Heidi Soosalu

Multiple melt injection along a spreading segment at Askja, Iceland

Lower crustal earthquakes (12–25 km depth) have been detected since August 2005 in the Askja volcanic system along the north Iceland rift, in the normally ductile part of the crust. The earthquakes occur in three clusters, which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust. Seismic velocity Vp/Vs ratios are consistent with the presence of partial melt in the lower crust at Askja. The spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active. This pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath Iceland compared to the rest of the Mid-Atlantic Ridge. However, the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano Askja (i.e. segment center). During the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano. Therefore on long time scales melt supply is probably greater at the segment center, with melt redistribution in the upper crust, even though there are multiple points of lower crustal injection along the segment.

Strike‐slip faulting during the 2014 Bárðarbunga‐Holuhraun dike intrusion, central Iceland

Over a 13?day period magma propagated laterally from the subglacial Barðarbunga volcano in the northern rift zone, Iceland. It created >?30,000 earthquakes at 5–7?km depth along a 48?km path before erupting on 29 August 2014. The seismicity, which tracked the dike propagation, advanced in short bursts at 0.3–4.7?km/h separated by pauses of up to 81?h. During each surge forward, seismicity behind the dike tip dropped. Moment tensor solutions from the leading edge show exclusively left-lateral strike-slip faulting subparallel to the advancing dike tip, releasing accumulated strain deficit in the brittle layer of the rift zone. Behind the leading edge, both left- and right-lateral strike-slip earthquakes are observed. The lack of non-double-couple earthquakes implies that the dike opening was aseismic.

Correction to “Multiple melt injection along a spreading segment at Askja, Iceland”

In the paper “ Multiple melt injection along a spreading segment at Askja, Iceland ” by Janet Key et al. (Geophysical Research Letters, 38, L05301, doi:10.1029/2010GL046264, 2011), we regret that an error was made in the calculation of the V p /V s ratios. The correct values are presented in this revised version of Figure 2 and its caption. This change does not affect either the hypocentral locations, for which we used the correct mean V p /V s value of 1.78, nor the overall conclusions of our paper. We still find evidence for relatively elevated V p /V s ratios from the lower crustal earthquakes, consistent with the presence of melt in the lower crust. However the following corrections to the text should be made in light of this discovery.

Habits of a glacier-covered volcano: Seismicity patterns and velocity structure of Katla volcano, Iceland

Abstract The Katla volcano, overlain by the Mýrdalsjökull glacier, is one of the most active and hazardous volcanoes in Iceland. Earthquakes show anomalous magnitude-frequency behaviour and mainly occur in two distinct areas: within the oval caldera and around Goðabunga, a bulge on its western flank. The seismicity differs between the areas; earthquakes in Goðabunga are low frequency and shallow whereas those beneath the caldera occur at greater depths and are volcano-tectonic. The seismicity shows seasonal variations but the rates peak at different times in the two areas. A snow budget model, which gives an estimate of the glacial loading, shows good correlation with seismic activity on an annual scale. Data recorded by the permanent network South Iceland Lowland (SIL), as well as by a temporary network, are used to invert for a 3D seismic velocity model underneath Eyjafjallajökull, Goðabunga and the Katla caldera. The tomography resolves a 15 km wide, aseismic, high-velocity structure at a depth of more than 4 km between the Eyjafjallajökull volcano in the west and the Katla volcano in the east. Anomalously low velocities are observed beneath the Katla caldera, which is interpreted as being a significantly fractured area of anomalously high temperature.