Mapping the Features of the Bárðarbunga 2014-2015 Rifting and Eruption Event; Magmatic Upwelling, Segmented Dyke, Caldera Subsidence

Abstract:

Bárðdarbunga volcano in Iceland had been showing signs of increasing microseismic activity for some years when in August 2014 the activity suddenly escalated. The seismicity, which had been confined NE of the caldera rim and in the fissure swarm NNE of the caldera, quickly spread south along the eastern rim and radially out of the SE corner of the caldera. At the outer margin of the central volcano the seismicity side-stepped and took a 90 degree turn towards NE and from there propagated 40 km in 12 days to Holuhraun, where an eruption started on the 14^thday and continued for 5 months. The seismicity was accompanied by several meters of spreading representing a rifting episode in the NE fissure swarm along a segmented dyke intrusion. The seismicity defines 8 main vertical dyke segments whose distinctively different orientations are controlled by the tensional stress regime of the region and by the overburden pressure (Sigmundsson et al., 2015; Heimisson et al., this conference). At the same time the caldera subsided by 60 m and over 70 M>5, very long-period events of complex mechanisms occurred at the southern and northern caldera rims.

Relative relocations and focal mechanisms of over 15 thousand microearthquakes are used to map details of the Bárðarbunga 2014-2015 episode, revealing (1) upwelling and pooling of magma under the southern end of the dyke, which may have played a critical role in enabling magma from Bárðarbunga to migrate into the dyke, (2) the lateral propagation of the dyke and its very detailed segmentation and (3) the fracture surfaces along the caldera rim extending from the near-surface down to several kilometers depth, defining a vertical plane along the southern rim, while at the northern margin the seismicity extends a few km outward from the rim, defining slightly outward dipping or northward stepping, segmented faulting surfaces. These surfaces may play a role in generating the complex mechanisms of the larger magnitude events.