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) 14–34 253.6. KatlaKatla volcano has had 20 eruptions since thesettlement period in 874 AD. (Larsen, 2000). Itscaldera, which is 600–700 m deep and encirclesan area of 100 km 2 (Björnsson et al., 2000) iscovered by the Mýrdalsjökull ice cap. The initialphase of its eruption is phreatomagmatic due to itsoccurrence at the base of the ice cap. Meltwaterfrom the eruption site is released as a jökulhlaup atthe ice margin. Such floods are characterized by arapid rise to maximum discharge within a period ofa few hours (Roberts et al., 2003). The most recentlarge eruption took place in 1918, generating amassive jökulhlaup. In 1955 a smaller jökulhlaupemerged from the eastern side of Mýrdalsjökull andtwo cauldrons were formed in the ice cap (Rist,1967). Guðmundsson et al. (2000) suggested thatthe jökulhlaup was caused by a small sub-glacialeruption that did not break the ice surface. In July1999 a sudden jökulhlaup occurred at the Sólheimajökulloutlet glacier (Fig. 12) on the south side ofthe Mýrdalsjökull ice cap (Sigurðsson et al., 2000).This was preceded by changes in earthquake activityand accompanied by tremor bursts (Einarsson,2000). A minor sub-glacial eruption or a shallowintrusion could have triggered this jökulhlaup(Guðmundsson et al., 2000).The Katla area has shown persistent seismic activitysince the beginning of seismograph observationsin the first half of the last century. Einarsson andBrandsdóttir (2000) analyzed seismic data from theperiod 1978 to 1985, and found that the seismicitywas mostly located in two areas, one within thecaldera and the other in the Goðabunga area to thewest of the caldera (Fig. 12). The seismicity has adistinct seasonal pattern, particularly in the Godabungacluster (Fig. 13), with a large majority ofearthquakes occurring in the second half of theyear. This pattern has persisted for at least fourdecades. The autumn activity in 2001 was intensebut within normal limits. Seismic activity, however,did not stop in the winter of 2002 as it would havein a normal year and seismicity has been high eversince. In a recent study by Soosalu et al. (in press) alocal seismic net was set up on and around the63º45'NHAMRENTAGo a-bungaThe KatlacalderaAUST63º30'N1 cmSólheimajökullSOHOHVOL10 km20ºW 19ºW19º15'WFig. 12. Observed horizontal and vertical displacements at Katla from June 2001 to March 2003, measured by GPS. The black arrows showhorizontal displacement, and the open arrows show vertical movements. All displacements are relative to the reference station at HAMR(Hamragarðar). This uplift and outward displacement from the centre of the caldera suggests magma inflow in a magma chamber, possiblelocated at 3–5 km depth. The vertical error is up to 7 mm.
26E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–346050M 0 *10E+13 [Nm]40302010Go abungaKatla caldera01997 1998 1999 2000 2001 2002 2003 2004 2005YearFig. 13. Accumulated seismic moment release from 1997 to 2004 in the Katla volcano. The area is divided into two sub-areas; the Katla calderaand the area west of the caldera, the Goðabunga area. The division is drawn along a north–south line at 198 15V W (see Fig. 12). The moment–magnitude relation log M 0 =M l +10 from Slunga (1991) is used, where M 0 is the moment (in Nm) and M l the magnitude, for the 2775earthquakes with M 1 N1.7 recorded since the beginning of 1997 (2554 eq. in Goðabunga, 221 in the caldera). The earthquakes under Goðabungashow a clear seasonal correlation with a distinct activity during the autumn. This pattern changed in 2002 to more continuous activity. Theearthquakes in the caldera have a swarm character, and the largest earthquakes occur there.glacier, with one station directly above the center ofthe activity in the Goðabunga area. This study gave amuch better depth constraint for the hypocentrallocations and suggests that the Goðabunga clusteris an expression of an ascending, dome-shaped structurecreated by accumulation of viscous magma atshallow depth (cryptodome).The July 1999 event at Katla spurred both tilt andcampaign GPS measurements. Tilt measurementsshowed no detectable displacements at the Katlavolcano, but the stations are located z12 km fromthe center of the Katla caldera. However, inflation ofthe neighboring volcano Eyjafjallajökull wasdetected (see the Eyjafjallajökull section). CampaignGPS measurements on nunataks in the ice cap, theENTA and AUST sites (Fig. 12), have providedimportant results. The first campaign measurementsthat include the nunatak stations were performed in1993, and repeated for the first time in July 2000.The two stations displayed outward displacementfrom the center of the caldera, during this period,indicating that magma accumulation had started(Sturkell et al., 2003b). The nunatak sites havebeen visited annually since year 2000, and recentlytwice a year. The GPS measurements for the period2001 to 2003 is shown in Fig. 12 and display upliftand outward horizontal displacement from the centreof the sub-glacial caldera. These displacements fitwell with a point source at 4.7 km depth within thecaldera and give an uplift rate of about 2 cm/a since2000 at the GPS point AUST (Sturkell et al.,2003c).The Katla GPS-network is complemented by twocontinuous GPS-stations, (SOHO and HVOL), whichwere installed in the autumn of 1999, at the southernflank (Fig. 12). After considering expected platemovements, they display residual outward displacements(southward) from the volcano centre (Fig. 14).This southward displacement has been observed sincethe year 2000. The southward displacement observedfrom the continuous stations is not uniform in timeand shows periods with accelerated southward displacements.This suggests eventual annual component ofdeformation related to variable ice loading. Altogether,the geodetic data suggest progressive calderainflation over time. From 2000, the measured sub-
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