Volcano geodesy and magma dynamics in Iceland - Acri-ST

More documents

Recommendations

Info

E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–34 21Cumulative vertical displacements [m]0.100.050.00-0.05-0.10-0.15-0.20(-(year - 1983) / 39.2)y = -49.8 + 45.7e1970 1975 1980 1985 1990 1995 2000YearFig. 7. Time series showing cumulative vertical displacements atAskja between benchmark A406 and A429 since 1968 (modifiedfrom Sturkell et al., submitted for publication). The time seriesprovides a measure of height change at the caldera centre, assuming95% of the deformation (height differences between A406 andA429) is due to the shallow Mogi source (Sturkell et al., submittedfor publication). The decay starting in 1983 is exponential. Tracingthe subsidence curve back and extrapolating the inflation trend, thechange from inflation to deflation occurred in the middle of 1973.The Krafla rifting episode started in December 1975.accounts for most of the observed displacements inthe main caldera and its immediate vicinity. At agreater distance, however, displacements observedwith GPS do not show the same good fit. A moreelaborate model is presented by Sturkell et al. (submittedfor publication) who invoke two Mogi sourcesto account for the far field displacements (Fig. 6). Theshallow source at 3 km depth and the deeper at 16 kmdepth. The shallow source contributes to an estimatedannual subsidence of 4.2 cm and the deeper sourceabout 1.0 cm. This model was also applied to theresults of a micro-gravity study for the period 1988–2003. A sub-surface mass decrease of 1.610 11 kg isderived (de Zeeuw-van Dalfsen et al., 2004b) indicatingthat magma drainage is an important contributor tothe subsurface mass decrease and therefore to thesurface deflation. Interferometric analysis of syntheticaperture radar images from radar satellites (InSAR)have also been conducted (Pagli et al., 2003), andshow clearly the ongoing subsidence at Askja. TheInSAR data are being further analyzed. The geometryof the deeper pressure source has not been resolved.Sturkell et al. (submitted for publication) suggest thatthe onset of deflation at Askja in 1973 is causallyrelated to the beginning of magma flow into the Kraflamagma chamber about 75 km to the north whichpreceded the rifting episode of 1975–1984. We alsopoint out that a series of large earthquakes began atthe Bárðarbunga volcano in 1974 that lasted until1996. The reverse faulting mechanisms of theseevents were interpreted by Einarsson (1991b) as theexpression of deflation of Bárðarbunga. The temporalcorrelation of deflation of Bárðarbunga and Askja andthe beginning of activity at Krafla suggests linksbetween the volcanoes, eventually by pressure connectionthrough the lower ductile crust in Iceland(Einarsson, 1991b).3.3. GrímsvötnThis highly active volcano most recently erupted inDecember 1998 and November 2004. Grímsvötn is asubglacial volcano, and deformation can only be measuredat a campaign GPS station located at the solenunatak on the caldera rim. Fig. 8 shows verticaldisplacement of the GPS point at Grímsvötn(GRIM). GPS measurements showed uplift beforethe 1998 eruption, followed by subsidence as thepressure dropped in the magma chamber. Measurementswere fitted to a Mogi model with the assumptionthat the source was located under the centre of theGrímsvötn caldera complex at a depth of at least 1.6km (Sturkell et al., 2003a). Inflation resumed atGrímsvötn following the 1998 eruption and it wasassumed that the pressure of the Mogi-source wouldhave to surpass a critical level before a new eruptionwas to be expected. The critical level of the verticalcomponent of the GRIM station was suggested to beat least 0.15–0.20 m above the minimum heightobtained immediately after the 1998 eruption (Sturkellet al., 2003a). A measurement in September 2004showed that the inflation had exceeded this level.Seismic activity increased markedly in the latter halfof the year 2003 (Fig. 9) showing that the stress in themagma chamber roof had increased. Further increaseof seismicity in late October and the onset of ajökulhlaup (a flood of melt water) from the Grímsvötncaldera lake on October 27 suggested that an eruptionmight be imminent, triggered by the pressure releaseof the flood. An eruption began in the evening of
22E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–340.15vertical displacement [m]0.100.050.00-0.05GjalpGrímsvötnGrímsvötn-0.101995 2000 2005YearFig. 8. The vertical displacement of the GPS statin at Grímsvötn (located at the top of the caldera rim) relative the reference point at Jökulheimar(JOKU in Fig. 1). The vertical displacement shows uplift prior to the 1998 eruption followed by subsidence as the pressure dropped in themagma chamber. Following the 1998-eruption pressure increase is observed, the 1997 inflation level was passed in 2003 and on 1 November2004 the volcano erupted. The timing of the eruptions at Gjálp in 1996 and Grímsvötn in 1998 and 2004 are shown by vertical bars. Modifiedfrom Sturkell et al. (2003a) with recent data.November 1 as indicated by the appearance of lowfrequencyeruption tremor on nearby seismographs.The eruption onset was preceded by an intense swarmof earthquakes. The geodetic and seismic precursorsto the eruption were recognized and formed the basisfor rather detailed eruption warnings issued to thecivil defense authorities, both in the long-term(months and year), intermediate-term (weeks anddays) and short-term (hours) (Vogfjörd et al., 2005).3.4. GjálpIn the beginning of October 1996, a sub-glacialeruption started on a fissure between the Bárðarbunga43start ofthe eruptionM l2102002 2003Year2004 2005Fig. 9. Magnitude of Grímsvötn earthquakes as a function of time, showing increased activity since a seismic station was installed at Grímsvötnin 2002, until the 25 November 2004. The Grímsvötn volcano erupted in the evening on 1 November 2004. All earthquakes larger thanmagnitude 1.0 that have been observed at four or more seismic stations are included.
Page 1: Journal of Volcanology and Geotherm
Page 4 and 5: E. Sturkell et al. / Journal of Vol

Volcano geodesy and magma dynamics in Iceland - Acri-ST

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?