The stress shadow induced by the Krafla rifting episode

The Krafla rifting episode (1975 - 1984) was a major stress-changing event in northern Iceland. It consisted of a series of about 20 dike intrusions, originating from the Krafla central volcano in the Northern Volcanic Zone, causing a cumulative spreading of about 8 - 9 meters at maximum and 3.5 meters in average along a 70 - 80 km part of the Krafla rift segment. A notable change in seismicity was observed in the region following the rifting episode, in particular in the near-by transform zone, called the Tjörnes Fracture Zone (TFZ), which consists of a two parallel transform structures, the Grímsey Oblique Rift (GOR) and the Húsavík-Flatey Fault (HFF). The rifting episode clearly triggered a magnitude 6.4 earthquake on the GOR while apparently putting most of the HFF into a stress shadow. Here we model the static stress perturbation due to the rifting episode and study the influence of the Coulomb stress shadow on the HFF. In the time frame 1995 - 2011, for which a high quality seismic catalog exists, we observe a relatively sharp seismicity rate increase along a large part of the fault. The change from low to high seismicity rate takes place gradually along the fault, with a later transition taking place closer to the rift axis. These locations coincide with areas on the Coulomb stress maps invested by negative Coulomb stresses of 0.1 - 0.4 MPa. Larger negative Coulomb stresses [> 0.5 MPa] are found on the easternmost part of the fault, the portion that is closest to the rift, and there the locking effect appears to persist today and no seismicity rate increase has taken place. In addition, we find significantly different b-values of the Gutenberg-Richter magnitude-frequency relation for time periods before and after the seismicity rate transition. Very low b-values (~ 0.7) are replaced after the increase in seismicity rate by values of ~1.0 or larger, which are more typical for a strike-slip regime with abundance of fluids, such as the TFZ. We interpret the low b-values and seismic rates as due to the strong static compression and clamping caused by the rifting episode, and the increases thereof as due to a gradual recovery to the pre-rifting state of stress. Our study implies that earthquake statistics and mechanics in the TFZ have been dramatically affected by the Krafla rifting episode, and that volcanic events of similar magnitude, which have a recurrence time of ~ 200 years, have the potential of triggering large earthquakes and overturning the current state of stress over the vast, partially populated area.