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Deep Magma Storage at Hekla Volcano, Iceland, Revealed by Insar Time Series Analysis Benedikt G

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Deep Magma Storage at Hekla Volcano, Iceland, Revealed by Insar Time Series Analysis Benedikt G JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, B05401, doi:10.1029/2010JB007576, 2011 Deep magma storage at Hekla volcano, Iceland, revealed by InSAR time series analysis Benedikt G. Ofeigsson,1 Andrew Hooper,2 Freysteinn Sigmundsson,1 Erik Sturkell,3 and Ronni Grapenthin4 Received 29 March 2010; revised 31 January 2011; accepted 14 February 2011; published 5 May 2011. [1] Hekla volcano is one of the most active volcanoes in Iceland. The most recent eruption occurred from 26 February to 8 March 2000 when about 0.19 km3 of magma was erupted. We present deformation data from multitemporal analyses of synthetic aperture radar (SAR) images acquired between 1993 and 2008, focusing on pixels with low‐phase variance (using persistent scatterer and small baseline approaches). Prior to and after the 2000 eruption, we find a broad area of inflation around the volcano (radius about 20 km), with satellite line‐of‐sight (LOS) shortening of up to 5 mm/yr. We interpret this signal as the result of pressure increase in a deep‐seated magma chamber, which we model as a spherical source at 14–20 km depth increasing in volume by 0.003–0.02 km3/yr. Within a ∼6 km radius of the summit of the volcano, a LOS lengthening is superimposed on the broad inflation signal, which correlates partly with recent lava flows. We interpret this signal as the result of thermally contracting lava flows, combined with viscoelastic yielding due to the load of the volcano and its lavas. Coeruptive deflation during the 2000 eruption was similar to the cumulative inflation from 1993 to 2000 and is consistent with a spherical magma chamber at 14–18 km depth that decreases in volume by 0.04–0.08 km3. Interferograms spanning the 2000 eruption show a local coeruptive deformation signal near the eruptive fissure. This is consistent with a dike opening from the surface to depths up to 5.8 km with a volume of 0.005–0.006 km3. Citation: Ofeigsson, B. G., A. Hooper, F. Sigmundsson, E. Sturkell, and R. Grapenthin (2011), Deep magma storage at Hekla volcano, Iceland, revealed by InSAR time series analysis, J. Geophys. Res., 116, B05401, doi:10.1029/2010JB007576. 1. Introduction Mt. Upptyppingar, at 14–22 km depth [Hooper et al., 2008]. ‐ A magma source north of Krafla has been suggested at [2] Iceland lies on the Mid Atlantic Ridge, the divergent about 21 km depth [de Zeeuw‐van Dalfsen et al., 2004], and boundary between the North American and Eurasian plates. beneath Bárdabunga at about 10 km depth [Pagli et al., High magmatic activity and relatively dense volcano mon- 2007]. Sturkell et al. [2006b] suggest a deep magma itoring has allowed various studies of magmatic plumbing source at 16 km for Askja. At Hekla volcano, evidence of a systems with geodetic techniques. At some of the more deep magma source is inferred but depth estimates are active volcanoes, including Grímsvötn, Katla, Krafla and uncertain due to complex and subtle deformation spread Askja, shallow magma chambers have been identified at 2– over a large area. 5 km depth [Sturkell et al., 2006a; Sigmundsson, 2006]. [3] In south Iceland, the plate boundary has two parallel However, geodetic evidence also suggests deeper (≥10 km) volcanic zones: the Western Volcanic Zone (WVZ) and the deformation sources in a number of areas, giving an Eastern Volcanic Zone (EVZ). These two branches of the opportunity to study magma migration in the lower crust. plate boundary are connected by an E‐W oriented transform, The best documented case is a 2007–2008 magma intrusion the South Iceland Seismic Zone (SISZ) [Einarsson, 1991]. into the lower crust east of Askja volcano, close to Hekla volcano is located near the intersection of the EVZ and SISZ (Figure 1). It is one of Iceland’s most active volcanoes with 18 summit eruptions during the last 1100 years. Until 1 Nordic Volcanological Center, Institute of Earth Sciences, University 1947, Hekla erupted 1–2 times every century, but since an of Iceland, Reykjavík, Iceland. 2Delft Institute of Earth Observation and Space Systems, Delft eruption in 1970 Hekla has erupted approximately every University of Technology, Delft, Netherlands. 10 years, in 1980–1981, 1991 and 2000. The most recent 3Department of Earth Sciences, University of Gothenburg, Gothenburg, eruption occurred from 26 February to 8 March 2000, when Sweden. 3 4 an estimated 0.19 km (dense rock equivalent, DRE) was Geophysical Institute, University of Alaska Fairbanks, Fairbanks, ejected [Höskuldsson et al., 2007]. Most seismicity at Hekla Alaska, USA. occurs prior to and during eruptions. Numerous small earth- Copyright 2011 by the American Geophysical Union. quakes were associated with the onset of the 2000 eruption. 0148‐0227/11/2010JB007576 The first earthquakes occurred approximately 80 min before B05401 1of15 B05401 OFEIGSSON ET AL.: INSAR STUDY OF HEKLA VOLCANO, ICELAND B05401 Figure 1. Hekla volcano and its surroundings in south Iceland. Hekla is an elongated ridge reaching an elevation of 1490 m. The black line along the summit indicates the 2000 eruptive fissure. The red triangle shows the optical leveling tilt station Næfurolt (NAEF) that has proven to be a long‐term indicator of pressure in a magma source under Hekla. The map also shows the outline of Torfajökull caldera, an active central volcano east of Hekla. Inset shows the general tectonic setting of Iceland. Volcanic systems with central volcanoes (black oval outlines) and fissure swarms (gray areas) are located along the divergent plate boundary [Einarsson and Sæmundsson, 1987]. Black arrow shows the full spreading vector of 19.5 mm/yr in the direction indicated by the REVEL model [Sella et al., 2002]. In the south the plate boundary divides up into the Western Volcanic Zone (WVZ) and the Eastern Volcanic Zone (EVZ). Hekla lies at the intersection of the EVZ and the South Iceland Seismic Zone (SISZ). The labeled volcanic systems are A, Askja; Bá, Bárdarbunga; Gr, Grímsvötn; ka, Katla; kr, Krafla. Red boxes indicate ERS track 52, blue boxes indicate ERS track 359, and green boxes indicate Envisat track 324. the eruption and were shallow (≤3 km). Around 30 min tremor, which starts when the magma reaches the surface before the eruption started, earthquakes with hypocenters and continues with diminishing magnitude until the end of from 6–14 km depth occurred. No propagation from depth the eruption [Soosalu et al., 2005]. toward the surface was observed. After the onset of most [4] Various seismic and geodetic studies have addressed Hekla eruptions, the dominant seismic signal is volcanic the existence and location of a magma chamber under Hekla. 2of15 B05401 OFEIGSSON ET AL.: INSAR STUDY OF HEKLA VOLCANO, ICELAND B05401 Geodetic studies from optical levelling tilt [Tryggvason,1994], and phase stability. The effect of atmospheric phase delay in EDM (electronic distance measurements) [Kjartansson and the reference SAR image is estimated as the signal present Grönvold, 1983], GPS measurements [e.g., Sigmundsson in every interferogram. Spatially correlated error in the et al., 1992] and borehole strain [Linde et al., 1993], have digital elevation model (DEM) and errors due to incorrect inferred a magma chamber in the range of 5–11 km depth, mapping of the DEM are estimated from their correlation with considerable associated uncertainty. On the other hand, with the perpendicular component of the interferometric Soosalu and Einarsson [2004] concluded that due to a lack baseline. We did not subtract slave atmospheric signal and of S wave attenuation under Hekla, a considerable molten orbit errors from individual interferograms (Figures 2–4). volume cannot exist in the 4–14 km depth range. Individual We did however estimate a bilinear ramp from the mean interferometric synthetic aperture radar images (conven- rates to account for orbit errors and long‐wavelength tional InSAR) have been used to infer a broad preeruptive atmospheric signal. The ramp is an estimation of a signal inflation centered on Hekla, about 40 km in diameter, with that has a wavelength much greater than the whole image, line‐of‐sight (LOS) shortening rates less than 10 mm/yr therefore not significantly influencing the observed infla- [Sigmundsson et al., 2001]. These also show that a LOS tion signal. We used SAR data from the ERS‐1 and ERS‐2 lengthening signal, most pronounced over recent lava fields, satellites collected during 1993–2000, with a total of is superimposed on the inflation signal and therefore masks 27 acquisitions from descending track 52, and 17 acquisi- uplift close to the summit. Previous geodetic studies in tions from ascending track 359. For the 2003–2008 period, relation to the 26 February to 8 March 2000 eruption of we used 15 acquisitions from descending track 324 of the Hekla indicate a deep seated magma source (≥10 km) and a Envisat satellite. The LOS range change derived from the shallow dike, connected by a narrow feeder channel data, is shown in Figures 2–4. The time series from each [Ágústsson et al., 2000; Sturkell et al., 2005]. The afore- track can be combined to reveal the mean LOS velocity for mentioned studies rely mostly on spatially sparse data sets, each of the three tracks, leading to a better estimate of making it difficult to constrain the complex and extensive crustal movements around Hekla. deformation field observed in the vicinity of the volcano. [6] Maps of mean LOS velocities for the 1993–1999 and Only part of the Hekla area remains coherent in inter- 2003–2008 periods for the three different tracks (Figure 5) ferograms, necessitating a more thorough analysis. We use all show a broad area of LOS shortening (inflation) interferometric time series analysis of SAR data to study approximately 20 km in radius, with LOS lengthening of up crustal deformation during 1993–2008.
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