Volcano geodesy and magma dynamics in Iceland - Acri-ST

28E. Sturkell et al. / Journal of Volcanology and Geothermal Research 150 (2006) 14–343.7. EyjafjallajökullThe volcano Eyjafjallajökull has an ice cap withthe same name. No volcanic activity is known fromthe time of settlement (874 AD) until 1612. Annals forthat year mention an eruption in Eyjafjallajökull orKatla, but it is not clear whether one volcano or bothwere active. A small eruption occurred in 1821–1823(Larsen, 1999). Prior to 1991, the Eyjafjallajökullvolcano showed insignificant seismic activity for atleast three decades (time of seismic coverage). Anincrease in activity started near the end of 1991 andpeaked with an earthquake swarm in 1994. Unrestcontinued, and a second earthquake swarm occurredin 1999. The seismic activity that started in the earlynineties continues with a higher background level thanbefore 1991 (Fig. 15).The 1994 and 1999 seismic episodes at Eyjafjallajökullwere associated with significant inflation of thevolcano indicating intrusions under its southern flank.The timing of the deformation is bracketed by tilt andGPS measurements (Sturkell et al., 2003b) for boththe intrusive events (Fig. 15). The deformation datafrom 1999 are modelled by a point pressure source at3.5 km depth beneath the flank of the volcano, about 4km south of the summit crater. Maximum uplift of themodel is 0.35 m (Sturkell et al., 2003b). A similarmodel also explains deformation associated with the1994 seismic crisis. However, its location is not thatwell constrained because only two GPS stations andone tilt station recorded the event. The 1994 intrusionhas also been modeled by Pedersen and Sigmundsson(2004) using InSAR images. Their study suggests thata sill with varying thickness accommodates the deformationsignal better than a single point pressuresource (Fig. 3). A similar sill model can be appliedto the 1999 event (Pedersen et al., 2004). The earthquakeactivity associated with the intrusions began afew months before measurable deformation occurred.A large part of it was located below the northern flankof the volcano (Dahm and Brandsdóttir, 1997) and atlarger depth than the inferred intrusions. We suggestthat this activity is related to the feeding channel ofthe intrusions.3.8. Hengill–HrómundartindurIn mid-1994 a very intense earthquake swarmstarted at the Hengill triple junction where the ReykjanesPeninsula oblique rift, The Western VolcanicZone and the South Iceland Seismic Zone meet. Theactivity was concentrated 5 km east of the Hengillvolcanic system, in the less pronounced Hrómundartindurvolcanic system. This enhanced earthquakeactivity continued until 1998. During the 1994–1998period about 85,000 earthquakes were recorded(Jakobsdóttir et al., 2002; Árnadóttir et al., 1999), andstill the Hengill triple junction is one of the mostpersistent source of microearthquakes in Iceland. Theincreased earthquake activity was associated with 243M l2101990 1992 1994 1996 1998 2000 2002 2004YearFig. 15. Magnitude of the earthquakes in Eyjafjallajökull volcano as a function of time, spanning the interval 1 Jan. 1990 to 31 Aug. 2004,including all events above magnitude 1.0. The seismic activity started in the autumn 1991 and it has continued with varying intensity,culminating in a swarm that started in the beginning of 1999. This swarm continued into 2000 and thereafter the earthquake activity has settledat a higher background level. The 1994 and 1999 periods of crustal deformation (intrusion) are shown as shaded bars (Sturkell et al., 2003b).