Háskóli Íslands

Doctoral defence in Geophysics - Ásdís Benediktsdóttir

Dissertation title: A Geophysical Study of Propagating Rifts at the Bight-Transform Fault on the southern part of the Reykjanes Ridge and in the Eastern Volcanic Zone of Iceland

Opponents:
Dr. Jason Phipps Morgan, Professor at Royal Holloway, University of London
Dr. Nicholas Rawlinson, Professor at the University of Cambridge

Advisors and Doctoral committee:

Ólafur Guðmundsson, Professor at the Department of Earth Sciences, Uppsala University, Sweden

Freysteinn Sigmundsson, Research Scientist at the Institute of Earth Sciences, University of Iceland

Bryndís Brandsdóttir, Research Scientist at the Institute of Earth Sciences, University of Iceland

Chair of Ceremony: Dr. Andri Stefánsson, Professor and the Head of the Faculty of Earth Sciences at the University of Iceland.

Abstract:

Iceland is located on the Mid-Atlantic Ridge and is the surface exposure of the ridge system on land. The influence of the plume has been shown to reach towards the south at least as far as the Bight Transform Fault. The ridge system in Iceland has also been evolving spatially and temporally during the life time of Iceland. One of the best documented and understood mechanism, where by plate boundaries evolve, is the propagating rift mechanism. Two such areas are studied in the dissertation; Eyjafjallajökull volcano in South Iceland and the area between the Bight-Transform Fault and 80 km south of it. These two areas are both located in front of the advancing limb of a propagating rift.
Two papers focus on the Eyjafjallajökull volcano area. In the first paper the velocity structure of the volcano is presented. A fairly new method is used where long-time series of small surface waves are used as a source for tomography. The signal source of the method is the ambient noise, originating in the oceans. If such persistent signal is observed over long enough time and then cross-correlated between two seismic stations, a signal emerges that contains information on the path between the two receivers. Using data from a 7 month long period from stations located around the volcano, ambient seismic tomography was possible around Eyjafjallajökull. Reliable phase dispersion curves were obtained between 1-7 s and sensitivity kernels showed resolution down to 10 kilometers depth. The results show two high-velocity zones elongated E-W on either side of caldera. In between a zone of relatively lower-velocity is observed. The lower-velocity zone coincides with an inferred magma pathway under the volcano as observed from earthquakes in the 2010 Eyjafjallajökull volcano eruption. The higher-velocity zones are interpreted as zones of intrusive rocks. 
The second paper on Eyjafjallajökull focuses on the tremor of the 2010 eruption. Tremor can be located in several different ways, e.g. if the source is isotropic a simple amplitude decay with distance can be used. This was not the case with the 2010 Eyjafjallajökull eruption. Tremor levels increased during the eruption and the tremor evolution during the eruption can be linked with other parameters, such as the eruption mechanism (i.e. effusive vs. explosive phase of the eruption). The paper addresses two main issues; location of the tremor source and analyses of the power ratios of different time series from different seismic stations. The location of the tremor was very stable in the 0.5-2 Hz frequency range during the entire span of the eruption. The location agrees well with the cauldrons formed in the eruption. Because the location was stable, many parameters in the amplitude decay equation stayed the same during the course of the eruption giving rise to a simple parameter-estimation problem with ample redundancy. Results suggests that the tremor was mostly composed of surface waves and that the Q values with in the volcano are in the range of about 15. For paths outside the volcano Q is between 40 and 50. 
The second study area, the first 80 km south of the Bight Transform Fault (BTF), mark the location where the spreading on the Mid-Atlantic Ridge changes from 30° oblique to the trend of the ridge to the north of the transform, to the expected, orthogonal spreading relative to the trend of the ridge. By analysing magnetic anomalies <6 Ma in the region, the kinematic evolution of the area was revealed. A series of small-scale propagators transfer lithosphere from the Eurasian Plate to the North-American Plate. This observed asymmetric accretion of lithosphere fits well with the asymmetric crustal accretion that is found to the north, on the Reykjanes Ridge and on Iceland. The asymmetry to the north of the BTF can be attributed to the movement of the ridge system relative to the Iceland hotspot. It can therefore, be concluded that the influence of the Iceland hotspot reaches farther south than the BTF, 1000 km south of the Iceland hotspot.

About the doctoral candidate:

Ásdís is born in Reykjavik in 1984. After she graduated from the Reykjavík Junior College in 2004 she started an her undergraduate studies and finished with a B.Sc. degree in geophysics from University of Iceland in 2007. She then moved from one hotspot to another and finished a Masters degree in geophysics from the University of Hawaii in 2011.

After moving back to Iceland, she started her Ph.D. work, with a few breaks. Ásdís has two daughters. In 2012 Aníta came into the world and in 2014 Tanja was added to the family. Ásdís is married to software engineer Tryggvi Björgvinsson. She works at Iceland Geosurvey and has been an employ there since March 2016. The waves found in the Earth are not the only ones that get her attention, as she enjoys singing most of the day.

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