Fracture movements and graben subsidence during the 2014 Bárdarbunga dike intrusion in Iceland

A dike propagated ~ 48 km from the Bárdarbunga central volcano into one of its fissure swarms in August and early September 2014. The dike intrusion was accompanied by caldera subsidence in the ice-covered Bárdarbunga central volcano, as well as a propagating earthquake swarm, graben subsidence, and fissure eruptions in the fissure swarm. Most of the dike was emplaced in the glacier-covered part of the fissure swarm, but a 10. km long section at the distal (northern) end of the dike is ice-free. New ground fractures were observed in the ice-free part during the dike propagation and prior to the outbreak of a major lava-producing eruption. The fractures delineate two graben structures in direct continuation of each other. Here we show the extent and depth of the grabens and maps of fractures and eruptive fissures that were formed or reactivated north of the Vatnajökull glacier during the dike intrusion. The active graben subsidence was identified 3 days after seismicity reached the ice-free area. Then fractures delineated a 700-1000 m wide and 5 km long graben visible from the northern edge of the Vatnajökull glacier. Farther north a narrower, 250-450. m wide graben was seen. A short-lived minor eruption occurred between these grabens on the 29th of August 2014, 2. days after the grabens were identified. The northern graben was extended southwards during or before the eruption, and included the eruptive fissure. On the 31st of August 2014, a much larger eruption began on the same eruptive fissure. The fissure quickly obtained its maximum length of 2. km. This eruption lasted 6. months. A minor eruption, lasting 2 days or less, occurred on the southern graben on the 5th of September 2014. The first two eruptions took place on the same eruptive fissure that formed the pre-existing Holuhraun lava, presumably in the 18th century. Measurements in January 2015 show that the southern graben was 3.5-5.5. m deep, but the northern graben had been covered by lava. From the depth and the opening of the graben, we estimate that the dip of the boundary faults of the graben is ~ 75°. The narrow grabens observed before the eruptions indicated that magma was at shallow depth, likely much less than 300. m, 2. days before it opened to the surface. Such observations can serve as a tool to estimate likelihood of eruptions during dike intrusions.