Stress interactions between earthquakes and volcanoes in South Iceland: Application to Eyjafjallajökull and Katla

Agust Gudmundsson, Trine Simmenes

South Iceland contains some of Iceland´s best-known volcanoes (Hekla, Katla, and Eyjafjallajökull) as well as one of its two main seismic zones, namely the South Iceland Seismic Zone (SISZ). The part of the SISZ that produces continuous microseismicity is a 70-km-long and 10-20-km wide zone, located between the active volcanic zones referred to as the West Volcanic Zone and East Volcanic Zones. The largest earthquakes in the SISZ, however, exceed M7, during which the N-S extension of the SISZ may be as wide as 60 km. The SISZ is partly covered with Holocene lava flows where the seismogenic faults occur as dextral NNE-trending and sinistral ENE-trending conjugate arrays. The same fault-segment trends occur in the Pleistocene rocks north of the Holocene lava flows. There are three main stratovolcanoes/central volcanoes located at lateral ends of the SISZ. These are Hekla, which is located in a quadrant of transtension, and Hengill and Eyjafjallajökull, which are located in quadrants of transpression. In the past decades, all these volcanoes have been subject to unrest and two have erupted, namely Hekla in 2000 and Eyjafjallajökull in 2010. The fourth volcano discussed here, Katla, is located just east of Eyjafjallajökull. Here we analyse the stress interaction between the SISZ and these four volcanoes. We provide new numerical models of the effects of seismogenic faulting in the SISZ on the state of stress in the volcanoes, with a particular focus on Eyjafjallajökull and Katla. The results indicate that the general stress transfer from the SISZ to the volcanoes Hengill, Eyjafjallajökull and Katla was primarily compressive and unfavourable for volcanic eruptions prior to the two June 2000 in the SISZ. Magma intrusions into Eyjafjallajökull in the 1990s thus resulted in sill formation and no eruption. There may also have been magma intrusion into Hengill in the 1990s, but no eruption occurred. Prior to the June 2000 earthquakes Hekla was subject to large tensile stress and had frequent eruptions, but the earthquakes resulted in the stress field becoming less tensile and Heklas has not erupted. By contrast, the stress field in Eyjafjallajökull became more tensile following the June 2000 earthquakes so as to favour dike propagation to the surface. Thus, renewed magma intrusion into the volcano in 2009 and early 2010 resulted in two eruptions in Eyjafjallajökull: an effusive eruption in March, and an explosive eruption in April 2010.