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 29cm/a of uplift and horizontal displacement between1993–1998, interpreted to be caused by magma accumulationat about 7 km depth (Sigmundsson et al.,1997b; Feigl et al., 2000). Hreinsdóttir (1999) measureda maximum uplift of 8 cm and outward horizontaldisplacement from the centre of Hrómundartindur volcanicsystem by GPS-geodesy. The geodetic monitoringof the Hengill triple junction utilized leveling, tilt,GPS and InSAR. An extensive GPS network isinstalled in the area. The unrest in this area (which islocated about 50 km east of the capital, Reykjavík)spurred the installation of 4 continuously recordingGPS stations, that were up and running in the springof 1999, transferring the data daily to the MeteorologicalOffice in Reykjavík. The activity culminated in1998 with a couple of magnitude 5 earthquakes (Cliftonet al., 2002) after which the unrest terminated.3.9. TorfajökullThe Torfajökull volcano is located at the rift–transformintersection where the Eastern Volcanic Zonemeets the South Iceland Seismic Zone. It is uniqueamong Icelandic volcanoes because of its abundanceof acidic lavas (e.g., Gunnarsson et al., 1998). Eruptionsare sometimes triggered by lateral injection ofbasaltic dykes from the rift zone to the NE, theVeiðivötn fissure swarm of the Bárðarbunga volcanicsystem (McGarvie, 1984). This appears to be true forthe last two eruptions, those of 871 and 1477 AD(McGarvie, 1984; Larsen, 1984). Torfajökull is apersistent source of small earthquakes, both highand low frequency. The distribution of the hypocentersof the high-frequency events led Soosalu andEinarsson (1997) to suggest that they were causedby the cooling of a hot body centered at 8 km depthbeneath the 12 km wide Torfajökull caldera. Tiltmeasurements seem to support this as slow deflationis discernible for the period 1991–2002. Our measurementof the tilt stations in 2003, however, shows thatthe subsidence has turned to inflation.4. DiscussionCrustal deformation studies of active volcanoes inIceland were initiated by Eysteinn Tryggvason in1966, when he set out the first part of the levelingline at the Askja volcano. In the following years, heinstalled short leveling lines at the Katla volcano andstarted to use mobile water-tube tilt meters at Hekla.The profile at Askja was extended in 1968 to 30benchmarks. Eysteinn concluded after a few years ofexperiences with mobile water-tube tilt meters thatoptical leveling gave the same precision and the measurementswere much easier to perform (Tryggvason,1994). With the start of the Krafla rifting episode1975–1984, crustal deformation studies became oneof the most important means to monitor the state ofthe volcano, and its shallow magma chamber. Theinstallation of the water-tube tilt meter (Fig. 4) inthe main building of the Krafla power station thengave a time-series of 11 years for the volcano.The installation of leveling lines and tilt stationssince the mid-sixties has made it possible to observelong time series at active volcanoes. Eysteinn Tryggvasoninstalled several long leveling lines and morethan fifty optical tilt stations (dry-tilt) around volcanoesin Iceland. Over the years some leveling linesand tilt stations have proved to be more sensitive tomagma movements under volcanoes than others. Agood example is the tilt station Næfurholt (Fig. 11) 11km due west of Hekla volcano, which shows inflation/deflation events clearly. Some tilt stations are notparticularly sensitive or are unstable with local conditionsmasking volcano signals. With the introductionof GPS and later InSAR techniques the studies ofcrustal deformation on volcanoes have taken a stepforward. Those techniques have much better spatialand temporal resolution and they are less weatherdependent.In the Hengill volcano a persistent swarm ofabout 85,000 earthquakes were recorded in 1994–1998. In May 1999 a permanent GPS station (OLKEin Fig. 1) was installed close to the measured centerof uplift in Hengill as determined by campaign GPS(Hreinsdóttir, 1999). At the time the continuousGPS-station at OLKE became operational in May1999, crustal deformation ceased. Based on tilt andGPS measurements it has been revealed that intrusionevents occurred within the Eyjafjallajökull volcanoin 1994 and 1999 (Sturkell et al., 2003b). Thetwo permanent GPS stations at the southern edge ofMýrdalsjökull that covers the Katla volcano havedetected displacements (Fig. 14) that are attributedto magma inflow under the Katla volcano (Sturkell
30E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–34et al., 2003c). The Icelandic Meteorological Officeprovides online access to real-time geophysical data,(see http://www.vedur.is/ja). Epicenters and straindata are displayed on this homepage and areuploaded automatically.Of the 35 active volcanic systems in Iceland, deformationdue to magma movements has been measuredat eight of them in the last 15 years. Using a combinationof the different geodetic techniques we have beenable to locate magma chambers with a better precision.The Askja, Krafla, Grímsvötn and Katla volcanoesappear to have a shallow magma chamber atabout 3 km depth. Recent observations suggest thatat least the Askja and Krafla volcanoes have deepermagma reservoirs, suggested to be at 16 and 20 kmdepth, respectively (Sturkell et al., submitted for publication;de Zeeuw-van Dalfsen et al., 2004a,b). Thelocation of the magma chamber under the Heklavolcano has been debated and depth estimates rangefrom 6.5 to 11 km depth. Petrological observationssuggest a magma reservoir under Hekla, as magmaundergoes differentiation with time. However, a studyof the attenuation of seismic waves beneath Heklasuggests that a significant volume of magma doesnot exist above 14 km depth (Soosalu and Einarsson,2004). The discrepancy between these data is thesubject of ongoing research.Comparison of seismicity and deformation duringvolcanic unrest demonstrates great variability. In theHengill area 85,000 earthquakes were recorded duringinflation, but the total uplift was 8 cm during the sametime period. The uplift in the Katla caldera since 1999has been 12 cm but only 221 earthquakes MwN 1.7were recorded within the caldera (Fig. 13). In theneighboring volcano Eyjafjallajökull, which experiencedintrusion events in 1994 and 1999, the earthquakeactivity began before the intrusion events. Thispattern could be observed for both intrusions (Fig.15). These observations suggest volcanoes behaveindividually, e.g., depending on prevailing stressfields.Over this period (the last 15 years), the volcanoesHengill, Hekla, Katla, Eyjafjallajökull, Torfajökulland Grímsvötn had inflow of magma from depthinto shallow-level magma chambers (Fig. 2). TheGrímsvötn volcano was the fastest inflating volcanowith a rate of 3 cm/a at the closest GPS point (Fig. 8)until it erupted 1 November 2004. It is likely thevolcano will start to re-inflate after the eruption in asimilar way as happened in 1998. The Grímsvötncaldera hosts a lake that can contain up to 2–3 km 3of water and can be drained in one flood (Björnsson,2002). Removing such amount of mass directly abovean inflating magma chamber may serve as an eruptiontrigger. The eruption of Grímsvötn on November 1,2004 was preceded by a jökulhlaup that began onOctober 27, suggesting that a pressure drop equivalentto 15 m of water was sufficient to trigger an eruptionfrom the inflated magma chamber. The center of theKatla volcano inflates with a steady rate of 3 cm/a.since year 2000. In addition to the uplift in the centerof the Katla caldera, uplift is observed in the Goðabungaarea (Fig. 12) with 2.6 cm in six months. Thisis interpreted to be in connection with an ascendingdome-shaped structure (Soosalu et al., submitted forpublication).Subsidence takes place at present time at theKrafla and Askja volcanoes. Cooling and consequentcrystallization of magma cause a volume reductionof 10–11%. This process is probably the most dominatingfor the 3 km deep magma camber at Krafla,but also magma draining may be a contributingfactor. The Krafla volcano began to subside in1989, initially at a rate of a few cm/a. Since around2000 the subsidence rate has decreased to a few mm/a. Currently the largest subsidence signal is causedby the extensive exploitation of the geothermal field.The extraction and increased water circulation maycontribute to the cooling of the shallow magmabody. Volcanic geodesy allows observations of thissubsidence and shows its decay rate. Combined withmicro-gravity, it gives an estimate of the amount ofcrystallization and the amount of draining from theshallow magma chamber (de Zeeuw-van Dalfsen etal., in press). The Askja volcano has subsided forover twenty years, and extrapolation of the verticalchange into the 1972–1983 period suggests a totalsubsidence from 1973 to 2003 of 1.9 m at the Askjacenter (Sturkell et al., submitted for publication). Acombination of magma drainage and cooling andcontraction of the shallow magma reservoir at 3km and the deeper one at 16 km depth is ourfavoured model, and is consistent with the integratedmicro-gravity and deformation observations (deZeeuw-van Dalfsen et al., 2004b; Sturkell et al.,submitted for publication). We suggest that exten-
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Volcano geodesy and magma dynamics in Iceland - Acri-ST

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