An intense earthquake sequence affected the area south of the Icelandic capital this week. Data indicate complex ruptures and possible magmatic triggering

Since February 24th, significant, swarm-like earthquake activity occurred near Iceland’s capital Reykjavik within an area of intense volcanism and tectonics. The strongest earthquake reached Magnitude 5.7 and marks one of the countries largest earthquakes for decades. Minor damage and rockfalls, but no significant impact was observed. However, earthquake statistics are rather impressive: More than 1000 small quakes just within a few hours, more than 2000 within 48 hours and a large number of missing smaller events due to the high event frequency. Even a volcanic eruption was considered possible near Reykjanes volcanic field. However, newest data indicate a more complex and more impressive origin.

InSAR image of the ongoing strong earthquake sequence near Reykjavík. Looks like ~30 km of the ENE-trending plate boundary moved left-laterally at depth with several coseismic N-S right-lateral strike-slip surface ruptures on top, including the one of the magnitude 5.7 earthquake pic.twitter.com/ZxQEQ2PiBL

— Sigurjón (Sjonni) Jónsson (@Sjonni_KAUST) February 26, 2021

Contrary to most other earthquake swarms, the Reykjanes sequence covers a rather wide area of 30 km along the plate boundary between Eurasia and North America. Usually, the extent of an earthquake sequence depends on the size of the mainshock rupture and thus on the mainshock magnitude. In this case, the maximum rupture length of the mainshock, which was magnitude 5.7, wouldn’t be more than just a few kilometers. In addition, the rupture of the mainshock extended in north-south direction, almost perpendicular to the plate boundary and the earthquake activity. This indicates that the “mainshock” and all related activity might be connected to an even larger process.

Beside the mainshock, a few other earthquake clusters are visible in the data of the Icelandic Meteorological Agency. While the mainshock cluster close to Krysuvik volcano seems to be the largest, several smaller ones are located to the west and east. These clusters seem more or less independent from each other as the seismicity in between is relatively low and might actually result from incorrect locations: Another indicator another trigger.

This possible trigger was now identified with InSAR satellite data (as published here and here on Twitter. It shows a broad area of slight horizontal deformation along the plate boundary with displacements of a few cm. Further, shorter but bigger ruptures are seen in north-south direction. The largest displacement can be seen close to the mainshock. All these ruptures correlate with the previously identified earthquake cluster extents.

The large rupture along the plate boundary caused a rather small surface rupture and therefore, was likely located in deeper parts of the crust. Possibly in a depth with high temperatures so that most of the rupture happened aseismically, without triggering any earthquakes. However, this deep rupture reactivated smaller faults close to the surface, whose ruptures caused the earthquake clusters that we can see. All these small sequences have characteristics of a typical foreshock-mainshock-aftershock sequence as the largest quake in each cluster happened rather early during the activity.

We can say that the previously assumed earthquake swarm actually consists of several simultaneous earthquake clusters, which occur independent of each other but are all caused by the same source: A rupture in deeper parts of the crust, similar to a typical Slow Slip Event. This aseismic rupture might be a result of the magmatic intrusions that took recently place beneath parts of the Reykjanes peninsula. The molten rock and the gases might have acted as lubricants for the fault. But, based on the currently available data this cannot be finally concluded.

However, this activity provides an excellent example of complex tectonic and magmatic processes, their interplay and their consequences within divergent plate boundaries. For the next couple of weeks, it is likely that this earthquake sequences will continue, maybe triggering other sequences. An earthquake warning, issued by Icelandic officials, is still active. In addition, potential volcanic activity that might follow the seismicity is still realistic. The aviation alert level for Krysuvik remains on yellow.