Abstract
A half century of monitoring of the Northern Volcanic Zone of Iceland, a branch of the North America—Eurasia plate boundary, shows that the seismicity is very unevenly distributed, both in time and space. The four central volcanoes at the boundary, Þeistareykir, Krafla, Fremrinámar, and Askja, show persistent but very low-level seismicity, spatially coinciding with their high-temperature geothermal systems. On their rift structures, on the other hand, seismicity is almost absent, except during rifting episodes. Krafla went through a rifting episode in 1975–1984 with inflation, interrupted by 20 diking events with extensive rifting, eruptive activity, and intense seismicity along an 80 km long section of the rift. During inflation periods, the seismicity was contained within the caldera of the volcano, reflecting the inflation level of the magma chamber. Diking events were marked by seismicity propagating away from the volcano into the fissure swarms to the south or north of the volcano, accompanied by rapid deflation of the caldera magma chamber. These events lasted from 1 day to 3 months, and the dike length varied between 1 and 60 km. The area around the Askja volcano is the only section of the Northern Volcanic Zone that shows persistent moderate seismicity. The largest events are located between fissure swarms of adjacent volcanic systems. Detailed relative locations of hypocenters reveal a system of vertical strike-slip faults, forming a conjugate system consistent with minimum principal stress in the direction of spreading across the plate boundary. A diking event into the lower crust was identified in the adjacent fissure swarm at Upptyppingar in 2007–2008. Four nests of anomalously deep earthquakes (10–34 km) have been identified in the Askja region, apparently associated with the movements of magma well below the brittle-ductile transition. Several processes have been pointed out as possible causes of earthquakes in the deformation zone around the plate boundary. These include inflation and deflation of central volcanoes, intrusion of propagating dikes, both laterally and vertically, strike-slip faulting on conjugate fault systems between overlapping fissure swarms, migration of magma in the lower, ductile crust, and geothermal heat mining.
Highlights
Volcanism in Iceland has its roots in two different processes
The Initial Rifting Event, December 1975–March 1976 A new phase of the activity began suddenly on December 20 when a small basaltic fissure eruption broke out in the center of the caldera, preceded by only 15 min of intense earthquake activity (Einarsson, 2018) that was detected by seismic stations throughout Iceland
Each dike or rifting event may only affect a limited length of one fissure swarm. This leads to accumulated horizontal shear stress between adjacent fissure swarms that is released in some kind of transfer motion on sets of conjugate strike-slip faulting. Two examples of such tectonism exist in the Icelandic rifts, the persistent seismicity east of the Askja volcano in the Northern Volcanic Zone, and the seismic activity in the Reykjanes Peninsula Oblique Rift (Hjaltadóttir, 2009; Parameswaran et al, 2020)
Summary
Volcanism in Iceland has its roots in two different processes. The mid-Atlantic plate boundary runs through the country, separating two of the largest lithospheric plates, the North America and the Eurasia plates and spreading at a rate of about 19 mm/year (e.g., Árnadóttir et al, 2009). The Initial Rifting Event, December 1975–March 1976 A new phase of the activity began suddenly on December 20 when a small basaltic fissure eruption broke out in the center of the caldera, preceded by only 15 min of intense earthquake activity (Einarsson, 2018) that was detected by seismic stations throughout Iceland. Intrusive Activity in the Southern Fissure Swarm, April and September 1977 The caldera seismicity was very low after the deflation of January 1977, only a few detected events per day. The rate quickly increased, and earthquakes propagated into the northern fissure swarm This event turned into the second largest dike intrusion of the Krafla rifting episode, in terms of volume, distance, and duration (Table 1). Four nests of deep events have been located in the Askja area, two in the area of the central volcano, one about 10 km east of the volcano and one about 15 km NNE of the volcano (Figure 6). Key et al (2011) suggested that these events represent magmatic injections along the fissure swarm of Askja, and Greenfield and White (2015) suggested that they represent magma transport through the lower crust
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