Deformation at Katla volcano, Iceland, 2003-2009: Disentangling surface displacements due to ice load reduction and magma movement using InSAR time series analysis Cover images from left to right: Lava flow from the 2010 Eyjafjallajökull eruption, by Kristján Freyr Þrastarson Interferogram of the 2010 Eyjafjallajökull eruption Ice floating in Jökulsárlon glacial lake, by Júlía Runólfsdóttir Deformation at Katla volcano, Iceland, 2003-2009: Disentangling surface displacements due to ice load reduction and magma movement using InSAR time series analysis M.Sc. Thesis Karsten H. Spaans March 2011 Delft University of Technology Faculty of Aerospace Engineering Department of Earth Observation and Space Systems Main supervisor: Dr. A.J. Hooper Promotor: Prof. dr. ir. R.F. Hanssen Committee member: Dr. R.C. Lindenbergh Copyright 2011, K.H. Spaans All rights reserved. PREFACE This report was written as part of the thesis work required for the Aerospace Engineering Master program at Delft University of Technology. I enrolled in the Earth and Planetary Observation master track in September 2008, after struggling through the Bachelor phase, which I had started in 2001. During the Bachelor program, the Earth and Planetary Ob- servation minor program is what rekindled my interest in finishing both my Bachelor and Master degrees at the faculty of Aerospace Engineering. Looking back, the decision to get into the field of Earth Observation was the single best decision I made during my University years. My interest mainly went out to remote sensing and its applications, specifically how ad- ditional layers of information can be extracted from seemingly very simple measurements. So when the time came to choose a thesis subject, it was radar interferometry that caught my eye. Although geophysical processes were initially not my main interest, a topic deal- ing with surface deformations around a volcano in Iceland, Katla, appealed to me most. The topic combined technical work on InSAR processing with interpretation work on a specific application, which was exactly what I was looking for. An added bonus of the thesis subject was that it could be combined with an internship in Iceland. The internship is also part of the Master program of Aerospace Engineering. Working at the University of Iceland for three months allowed me to improve my geophys- ical understanding of volcanic and tectonic processes, as well as get experience in doing fieldwork. The combined work done during my internship and thesis forms the culmination of all my years spent at the faculty of Aerospace Engineering, and gave me the sense of achievement often missing during the previous years. This report contains the account of the achievements obtained during my thesis work. i The guidance and support of my supervisor, Dr. Andy Hooper, was invaluable to com- plete the project successfully. Also, I am grateful to Dr. Freysteinn Sigmundsson and Prof. Ramon Hanssen for their advice throughout the project. I would like to thank Peter Schmidt and Björn Lund from Uppsala University, as well as Þóra Árnadóttir from the University of Iceland for letting me use their glacio-isostacy model. Halldór Geirsson from the Iceland Meteorological Office, now at Penn State University, kindly provided me with processed GPS data from three continuous GPS stations in Iceland, for which I am grateful. For the wonderful discussions and overall great time I would like to thank all the people at the Nordic Volcanological Institute. All the people at the radar group in Delft have my grat- itude for their input, advice and support, as well as the very enjoyable radar meetings. A special mention is reserved here for my officemates during my thesis work, David Bekaert, Lennert van den Berg, Jochgem Gunneman, Anneleen Oyen and Piers van der Torren. The MGP department is thanked for funding my visit to the Dutch Earthscience Conference (NAC10), and allowing me to present my work there. A very special thank you goes out to my parents, for keeping their faith in me, even when no progress was being made, during my years at University, and for supporting me throughout. Finally I would like to thank Amandine, who had the misfortune of becoming my girlfriend during the last months of my thesis. Her unwavering support and endless reviewing of my thesis have helped me keep my sanity through difficult days of coding and writing. Delft, The Netherlands March 2010 Karsten H. Spaans ii CONTENTS Preface i Abstract v Nomenclature ix 1 Introduction 1 2 Study area 5 2.1 Iceland . 5 2.1.1 Tectonic setting and volcanic zones . 6 2.1.2 Volcanism . 6 2.1.3 Glacio-Isostatic Adjustment . 8 2.2 Katla . 8 3 Radar interferometry 11 3.1 Real and Synthetic Aperture Radar . 12 3.2 InSAR . 14 3.2.1 DORIS processing . 17 3.2.2 StaMPS processing . 19 4 Partial persistent scatterer processing methodology 27 4.1 Motivation . 28 4.2 Interferogram selection . 29 4.3 Partial PS selection . 31 4.3.1 Gamma estimation . 31 4.3.2 Selection . 32 iii 4.3.3 Random point reduction . 34 4.3.4 Weeding . 35 4.4 Displacement estimation . 36 4.4.1 Phase unwrapping . 37 4.4.2 Nuisance term estimation . 38 5 Partial Persistent Scatterer testcases 41 5.1 Simulated data . 42 5.1.1 Simulated dataset . 42 5.1.2 Processing . 42 5.1.3 Selection comparison . 46 5.1.4 Phase unwrapping . 53 5.2 Eyjafjallajökull eruption . 57 5.2.1 TerraSAR-X satellite and data . 57 5.2.2 Processing . 59 5.2.3 Selection comparison . 60 5.2.4 Phase unwrapping . 69 6 Katla 73 6.1 Data selection . 74 6.2 InSAR processing . 77 6.3 Long wavelength signals . 83 6.4 Comparison to GPS data . 87 6.5 Discussion . 91 7 Conclusions and recommendations 97 A Line-of-sight conversion 101 A.1 Incidence angle . 101 A.2 Bearing . 101 A.3 LOS unit vector . 103 iv ABSTRACT The Katla volcano is one of the most active volcanoes in Iceland. It is partly covered by Mýrdalsjökull icecap, and is in close proximity to three smaller ones. Furthermore, Vatna- jökull icecap, the largest in Europe, is only 80km away. Ongoing ice mass loss from these icecaps, which started over a century ago, has resulted in an uplift of up to 15 mm/yr in the area around Katla, accompanied by smaller horizontal displacements. This displacement signal sums with deformation caused by magma movements, landslides and plate spread- ing. Disentangling the different contributions in the surface deformation signal around Katla volcano is essential to asses volcanic processes occurring beneath the surface. Measurements at two continuous GPS stations on Katla’s south flank have shown un- expected horizontal movements, compared to the expected plate spreading rates. These movements were initially attributed to a magma source beneath Katla’s icecap. This hy- pothesis has since become less plausible, due to the long timespan in which this signal was detected, from 2000 up to the summer of 2009. After this, deformations related to the 2010 Eyjafjallajökull eruptions affected the measurements. Alternative hypotheses for the residual horizontal movements are surface displacements due to ice load reduction, lo- cal landslides and gravitational sliding. The spatial extent of deformation signals provides valuable clues to the origin of the displacements. GPS measurements in the area how- ever do not have sufficient spatial sampling to differentiate between different deformation sources. In this study, I applied radar interferometry techniques on data over the Katla area, using its superior spatial sampling to discriminate between different contributions to the surface deformation signal in the area. Not only is it important to determine if the resid- ual horizontal motions indicate increased volcanic activity of Katla, the results also add to the understanding of ice mass unloading and other processes going on around Katla and v the surrounding volcanic systems. Satellite radar interferometry is a technique that uses two or more radar images to infer surface deformations with centimeter to millimeter accuracy. After processing, the spatial sampling of the deformation measurements is in the order of tens of meters in many areas, allowing for a detailed overview of the spatial behaviour of deformation signals. Current InSAR techniques allow the use of timeseries analysis of interferometry data to increase the quality of results and evaluate deformation behaviour in space and time. Radar im- ages taken over Iceland during (near-)winter time are often affected by snow cover, which reduces the quality of the results significantly. Not only are the affected measurements themselves noisier, they also influence the overall results, reducing the number of accepted measurements in all epochs. I mitigated these effects by processing the high quality data in the conventional way, and developed an extension to the current processing methodology, which allowed me to process the low quality data separately, extracting as much defor- mation information from them as possible. In doing this, I obtained 21 epochs of surface displacement measurements around Katla with dense sampling and large coverage. During the analysis of the interferometry results, I used a model of the ice load reduction defor- mation around Iceland to remove this signal from the displacement measurements, leaving only deformation signals that had a relatively small spatial extent. The interferometry re- sults were validated using GPS data from three continuous stations, located south of Katla. I find no indications of deformation signals that could be related to magma movements on Katla’s south flank, nor do I find these on any other part of the volcano. Furthermore, lo- cal causes of the residual movements at the two stations, like land sliding, are not probable, as I detect no variations in the deformation signal in the area surrounding the stations.