Micrometre-scale deformation observations reveal fundamental controls on geological rifting

Many of the world's largest volcanic eruptions are associated with geological rifting where major fractures open at the Earth's surface, yet fundamental controls on the near-surface response to the rifting process are lacking. New high resolution observations gleaned from seismometer data during the 2014 Bárðarbunga basaltic dyke intrusion in Iceland allow us unprecedented access to the associated graben formation process on both sub-second and micrometre scales. We find that what appears as quasi steady-state near-surface rifting on lower resolution GPS observation comprises discrete staccato-like deformation steps as the upper crust unzips through repetitive low magnitude (M_W < 0) failures on fracture patches estimated between 300 m² and 1200 m² in size. Stress drops for these events are one to two orders of magnitude smaller than expected for tectonic earthquakes, demonstrating that the uppermost crust in the rift zone is exceptionally weak.