Icelandic Meteorological Office (IMO) reported that intense seismicity and deformation at the Reykjanes volcanic system, caused by a magmatic dike intrusion with no surface eruption, was ongoing during 15-21 November. Seismicity during the week was relatively stable with 1,500-2,000 daily earthquakes; the number of events decreased during 20-21 November with only 165 recorded during 0000-1530. Earthquakes were mostly located at depths averaging 4 km.

Most of the earthquakes were located near the middle of the dike, near Hagafell, 3.5 km NNE of Grindavík, and near Sundhnúk, about 1 km NE of Hagafell and about 2 km ENE of Mt. Thorbjorn. Most earthquakes were less than M 2 during 15-16 November and less than M 1 during 16-17 November; the largest event during the week was an M 3 recorded on 17 November. On 16 November sulfur dioxide gas was measured from a borehole located at Svartsengi, N of Mt. Thorbjorn, and extended E to a notable depth. The presence of sulfur dioxide was another indication of the magma intrusion N of Hagafell. In addition to earthquake detected by the seismic network, new technology using the high-sensitivity fiber optic cable that runs from Svartsengi, W of Mt. Thorbjorn, to Arfadalsvík was also collecting seismic data.

Deformation data was consistent with magma flowing into the dyke at depths greater than 5 km. On 17 November GPS data from instruments in and around Grindavík, near the center of the subsidence zone, indicated about 3-4 cm of subsidence per day. Analysis of COSMO-SkyMed radar interferogram data from 18-19 November showed that 30 mm of uplift was centered in the vicinity of Svartsengi, about 2 km N of Hagafell. Uplift was recorded in that same area before the 10 November magmatic intrusion, thought the rate had accelerated. The uplift aligned with the margins of the intrusion, whereas subsidence was located above the intrusion itself. The deformation and seismic data indicated that Hagafell, where the intrusion was modeled to be the widest, was the most likely location for an eruption.

The Blue Lagoon geothermal pool was closed on 9 November and planned to remain closed at least until 30 November. Residents of Grindavík evacuated on 10 November, due to the uncertainty of an eruption and the onset of ground cracking and damaged infrastructure; access to the town continued to be restricted with only periodic entry allowed for residents to collect belongings. During the week ground cracks and sinkholes opened in and around Grindavík, affecting streets and buildings. Construction of earthen barriers began by 15 November to protect the Svartsengi power station, which supplies tens of thousands of people with electricity and hot water; new wells were being drilled to produce a back-up water supply. An 18 November news report indicated that most of the houses in Grindavík were undamaged, but some had been damaged along a big crack that goes through the town; a few homes were destroyed.

Geological Summary. The Reykjanes volcanic system at the SW tip of the Reykjanes Peninsula, where the Mid-Atlantic Ridge rises above sea level, comprises a broad area of postglacial basaltic crater rows and small shield volcanoes. The submarine Reykjaneshryggur volcanic system is contiguous with and is considered part of the Reykjanes volcanic system, which is the westernmost of a series of four closely-spaced en-echelon fissure systems that extend diagonally across the Reykjanes Peninsula. Most of the subaerial part of the system (also known as the Reykjanes/Svartsengi volcanic system) is covered by Holocene lavas. Subaerial eruptions have occurred in historical time during the 13th century at several locations on the NE-SW-trending fissure system, and numerous submarine eruptions dating back to the 12th century have been observed during historical time, some of which have formed ephemeral islands. Basaltic rocks of probable Holocene age have been recovered during dredging operations, and tephra deposits from earlier Holocene eruptions are preserved on the nearby Reykjanes Peninsula.

Sources: Icelandic Meteorological Office (IMO), Icelandic National Broadcasting Service (RUV), Almannavarnadeild ríkislögreglustjóra (National Commissioner of the Icelandic Police and Department of Civil Protection and Emergency Management), mbl.is