 1989. After thatthe caldera region has been slowly subsiding, a fewcentimeters per year, as determined by InSAR (Sigmundssonet al., 1997a) and a combined geodetic andmicro-gravity study (Rymer et al., 1998). In the postriftingperiod, GPS revealed higher than averagespreading rates. The post-rifting deformation is fitby an elastic upper crust overlying a visco-elasticlower crust and even weaker visco-elastic upper mantle(e.g., Foulger et al., 1992; Pollitz and Sacks, 1996).A recent InSAR study (de Zeeuw-van Dalfsen etal., 2004a) has revealed uplift over an about 50-kmwidearea at the Krafla volcanic system (8 cm from1993 to 1999), in addition to deformation caused bysubsidence above the shallow magma chamber, andeffects of plate spreading and post-rifting adjustment(Fig. 5). The favored explanation of de Zeeuw-vanDalfsen et al. (2004a) for the widespread uplift is thatit results from magma accumulation at the crust–mantleboundary at 20 km depth, rather than being causedby post-rifting adjustment.3.2. AskjaThe Askja volcano in the Northern Volcanic Zone(Fig. 1) has three overlapping and nested calderas.The main caldera, formed in the early Holocene, is 8km in diameter. An older caldera is discernable but ithas been filled by later lava flows. The most recentcaldera, Öskjuvatn, is 4.5 km in diameter and is filledby a lake. It was formed after the Plinian eruption of1875. The most recent eruption of Askja took place in1961 when a short E–W fissure opened up near theeastern caldera fault of the main caldera. A shortleveling line was installed in the main caldera in
20E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–34AH66NM1GM2KL10 km16WB 93-99curve with a decay constant of 39 years (Sturkell etal., submitted for publication). The geodetic data havebeen modeled and interpreted in terms of a singleMogi-type pressure source (Mogi, 1958) locatedclose to the centre of the main Askja caldera (Tryggvason,1989; Rymer and Tryggvason, 1993; Sturkelland Sigmundsson, 2000). All these authors placed thepoint source at 1.5 to 3.5 km depth. This modelA4291 km66NA406A404DYNG10 kmFig. 5. A) A InSAR amplitude image which includes the outline ofthe Krafla fissure swarm (dotted line) the Krafla central volcano(solid white line), the caldera (white dashed line) and the location oftwo model sources, a shallow magma chamber (M2) and about 20km deep source (M1). The location of the area is given in Fig. 1. B)An interferogram spanning the period from 1993 to 1999 showingseveral features including subsidence above a shallow magmachamber (M2), inflation due to suggested deep magma accumulation(M1) and a N–S linear feature relating to plate spreading.Modified from de Zeeuw-van Dalfsen et al. (2004a).65º10'N65º05'N í1966. The line was extended to 1.7 km length (Fig. 6)in 1968 and measured annually until 1972. Annualmeasurements were resumed in 1983 and the line wasalso extended. Because not all benchmarks are foundevery year due to snow cover, and because the entireline lies within the deformation field of the volcano,benchmark 404 is arbitrarily set to zero. This point hasin recent years been measured annually by GPS.To visualize the changes along this line we haveplotted the height difference between the end points,A429 and A406, as a function of time (Fig. 7). StationA406 is 1 km closer to the caldera centre than A429.The data show that the volcano was inflating during1970–1972. This had turned to deflation when measurementswere resumed in 1983, a trend that hascontinued since. The deflation follows an exponential65º00'Nkm0 5 10Öskjuvatn17º 00’W 16º 50’W 16º 40’W5 cmFig. 6. Horizontal GPS-displacements 1993–1998 in the Askjavolcano. The model with the two Mogi sources gave an estimateof the displacement of the reference station DYNG (white arrow).The horizontal displacements are adjusted for the displacement ofthe reference station. (Modified from Sturkell et al., submitted forpublication). Inset shows the location of leveling benchmarksdescribed in the text.
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