Abstract
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 (MW < 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.
Highlights
In the afternoon of 30 August 2014, we installed a small profile of three 3-component broadband seismometers (Guralp 6TD 30 s) perpendicular to the graben and inferred dyke (Fig. 1a), with the closest station (DY3) directly at the western shoulder of one of the large graben boundary faults and the other two stations approximately 1 km (DY1) and 2 km (DY2) from DY3
In a recent study[14], we presented a new data processing approach that allows for the recovery of micrometre-scale displacement steps from instrument-corrected seismograms
It is based on long-period noise removal using median filters and its performance was confirmed by laboratory experiments. We apply this method to the event (Fig. 2c–e) and observe displacement steps on all three components of the instrument, i.e. the station was displaced by approximately 125 μm in a northwest, slightly upward direction
Summary
In the afternoon of 30 August 2014, we installed a small profile of three 3-component broadband seismometers (Guralp 6TD 30 s) perpendicular to the graben and inferred dyke (Fig. 1a), with the closest station (DY3) directly at the western shoulder of one of the large graben boundary faults and the other two stations approximately 1 km (DY1) and 2 km (DY2) from DY3. If a purely tensile source mechanism is considered, the slip displacement on the same patch is reduced by a factor of 2, showing the results are robust for a deviation from the pure normal faulting source Both results are in agreement with Liu-Zeng et al.[32], who model the slip-to-length ratio and obtain equivalent values for small faults with rough fault surfaces. They are consistent with the lack of shallow “standard” earthquakes associated with such a large rifting event Such small stress drops are in striking agreement with the value of Δσ = 0.01 MPa obtained for shallow long period seismicity on Mt Etna, Italy[35], attributed to the presence of exceptionally weak near surface volcanic material that could not sustain high shear or tensile stresses and failed at exceptionally low earthquakes magnitudes
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