Rikke Pedersen

Post-earthquake ground movements correlated to pore-pressure transients

Large earthquakes alter the stress in the surrounding crust, leading to triggered earthquakes and aftershocks. A number of time-dependent processes, including afterslip, pore-fluid flow and viscous relaxation of the lower crust and upper mantle, further modify the stress and pore pressure near the fault, and hence the tendency for triggered earthquakes. It has proved difficult, however, to distinguish between these processes on the basis of direct field observations, despite considerable effort. Here we present a unique combination of measurements consisting of satellite radar interferograms and water-level changes in geothermal wells following two magnitude-6.5 earthquakes in the south Iceland seismic zone. The deformation recorded in the interferograms cannot be explained by either afterslip or visco-elastic relaxation, but is consistent with rebound of a porous elastic material in the first 1–2 months following the earthquakes. This interpretation is confirmed by direct measurements which show rapid (1–2-month) recovery of the earthquake-induced water-level changes. In contrast, the duration of the aftershock sequence is projected to be ∼3.5 years, suggesting that pore-fluid flow does not control aftershock duration. But because the surface strains are dominated by pore-pressure changes in the shallow crust, we cannot rule out a longer pore-pressure transient at the depth of the aftershocks. The aftershock duration is consistent with models of seismicity rate variations based on rate- and state-dependent friction laws.

Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During such eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallajökull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic, closing airspace over much of Europe for days. This eruption was preceded by an effusive flank eruption of basalt from 20 March to 12 April 2010. The 2010 eruptions are the culmination of 18 years of intermittent volcanic unrest. Here we show that deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single magma chamber. Deformation was rapid before the first eruption (>5 mm per day after 4 March), but negligible during it. Lack of distinct co-eruptive deflation indicates that the net volume of magma drained from shallow depth during this eruption was small; rather, magma flowed from considerable depth. Before the eruption, a ∼0.05 km3 magmatic intrusion grew over a period of three months, in a temporally and spatially complex manner, as revealed by GPS (Global Positioning System) geodetic measurements and interferometric analysis of satellite radar images. The second eruption occurred within the ice-capped caldera of the volcano, with explosivity amplified by magma–ice interaction. Gradual contraction of a source, distinct from the pre-eruptive inflation sources, is evident from geodetic data. Eyjafjallajökull’s behaviour can be attributed to its off-rift setting with a ‘cold’ subsurface structure and limited magma at shallow depth, as may be typical for moderately active volcanoes. Clear signs of volcanic unrest signals over years to weeks may indicate reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

Fault slip distribution of two June 2000 MW6.5 earthquakes in South Iceland estimated from joint inversion of InSAR and GPS measurements

Abstract We present the first detailed estimates of co-seismic slip distribution on faults in the South Iceland Seismic Zone (SISZ), an area of bookshelf tectonics. We have estimated source parameters for two M W 6.5 earthquakes in the SISZ on June 17 and 21, 2000 through a joint inversion of InSAR and GPS measurements. Our preferred model indicates two simple 15 km long, near vertical faults extending from the surface to approximately 10 km depth. The geometry is in good agreement with the aftershock distribution. The dislocations experienced pure right-lateral strike-slip, reaching maxima of 2.6 m and 2.9 m for the June 17 and 21 events, respectively. We find that the distribution of slip with depth may be correlated to crustal layering, with more than 80% of the total geometric moment release occurring in the uppermost 6 km. According to the distributed slip model the middle and upper crust appears to be more apt to generate large displacements than the lower crust. The geodetic estimates of seismic moments are 4.4×10 18 Nm ( M W 6.4) and 5.0×10 18 Nm ( M W 6.5). The total moment released by the two events equals that generated by several decades of plate motion in the area, but is only a fraction of the moment accumulated in the area since the last major earthquake in 1912.

Eruptions of Eyjafjallajökull Volcano, Iceland

The April 2010 eruption of Eyjafjallajokull volcano (Figure 1), located on Icelands southern coast, created unprecedented disruptions to European air traffic during 15–20 April, costing the aviation industry an estimated

Coseismic interferograms of two MS=6.6 earthquakes in the South Iceland Seismic Zone, June 2000

250 million per day (see the related news item in this issue). This cost brings into focus how volcanoes can affect communities thousands of miles away. Eyjafjallajokull rises to 1666 meters above sea level and hosts agricultural land on its southern slopes, with farms located as close as 7 kilometers from the summit caldera. In the past 1500 years, Eyjafjallajokull has produced four comparatively small eruptions. The eruption previous to 2010 began in December 1821 and lasted for over a year, with intermittent explosive activity spreading a thin layer of tephra (ash and larger ejected clasts) over the surrounding region. In contrast, the explosive 2010 eruption, sourced within the ice-capped summit of the volcano, so far is larger and characterized by magma of a slightly different composition. This may suggest that deep within the volcano, the 1821 magma source is mixing with new melt, or that residual melt from past intrusive events is being pushed out by new magma.

Geodetic data shed light on ongoing caldera subsidence at Askja, Iceland

We present InSAR observations of deformation due to two MS=6.6 earthquakes in the South Iceland Seismic Zone in June 2000. Coseismic deformation predominates in the ERS interferograms. Range change, due to mainly right-lateral strike-slip on N-S striking faults, amounting to more than 15 cm is observed, although displacement is mainly perpendicular to the satellite look direction. Using elastic dislocation models in a trial-and-error scheme, we find a best-fitting model that agrees well with the aftershock locations and moment magnitudes estimated from seismograms. The June 17 model has a fault patch 16 km long, 10 km deep, striking N05°E, dipping 86°, with a slip maximum of 2.40 m. The June 21 model has a vertical patch 15 km long, 9 km deep, striking N01°W, with a slip maximum of 2.15 m.

Interpretation of Rayleigh-wave ellipticity observed with multicomponent passive seismic interferometry at Hekla Volcano, Iceland

Subsidence within the main caldera of Askja volcano in the North of Iceland has been in progress since 1983. Here, we present new ground- and satellite-based deformation data, which we interpret together with new and existing micro-gravity data, to help understand which processes may be responsible for the unrest. From 2003 to 2007, we observe a net micro-gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro-gravity increase while the subsidence continues. We infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary. Mass movements at active volcanoes can be caused by several processes, including water table/lake level movements, hydrothermal activity and magma movements. We suggest that, here, magma movement and/or a steam cap in the geothermal system of Askja at depth are responsible for the observed micro-gravity variations. In this respect, we rule out the possibility of a shallow intrusion as an explanation for the observed micro-gravity increase but suggest magma may have flowed into the residing shallow magma chamber at Askja despite continued subsidence. In particular, variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation.

Magma Movements in Volcanic Plumbing Systems and their Associated Ground Deformation and Seismic Patterns

The 2010 eruption of Eyjafjallajokull has drawn increased attention to Icelands Eastern Volcanic Zone (EVZ) due to the threat it poses to the heavily used air-traffic corridors of the northern Atlantic Ocean. Within the EVZ, Hekla is historically one of the most active volcanoes and has exhibited a decadal eruption pattern for the past 40 years. Hekla most recently erupted in 2000 and is thus ripe for another decadal eruption. Because Hekla is generally aseismic, except for a brief time period (hours) leading up to an eruption, monitoring has previously depended on precursory deformation signals (Linde et al., 1993). As a result, seismic tomography of the internal structure of the volcano using phase arrivals of local earthquakes is not possible. Motivated by Heklas practically aseismic behavior in inter-eruptive periods, we installed a temporary network of four broadband seismometers around the volcanic edifice in late August 2010 with the intention of investigating the applicability of passive seismic...

Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland

Abstract Improving our understanding of volcanic hazards requires better knowledge of the location, volume and properties of magma bodies in the roots of active volcanoes, as well as information on melt supply and magma transfer. This requires a good understanding of both the geometric structure of the volcanic and igneous plumbing system, as well as observations of sub-surface magma movements and their interpretation. Arrival of new magma in volcano roots often causes volcanic unrest expressed by one or more of the following: increased seismicity, ground deformation, volcanic gas release and ground temperature changes. Recent eruptions and magmatic events in Iceland have provided opportunities to apply repeated geodetic observations at volcanoes to measure ground deformation and interpret these measurements together with seismic observations in terms of subsurface magmatic processes.