Nucleation, propagation and arrest of seismic swarms in the Tjörnes Fracture Zone (North Iceland)

Abstract:

The Tjörnes Fracture Zone (TFZ) connects the Northern Volcanic Zone to the Mid-Atlantic ridge north of Iceland. It primarily consists of two transform structures, the Húsavík-Flatey Fault (HFF) and the Grímsey Oblique Rift (GOR), which together have experienced about ten M>6 earthquakes since 1750. There is growing concern that a large earthquake may be due in the TFZ. The GOR and the northwestern part of the HFF have been seismically very active during the past two decades, often in the form of seismic swarms of various intensity and duration. The most energetic swarms during this period took place in October 2012 and March 2013, with several M>5 earthquakes. These and previous swarms occurred offshore and are poorly understood, in particular regarding the physical mechanism behind their generation and to what extent the swarm-like activity might temporally or permanently modify the hazard in the entire TFZ.

Here we study the spatio-temporal pattern of earthquake swarms occurring in the TFZ since 1996. We find that the swarms show spatial complementarity, i.e., in general they do not overlap spatially with earlier swarms. Moreover, together they have progressively filled up the entire GOR and the western half of the HFF. Each swarm shows a clear migration of hypocenters on their respective fault planes. This is particularly visible in the September-October 2012 and March 2013 sequences, where the earthquakes expanded concentrically from a central, focused patch on the fault to the entire fault area excited by seismicity. The swarms typically start as very localized microseismicity, lasting for a few hours to a few days. Then, the hypocenter area starts to expand, with earthquakes migrating at velocities ranging from 1 km/day up to 1 km/h. The migration sometimes accelerates or decelerates, depending on the case, and then comes to a sudden stop, with microseismicity continuing on the fault plane for a few days or weeks. The estimated rupture duration and rupture velocity scale with the estimated seismic moment as previously identified slow slip events in subduction zones or on transform faults. The properties of the swarms in the TFZ therefore point to an interesting sequence of nucleation, propagation and arrest of transient slip in a fault zone with a complex rheology and to an interesting form of strain release and seismic cycle.