More than 6000 earthquakes have hit the Reykjanes peninsula in southwestern Iceland over the past few days, leading to rockfalls, damaged buildings, and widespread tension among residents. Shaky days, especially for the capital Reykjavik. A new episode of magmatic activity beneath the volcano Fagradalsfjall that erupted in 2021 is responsible for several earthquake clusters that cover an area of more than 150 square miles. While only a few of these events are actually directly connected to rising magma, most events result from the significant shifting of rock layers. An analysis, why Iceland is shaking and why only some specific areas are affected.

Overview of the ongoing seismic crisis in Iceland
Schematic overview of ongoing volcanic and tectonic processes associated with the renewed activity of Fagradalsfjall volcano. Black spots mark the earthquakes of 2021 associated with the volcanic eruption. Bigger, colored circles show the epicenters of the new earthquakes: Red the most recent, blue the oldest. Earthquake data: . Note that only epicenters are plotted which have been manually revised by scientists. Automatic locations of several thousand more events are uncertain and therefore not useful for this purpose.

1. Magma

Everything has started with the rise of new magma beneath the Fagradalsfjall volcano on July 30th. Along a fracture zone that strikes southwest-northeast, a body of melted rock (Intrusion) is present in depths of 3 – 7 kilometers. This magma can lead to a new phase of volcanic activity, similar to the eruption that caught global interest last year and attracted thousands of tourists. While a new eruption would probably not lead to significant damage or losses of life due to its remote location, earthquakes triggered by magma movement can.
Along the fracture zone where the intrusion is ongoing, thousands of small earthquakes, strongest so far with magnitude 4.7, were caused as the magma breaks its way to the Earth’s crust. Seismicity is directly linked to this magmatism. But this movement of rocks is not limited to the vicinity of the magma.

2. Ground deformation

Since the magma is opening a path of several inches in width, surrounding masses of rock have to be pushed aside. This deformation is still detectable miles away and is the main reason why earthquakes are happening away from the magma. Pressure from the volcano affects existing tectonic faults. The area is located on the Mid Atlantic Ridge (MAR), an active plate boundary between the European and the North American continents. High seismicity and several active fault zones are present there. Many faults are only waiting for the next bigger quake. A slight push from a magma body nearby is enough to reach the critical point.

Two of these fault zones have already reacted to larger earthquakes: A normal fault near the town of Grindavik and another near the lake Kleifarvatn.

The Grindavik earthquake happened on the evening of July 31st. With a Magnitude 5.4, it was one of the strongest earthquakes in this area in decades. A shallow depth of less than two kilometers and the direct proximity to residential areas lead to minor damage across the town. Cracks in buildings formed, water pipes broke, and objects were knocked off shelves.

Similar situation 36 hours later: In the early morning of August 2nd, eight quakes between magnitude 4 and 5 occurred on a fault zone along the western shore of Kleifarvatn, only 15 kilometers south of Reykjavik. Besides a sleepless night for capital residents, the two strongest quakes (M4.8 and 5.0) caused minor damage to objects and even damage to a ceiling of a shopping mall.

The activation of present faults is currently only taking place east and west of the intrusion. These areas are where the deformation is overlapping with the most active part of the MAR (see map above). Therefore, the chance for triggering a “loaded” fault is highest there. Additionally, the angle of faults, which usually have a north-south orientation (in contrast to the magmatic fracture zones which have a southwest-northeast orientation), is favorable for triggering due to this deformation pattern. But it is not only tectonic tension that receives additional loading.

3. More magma

Over the past few years, the whole area of the Reykjanes peninsula has witnessed a magmatic revival. Several episodes of magmatic intrusions were recorded across the volcanic systems. While only one has actively led to a volcanic eruption (Fagradalsfjall 2021), others were also accompanied by significant ground uplift and of course seismicity. The most recent just happened two months ago near the Thorbjorn volcano northwest of Grindavik. Over several weeks in May 2022, thousands of small earthquakes happened west of the volcano, and ground uplift of up to 45 mm was detected by satellites.

This area of previous magmatic unrest has also shown signs of activity on July 31st, following the Fagradalsfjall intrusion. Ground deformation has in this case led to additional tension in a fracture zone that was previously weakened by intruding (and possibly still present) magma. A new swarm of minor earthquakes followed but meanwhile stopped.

4. Remnants (and hopes) of 2021

Similar triggering mechanisms operated during the intrusion preceding the 2021 eruption. Small black dots on the map above represent the earthquakes that happened at that time. A cluster of events even further to the west marks an area on the western tip of the peninsular where another intrusion was present during previous years. Smaller clusters of quakes were also recorded near Grindavik, Thorbjorn, and also along the tectonic fracture zones near Kleifarvatn. However, there is no noteworthy overlapping visible between 2021 and current seismicity. The new ongoing magmatic deformation episode seems to trigger those areas which haven’t been affected last year. Either because the tectonic loading wasn’t enough yet back then, or because the deformation pattern was not favorable to trigger these faults.

Anyway, the general mechanisms and zones of triggering are almost identical. The only big difference (so far): An eruption hasn’t happened. As long as magma is still breaking and shifting the crust, more earthquakes and possibly even strong ones are likely. The earthquake hazard is present, and authorities have warned the whole area of possible quakes that might lead to dangerous rockfalls or even building damage. A sudden release of pressure in form of a volcanic eruption would significantly reduce the seismicity. So it happened last year. A volcanic eruption, spectacular, tourists attracting, and likely not dangerous is what most Icelanders are hoping for. A firework to end the shaky days.