Why is Extension in the Northern Rift Zone of Iceland Accompanied Predominantly by Strike-slip Seismicity?

Thursday, 17 December 2015: 16:00
304 (Moscone South)
Robert S White1, Jennifer Woods2, Thorbjörg Ágústsdóttir2, Robert George Green3, Tim S Greenfield1, Bryndis Brandsdottir4 and Simon A T Redfern5, (1)Bullard Laboratories, Cambridge, United Kingdom, (2)University of Cambridge, Bullard Laboratories - Department of Earth Sciences, Cambridge, United Kingdom, (3)Bullard Laboratories, Cambridge, CB3, United Kingdom, (4)University of Iceland, Reykjavik, Iceland, (5)University of Cambridge, Cambridge, United Kingdom
It might be expected that in rift zones most earthquakes would show normal faulting mechanisms as extension is accommodated. However, fault plane solutions we have recorded over the past decade in the eastern neovolcanic rift zone of Iceland are almost entirely strike-slip. This is true both of earthquakes accompanying a 2014 dike intrusion along the rift zone and of tectonic events not associated with intrusion or volcanism.

 Our network of 65 seismometers provides excellent locations and coverage of the focal sphere of earthquakes in central Iceland. The northern rift zone is part of the mid-Atlantic Ridge which here is spreading at 19 mm/a, at an angle slightly oblique to the rift-normal direction. Persistent seismicity occurs on a series of sub-parallel zones aligned with two overlapping volcanic rifts, the Askja and Kverkfjöll segments. In the overlap region, extension is transferred from one segment to the next by left-lateral strike-slip faulting. We infer that it is energetically favourable to utilize the strong rift fabric in strike-slip faults rather than break new faults.

Over 30,000 earthquakes were recorded in August 2014 as a dike intruded 46 km northward from Bárðarbunga volcano before erupting two weeks later at Holuhraun. Although up to 6 metres of dike opening occurred, almost all the seismicity was strike-slip, with fault planes aligned parallel to the dike. We interpret the seismicity as caused by fracturing at the dike tip along pre-existing fabric: the relative orientation of the dike and the spreading direction caused ubiquitous left-lateral faulting. Less common right-lateral faulting, also with fault planes orientated parallel to the dike, occurred behind the dike tip; most likely caused by breakage of solidified magma within the dike as the pressure of magma flow continued. The opening of the dike as magma intruded and filled the space to accommodate plate spreading occurred aseismically.