A Magmatic Origin for the 2007 Micro-Earthquake Swarms at Upptyppingar, Iceland?

Micro-seismicity is common within the rifting zones that intersect Iceland. However, between February and August 2007, anomalous swarms of tectonic earthquakes -- amounting to at least 3,350 epicentres -- have been detected over a 36 km2 area near to Upptyppingar (16.2° W; 65° N), which forms part of the Kverkfj~{o}ll volcano system, extending from the northern flank of the Vatnaj~{o}kull ice cap. Using results from the SIL seismic network, we summarise the spatial and temporal changes in ongoing seismicity that began in February 2007; additionally, we consider the processes underlying the escalated activity. Besides displaying spatial clustering, the Upptyppingar micro-earthquakes are noteworthy because (i) they concentrate at focal depths ranging from 15 to 18 km; (ii) the swarms comprise earthquakes <2 in magnitude; and (iii) several of the swarms originate at focal depths exceeding 18 km. Each swarm has been confined to a small surface area and focal depths have remained mostly consistent, both within and between swarms. Spatially, different parts of the affected region have exhibited seismicity at different times, with swarm sites alternating between distinct areas. Curiously, few earthquakes have been detected at shallow depths within the zone of unrest. A likely reason for the sustained clustering of earthquakes near to Upptyppingar is inflow of magma into the base of the crust; such an hypothesis would account for the depth, intensity, and duration of the swarms. Based on observations elsewhere in Iceland, a candidate explanation is the emplacement of a dyke. Interestingly, the up-surge in seismicity from the Upptyppingar region coincides with the ongoing formation of a 2,000 GL reservoir for water, located 21 km south-east of Upptyppingar. Nevertheless, the possibility of induced seismicity remains unclear.