Jökull - 01.01.2013, Blaðsíða 17

Reviewed research article
Evidence of recent fault movements in the Tungnafellsjökull
fissure swarm in the Central Volcanic Zone, Iceland
Þórhildur Björnsdóttir1,2 and Páll Einarsson1
1Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
2now at Akureyri Junior College, Eyrarlandsvegur 28, 600 Akureyri, Iceland
thorhildur@ma.is, palli@hi.is
Abstract — The volcanic system of Tungnafellsjökull lies in the Central Iceland volcanic zone near the center
of the hot spot and the triple junction where the Eurasian Plate, the North-American Plate and the Hreppar
Microplate meet. Holcene activity in the Tungnafellsjökull system has been very low, only two small lavas are
associated with the system. The Tungnafellsjökull fissure swarm is rather short and wide compared with fissure
swarms of other volcanic systems at the divergent plate boundary, 40 km long and 20 km wide. Earthquakes are
not common, with usually fewer than 10 being registered per year. Due to these facts, it came as a surprise when
InSAR measurements detected movements on faults in the fissure swarm of Tungnafellsjökull during the Gjálp
eruption in Vatnajökull in 1996 at a distance of around 37 km from the eruption site. Ground check in 2009 and
2010 revealed evidence of recent movements on faults in the area in the form of fresh sinkholes and fractures,
some of which had moved as recently as the spring of 2010. Fresh sinkholes are known to form mostly during
faulting events. They are formed when surface soil is washed into underlying, widening cracks in the bedrock.
Based on earthquake data and InSAR images these fault movements occurred during three tectonic events, in
October 1996 during the Gjálp eruption, in August 2008 and in November 2009. The events are expressed by
increased seismicity in the Tungnafellsjökull area, both in terms of number of recorded earthquakes as well
as rate of seismic moment release. The earthquakes were all small. The total released seismic moment is
equivalent to that of a single earthquake of magnitude 3.4.The widespread evidence of recent fault movements
and the small magnitude of the earthquakes suggests that the fault activity is related to magma movements
rather than tectonic faulting.
INTRODUCTION
Iceland is located at the mid-Atlantic plate bound-
ary where two major plates meet, the North-American
plate and the Eurasian plate. In addition, a microplate
has been defined between two branches of the bound-
ary, called the Hreppar microplate (Einarsson, 1991,
2008). The country provides the only place where a
divergent part of this plate boundary can be studied on
land. Being hot-spot influenced the tectonic picture
of the plate boundary in Iceland is more complicated
than most other mid-oceanic plate boundaries. The
relative motion of the Mid-Atlantic Ridge with re-
spect to the hot spot leads to ridge jumps, propagating
rifts and microplate complexities. Complex fracture
zones are found in the north (Tjörnes Fracture Zone)
and south of the island (the South Iceland Seismic
Zone), as well as volcanic zones (Reykjanes Peninsula
Rift, Western Volcanic Zone, Eastern Volcanic Zone,
Central Iceland Volcanic Zone and Northern Volcanic
Zone) (Einarsson, 2008). The volcanic zones are
made up of structural units called volcanic systems,
which consist of a central volcano and transecting rift
zones or fissure swarms (Sæmundsson, 1974; Jakobs-
son, 1979a). A central volcano is a centrally situated
JÖKULL No. 63, 2013 17