EXTENDING THE TEMPORAL COVERAGE OF ICELANDIC ...

EXTENDING THE TEMPORAL COVERAGE OF ICELANDIC CRUSTAL DEFORMATION MEASUREMENTS THROUGH ENVISAT InSAR IMAGES 1

Rikke Pedersen, 1Freysteinn Sigmundsson, 1Andrew J. Hooper, and 2Kurt L. Feigl

1

Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland 2 Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, USA Contact: [email protected]; phone: +354-525-5483; fax: +354-562-9767

ABSTRACT Iceland has for decades been an international target for geodetic studies, as it provides opportunity to study a wide variety of crustal deformation processes, including plate tectonics, magmatism, and responses of the crust due to load variations. We here present results from crustal deformation surveillance spanning 2003-2006, obtained from interferometric processing of ASAR ENVISAT images, contributing to an extension of the time series of ongoing deformation processes in Iceland. More than 100 ASAR interferograms (ha>50 m), have been produced so far, spatially covering the entire neotectonic zone. The interferograms show three areas of significant deformation. Hekla and Askja volcanic systems continue to deform in a similar manner as observed with ERS interferometry 1992-2000. At the Krafla volcanic system subsidence due to processes in a shallow magma chambers appears to have stopped, while other processes related to plate spreading and possible deep accumulation of magma continue.

1. INTRODUCTION Iceland is a unique laboratory for geodetic studies of crustal deformation, as it is the largest part of the midocean ridge system above sea level. A series of volcanic and seismic zones accommodate the 2 cm/yr spreading between the North-American and Eurasian plates. Interferometric combination of radar satellite images (InSAR) is by now an important and extensively used way of observing sources of crustal deformation within Iceland [e.g. 1-15]. The technique frequently supplements other geodetic data such as campaign style as well as continuous GPS, borehole strain, optical leveling and tilt measurements. However, under certain circumstances, InSAR images are the only form of deformation recordings available, typically for small scale, remote and anomalous deformation events. Furthermore, the spatial resolution of interferograms is unparalleled by any other deformation measurement technique. InSAR has therefore been important for obtaining a complete picture of the complex processes responsible for the buildup and release of stresses in the Icelandic crust. The unique deformation observations made by InSAR demonstrate clearly the importance of _____________________________________________________ Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

repeated, regular collection of SAR images within highly dynamic areas such as Iceland.

2. DATA The ASAR instrument onboard the ENVISAT satellite provides an opportunity of continuing, and possibly expanding, already well established research projects carried out in Iceland, with emphasis on advancing understanding of fundamental processes shaping the Earths crust. The projects are based on an extensive collection of SAR images from the ERS1 and ERS2 satellites, kindly provided by ESA through a number of Cat-1 and AOE projects. Some limitations exist in the application of InSAR in Iceland. So far, mostly descending tracks have been exploited due to the very northerly location of the target areas. The high latitude may cause an elevated level of ionospheric noise in the night acquisitions of ascending tracks. Furthermore, only images acquired during summer and early fall months can be used for interferometry due to the presence of snow cover in the winter and spring. Effectively this means that images acquired from June through October render favorable for interferometry, though local conditions may vary between years. One advantage of the northerly location of Iceland is the extensive overlap between neighboring tracks. By utilizing data from neighboring track frames, improved temporal resolution of transient crustal deformation events can be obtained, relative to the 35 day recurrence time of the satellite [e.g. 13, 15]. Furthermore, the barren landscape provides good coherence over wide areas in absence of snow cover.

2.1. ERS SAR archive The data archive at the Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland (NORDVULK) holds SAR images acquired by the ERS satellites spanning the years 1992 or 1993 to 2000. In a few unique cases the temporal coverage can be extended to 2002.

The NORDVULK ERS archive consists of 347 images, distributed on 19 individual track frames. 96% of the images are from descending tracks and 4% from ascending. A reasonable temporal coverage (average at least 2 images/year) exists for all of the 14 selected surveillance track frames. The remaining 5 track frames hold only a small number of images which have been requested for specific research projects (this applies to all the ascending data in the archive).

2.2. ENVISAT SAR archive By spring 2007 the NORDVULK ASAR image archive consists of 155 individual images, distributed on 17 track frames (Fig. 1; Tab. 1). 88% of the images are from descending tracks and 12% from ascending. Regrettably, a reasonable temporal coverage (average at least 2 images/year) exists for less than 60% of the track frames. The limited availability of ENVISAT SAR data imposes serious restrictions on the applicability of interferometry as a reliable aid in surveillance of crustal deformation in volcanically and tectonically active areas, and furthermore seriously restrains the successful continuation of well established research projects based on the excellent ERS SAR archive.

Figure 1: Volcanic systems of Iceland with their fissure swarms (filled dark red), central volcanoes (red outline) and calderas (black stippled). Modified from [16]. Permanent icecaps are shown in white. Outlines (boxes) of 17 ENVISAT ASAR track frames selected for crustal deformation surveillance covering the entire neotectonic zone in Iceland are plotted.

3. RESULTS A range of crustal deformation processes have previously been detected in Iceland through interferometric processing of ERS SAR images. Results have been reported in a series of papers [1-9, 11, 13,

15], PhD thesis [10, 12] and a book providing an overview of Icelandic geodynamics [14]. These observations however are largely limited to the 1990ies, though in a few cases the time series have been successfully extended until 2002. We here present results from crustal deformation surveillance spanning 2003-2006, obtained from processing of all favorable combinations of images held in the NORDVULK ASAR image archive. Through the ASAR images, an extension of the time series of ongoing deformation processes has been facilitated. The resulting ASAR interferograms amount in total to 116 (altitude of ambiguity ha>50 m), spatially covering the entire neo-tectonic zone in Iceland.

3.1. Observed crustal deformation Three areas experiencing significant crustal deformation have been detected in ASAR interferograms. All are related to volcanic centers, and have been recognized previously through ERS interferometry, hence the ASAR images provide an extension of the temporal coverage of earlier deformation measurements. 3.1.1. Krafla spreading segment Based on processing of favorable interferometric combinations of 23 ERS SAR images spanning the years 1993-2002, three distinct deformation sources have been identified associated with the Krafla spreading segment in the northern volcanic zone (Fig. 2A). 1: A local subsidence signal about 3 km across has been observed coinciding with the location of a shallow magma chamber at the central volcano Krafla [8]. According to the most recent favorable ERS image available (from 2002) a decline in the rate of subsidence at the shallow magma chamber is clearly evident. 2: A linear fringe pattern striking about N010°E aligned along the rift axis can be observed in the images, extending for more than 20 km. The signal is interpreted to originate from the combined effect of plate-spreading and post-rifting relaxation, following the Krafla fires (1975 to 1984). 3: Widespread concentric fringes in an area about 50 km in diameter appear as a significant feature in most images. The signal amounts to about 1 cm of uplift pr year. The center of uplift is located about 15 km north of Krafla and is due to deep pressure increase. It has been interpreted as a sign of deep magma accumulation, most likely at the crust-mantle boundary (21 km depth) [8]. The ASAR interferograms temporally spans 2003-2006 (14 individual SAR images, 18 interferograms). They confirm that the style of plate spreading deformation and the widespread uplift is continuing in a similar manner as before, whereas the shallow magma chamber appears to be completely cooled down by now or in

pressure equilibrium, as no signal due to it can be detected (Fig. 2B). Unfortunately, all images acquired in 2004 appear to be severely disturbed by atmospheric noise, and the temporal resolution is therefore lower than what could be expected from the number of images. PS processing of the area, including atmospheric noise estimation as described by [17], is in progress and may provide further information, in the form of a more detailed temporal and spatial resolution of the deformation patterns.

10 km

A

00

[cm]

2,8 2.8

B Figure 2: Interferometric data covering the Krafla spreading segment in north Iceland. Black lines outline the fissure swarms in the area, circular outlines show the location of central volcanoes and the black stippled outline show the location of the Krafla caldera. The white line is the coastline. A: ERS data spanning 2 years (07/1993-06/1995). Three deformation sources can be detected; see text for detailed description. B: ENVISAT data spanning 2 years (09/2003-08/2005). It is evident from the data that the shallow Krafla magma chamber has now reached pressure equilibrium, as the circular subsidence feature observed in panel A does not appear in panel B.

3.1.2. Askja volcanic system At the Askja central volcano subsidence has been recorded by various geodetic methods for more than two decades (since 1983). The accumulated deformation in the center of the Askja caldera amounts to at least 75 cm from 1983 to 1998, without any eruptive activity [11]. A large number of ERS interferograms spanning 19922002 (3 different track frames utilized) confirms the deflation of the Askja volcano. From 1992-2000 the mean subsidence rate was about 5 cm/yr [11]. The interferometric data adds detailed information on the spatial pattern of deformation. The center of subsidence is located within a nested caldera complex as also indicated by other geodetic methods, and furthermore some subsidence takes place along a fissure swarm that transects the caldera complex [11]. This signal has previously not been well recognized with other geodetic methods, due to the relatively sparse distribution of measuring points in the remote and harsh environment. The subsidence within the fissure swarm fades away at a distance of about 25 km from the center of the caldera. The ASAR interferograms temporally spans 2003-2006 (17 individual SAR images, 21 interferograms). They confirm that subsidence is continuing within the Askja caldera though possibly at a slightly lower rate than before. They furthermore reveal that the subtle subsidence within the fissure swarm continues similarly as in previous years. 3.1.3. Hekla volcano A time series of ERS interferograms (1992-2000) covering the Hekla volcano and immediate surroundings display a composite deformation pattern. During intereruptive periods an area approximately 20 km in diameter (centered at the summit) subsides. The subsidence peaks on the most recent lava flows, due to cooling and compaction of the erupted material. Subsidence is however not confined within the areas covered by recent lava flows, but extends over a broader region. A subtle uplift signal of about 1cm/yr is seen circumscribing the subsidence. The uplift has a diameter of approximately 40 km, inverting to subsidence at about 20 km distance from the summit, as previously mentioned. Gravitational loading by eruptive products in the summit area may be a cause of the observed subsidence, whereas the uplift may be interpreted as either due to recharging of a deep-seated magma chamber, or a flexural response of the elastic crust on which the summit is resting [18]. The ASAR interferograms temporally spans 2003-2006 (3 tracks utilized, 21 individual SAR images, 13 interferograms). These confirm that the composite pattern of deformation observed during the 1990ies persists subsequent to the February 2000 eruption of Hekla.

Table 1. Table of ENVISAT ASAR orbit, track and frame numbers contained in the archive at the Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland. D: descending track; A: ascending track;. i.p.: in production. Track Frame 2003 May June July Aug Sept Oct

D9

D52

D95

D138

D324

D367

D453

A87

2259

2277

2295

2113

2295

2113

2295

2113

2295

2113

2295

2113

2295

2113

2295

1287

1269

-

7578 8079 -

7077 7578 8079 -

7077 7578 8079 -

-

-

-

-

-

7707 8208 8709

7893 8394

7893 8394

-

-

-

7155 -

7155 -

12588 -

12588 13089 -

12087 12588 13089 13590

12087 12588 13089 13590

12631 -

12631 -

12674 -

12674 13175 -

12216 12717 -

12216 12717 13218 -

12402 12903 13404 -

12402 12903 13404 -

12946 -

-

12531 13032 -

11664 12165 12666 13668

11664 12165 12666 13668

18099 18600 19101

17097 17598 18099 18600 19101

17097 17598 18099 18600 19101

17097 17598 18099 18600 19101

18142 18643 -

18142 18643 -

17684 18185 18686 -

17183 17684 18185 18686 -

18228 18729 -

17226 17727 18228 18729 -

17913 18915

17913 18915

17455 17956 18457 -

17455 17956 18457 -

17040 18042 18543 -

17175 17676 18678 -

17175 17676 18177 18678 -

22107 22608 23109 23610 -

22107 22608 23109 23610 -

22107 22608 23109 23610 -

22107 22608 23109 23610 -

-

-

22694 23195 i p.?

22694 23195 -

22737 23238 23739 -

22737 23238 23739 -

-

22923 -

22465 23467 23968 -

22465 22966 23467 23968 -

23553 i p.?

i p.?

24189

9

14

17

17

4

4

6 (7?)

9

8

14

7

8

7

7

6 (7?)

8 (9?)

10

2004 May June July Aug Sept Oct

2005 May June July Aug Sept Oct

2006 May June July Aug Sept Oct

Total

3.2. Stable areas Important insight into tectonic, magmatism and the volcano and plate boundary deformation cycles can also be obtained from observations of zero deformation. One theory is that volcanoes may show evidence of a decrease in the rate of uplift immediately prior to intrusive or extrusive events, as the pressure threshold of the magma chamber walls is approached. This theory was confirmed many times during the Krafla fires from 1975-1984. Furthermore, the processes behind magmatic sources which show evidence of subsidence may be restricted by combining various observation methods. One example is combining deformation and gravity measurements to constrain whether subsurface mass movements are required to produce the subsidence observed or simple cooling and contraction of a volume is sufficient. One example of a zero deformation result from the ASAR interferograms of Iceland is the shallow Krafla magma chamber, which apparently has ceased cooling or reached equilibrium, as no subsidence has been detected in an area known to be continuously subsiding since 1986. Information about the stress state of the crust can also be approached through ASAR results. The Herdubreid area, close to the Askja central volcano, where strain release in the upper crust has been detected in the form

of micro-seismic swarm activity shows no signs of crustal deformation in the period 2003 to 2006. ERS interferograms of the area have previously shown that fault slip or creep associated to such periods of slightly elevated seismicity may occur episodically. Finally, no unexpected deformation signals have been detected in ice-free areas in the ASAR interferograms, which cover the entire neo-volcanic zone in Iceland, indicating that no significant shallow magma movements have occurred there in the period 2003/2004 to 2006.

3. CONCLUSIONS The results from ASAR interferograms add information to previous ERS and terrestrial geodetic results, providing an array of relatively long time series of crustal deformation, though gaps in the time series do exist. Various types of deformation expected within the study areas have been confirmed, including magma movements, plate spreading, and possibly responses to variable surface loadings. More regular acquisition of images in the selected track frames are required in order to use interferometry as a surveillance method. Additionally, further investigation of the utilization of ASAR images acquired by the satellite in ascending

tracks is needed, to be able to determine if expansion of the current selection of surveillance track frames to include ascending tracks may render favourable results. This work has already been initiated.

8.

de Zeeuw-van Dalfsen, E., Pedersen, R., Sigmundsson, F. & Pagli, C. (2004). Satellite radar interferometry 1993-1999 suggests deep accumulation of magma near the crust-mantle boundary at the Krafla volcanic system, Iceland. Geophys. Res. Lett. 31(L13611), doi:10.1029/ 2004GL020059.

9.

Pedersen R. & Sigmundsson, F. (2004). InSAR based sill model links spatially offset areas of deformation and seismicity for the 1994 unrest episode at Eyjafjallajökull volcano, Iceland. Geophys. Res. Lett. 31(L14610): doi:10.1029/ 2004GL020368.

4. ACKNOWLEDGEMENTS We thank ESA for providing all ERS SAR and ENVISAT ASAR data for these studies, through a number of Cat-1 and AOE projects (C1P.2804, AOE.212, AOE.711). We furthermore thank the University of Iceland Research Fund for financial support.

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2.

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