Jökull - 01.01.2010, Blaðsíða 107

The Kerlingar fault, Northeast Iceland
RESULTS
The fault region
The Kerlingar fault is located within the Dimmifjall-
garður mountains, a series of arcuate hyaloclastite
ridges along the NVZ-EFB boundary (Figure 1),
(Helgason 1987; Sæmundsson 1977; Vilmundardóttir
1997). Various structures related to tectonic activity
are present in this region. However, due to the com-
plexity of the area, we have only mapped the most
prominent features. The fractures which we mapped
are of various sizes, sharpness and orientations (Fig-
ure 3). Many of these fractures, including the Kerl-
ingar fault, form a structural pattern which is oriented
parallel with the EFB-NVZ boundary. However, no
Holocene volcanic fissures have been mapped in the
area close to the Kerlingar fault.
Fault orientation
The Kerlingar fault is oriented NNW-SSE; therefore,
it is not parallel with fractures in the NVZ fissure
swarms at this latitude, which are generally N or NNE
oriented (i.e. ∼perpendicular to the 106◦ plate spread-
ing vector (DeMets et al. 1994)) (Figure 1). The fault
is, however, parallel with the line of central volca-
noes that extends along the axis of the NVZ: Frem-
rinámar, Krafla, and Þeistareykir (Figure 1). Although
the Kerlingar fault has a general NNW orientation, it
curves along its length from a NNW orientation at its
southern end to a NW orientation at its northern end.
Generally, the strike of different fault segments ranges
from 350◦ in the south, to 336◦ in the north.
Fault dip and vertical offset
Our field observations show that the eastern side of
the Kerlingar fault is downthrown. Assuming a nor-
mal fault, this indicates that the fault is east-dipping,
i.e. it dips away from the NVZ (Figures 4–8). This is
not obvious everywhere, particularly where the fault
cuts westward sloping surfaces. The throw of the fault
varies along the fault. Our field measurements ranged
from 2 to 9 m in vertical offset. As can be seen from
Figure 7, snow accumulates and stays longer in some
parts of the fault. The meltwater from this snow may
erode the fault scarp, which makes precise measure-
ments of vertical offset due to fault movements diffi-
cult. Therefore, 9 m may be an overestimate, at least
in terms of Holocene movements.
Fault length
The Kerlingar fault is at least 30 km long, interrupted
by a few E-W oriented gullies. It may even extend
further, if we assume that the NW oriented fractures
north of the Kerlingar fault are a part of the fault.
The Kerlingar fault is therefore an unusually long fea-
ture, compared with faults within the fissure swarms
of the NVZ. As a comparison, the majority of frac-
tures within the Askja fissure swarm are less than 1
km long (Hjartardottir et al. 2009).
Earthquakes
Although some faults (including the Kerlingar fault)
have been previously identified in the Dimmifjall-
garður mountains (Sæmundsson 1977), earthquakes
have not been detected there by seismograph net-
works that have been in operation in the area since
1974 (Einarsson 1989; Einarsson 1991; Jakobsdóttir
2008). This indicates that the activity of the area is
not steady-state. Although it is more likely that the
fault ruptured in several smaller earthquakes, we cal-
culate the maximum size of an earthquake, assuming
that the total length of the fault ruptured in one event.
The seismic moment is calculated according to:
M0=µu¯
F (1)
We assume that the shear modulus (µ) is 10 GPa and
the mean displacement (u
¯
) is 4 m. The area of the
fault (F) is calculated assuming that the fault has a
dip of 60◦, that the length of the rupture is 30 km and
that the rupture reaches 7 km depth, i.e. the approxi-
mate bottom of the seismogenic crust (Einarsson et al.
1977; Soosalu et al. 2010). The moment magnitude is
calculated according to:
Mw = 2/3 log M0–6.0 (2)
From this, we conclude that the fault could have gen-
erated an earthquake of moment magnitude up to
Mw≈6.7. If we increase the depth of the fault to 10
km, we get an earthquake of magnitude Mw≈6.8.
JÖKULL No. 60 107