IMO reported that an intense earthquake swarm on the Reykjanes Peninsula began on 24 October. By 1700 on 26 October more than 4,000 earthquakes had been located at depths of 2-6 km. A total of 14 earthquakes had a magnitude over M 3; the largest event, a M 4.5, was recorded at 0818 on 25 October. Most of the activity occurred between Stóra-Skogafell and an area NE of Eldvörp. No ground deformation was recorded, though a single GPS station (FEFC), E of Festarfjall, recorded localized movement to the SE. During 25-26 October the displacement recorded by the FEFC station totaled about 2 cm and movement was also detected at a station in Selatangar. The swarm continued and by 1400 on 27 October more than 5,800 earthquakes had been recorded; a M 4 earthquake occurred at 0402 on 27 October and was located about 2 km N of Grindavík.

Seismicity decreased considerably by 1330 on 28 October, though the swarm was ongoing with a total of more than 7,000 earthquakes. Uplift centered around Svartsengi, 1.5 km NW of Mt. Thorbjorn, was clearly evident in satellite radar and GPS data. The uplift had begun at some point the previous day and likely signified a magmatic intrusion at depth. IMO raised the Aviation Color Code for Reykjanes to Yellow (the second level on a four-color scale) at 1518 on 28 October. During 1130 on 29 October to 1130 on 30 October about 1,300 earthquakes occurred at depths of 2-4 km. The largest event was a M 2.7 at 1140 on 29 October. Uplift continued during 28-31 October, though the rates began to decrease. Modeling suggested that magma was accumulating at a depth of about 4 km. An earthquake swarm began at 0840 on 31 October and lasted about two hours. The events were located at depths of 1.5-5 km and indicted that magma was moving.

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.

Source: Icelandic Meteorological Office (IMO)