Brief introduction to the tectonics of Iceland

Iceland is situated across the Mid-Atlantic ridge on top of a mantle plume, Figure 15.

Figure: Map showing the rift and transform zones in Iceland. WRZ is the Western Rift Zone, SISZ, the South Iceland Seismic Zone, ERZ the Eastern Rift Zone and NRZ the Northern Rift Zone. The rate and direction of spreading is from Sigmundsson et al. [1995]. SIL seismic stations are shown as solid triangles. Small dots are earthquakes above magnitude 2.

The center of the mantle plume is inferred to be beneath the east-central part of Iceland [e.g. Tryggvason et al., 1983; Allen et al., 1999] and due to the westward drift of the plate boundary with respect to the hotspot, the spreading zones successively jump eastward [Einarsson, 1991]. The divergent plate boundary in Iceland is thus offset to the east compared to the Mid-Atlantic ridge offshore to the north and south. From the south, see Figure 15, the Mid-Atlantic Ridge connects to the Reykjanes peninsula and continues onshore to the Hengill triple junction, where the spreading zone continues north along the Western Rift Zone (WRZ). East from the triple junction is a transform zone, the South Iceland Seismic Zone (SISZ), which connects the WRZ with the Eastern Rift Zone (ERZ). The ERZ extends northward to the Vatnajökull glacier and when the rift zone emanates from under the glacier in the north it is named the Northern Rift Zone (NRZ) and extends to the coast in the north. The Tjörnes fracture zone again connects the plate boundary to the Mid-Atlantic ridge. The spreading rate across Iceland is predicted by global plate motions, averaged over the last few million years, in the NUVEL-1A model [DeMets et al., 1994] to approximately 19 mm/yr in the direction $\mathrm{N}{76}^{\circ}\mathrm{W}$. The observed relative velocity of the North American plate (west of the WRZ) and the Eurasian plate (at the southern tip of the ERZ) is 21$\pm$4 mm/yr in the direction $\mathrm{N}{63}^{\circ}\mathrm{W}$ $\pm$ ${11}^{\circ}$ [Sigmundsson et al., 1995]. The Global Positioning System (GPS) measurements of Sigmundsson et al. [1995] indicate that about $85\pm 15\%$ of the relative plate motion is accommodated by the SISZ in a 20 km to 30 km wide band that correlates well with the seismic activity in the SISZ.

The South Iceland Seismic Zone takes up the transform motion between the Reykjanes peninsula and the ERZ. There is, however, no large, east-west oriented transform fault in the SISZ, instead the transform motion is accommodated on many parallel, north-south right-lateral faults. The SISZ has seen a number of major earthquake sequences and large localized events such as the 1912 magnitude 7 earthquake west of Hekla [Einarsson, 1991]. Most of the large events are inferred to have occurred on N-S striking faults and much of the seismicity in the SISZ have strike-slip focal mechanisms [Einarsson, 1991; Stefánsson et al., 1993]. Surface mapping in the SISZ reveals a multitude of NNE and ENE striking faults, many with a strike-slip component, and NE striking normal faults [Passerini et al., 1997; Guðmundsson, 1995; Bergerat et al., 1998].

The Ölfus area is situated immediately south of the Hengill triple junction, in a transition region between the Reykjanes peninsula and the SISZ. In Reykjanes, earthquakes are mostly normal faulting, characterized by $\sigma_{3}$ being consistently horizontal, trending NW, and $\sigma_{1}$ rotating between vertical and horizontal, trending NE [Einarsson, 1991]. Earthquakes in the Ölfus region are predominantly strike-slip, on N-S striking subvertical faults [Rögnvaldsson et al., 1998] and surface fault and fissure mapping in the Ölfus region show that most faults strike N-S to NE-SW [Sæmundsson, 1995]. The Ölfus area will be discussed further in Papers II to IV.