Jökull - 01.12.1982, Blaðsíða 120

Fig. 4. Earthquake fractures near the farm Sel-
sund. Part of the 1912 fractures shown in Fig. 3.
Mynd 4. Jaröskjálftasprungur við Selsund. Hluti af
sprungukerfinu á mynd 3.
admittedly do arise, especially where the fault trac-
es merge with brooks, catde tracks and old bridle
paths. Runoff water and animals seem to have a
tendency to choose their paths along fresh fault
traces. Flow structures and cooling cracks in the
lavas can also be deceptive.
The most prominent features of the faults are
open fissures that are arranged in en echelon arrays.
In many instances the turílayer has not ruptured,
but instead sagged down to form an elongate de-
pression. Individual fissures may be tens of centi-
metres to tens of metres long, and very rarely exceed
100 m. The fissures themselves strike in a north-
easterly direction, variable between due N and
N60°E. The en echelon arrays, on the other hand,
have a more northerly direction. Most of them
strike due north, a few have a more easterly trend,
as much as N20°E (Fig. 2).The en echelon pattern
can be seen on many scales, ranging from metres to
a kilometre, even along the same fault and super-
imposed on each other. The 1912 earthquake, for
example, was accompanied by faulting along sever-
al fault segments of a few hundred metres to over a
kilometre in length (Fig. 3). Six of the seven seg-
ments shown in Fig. 3 are arranged en echelon in a
north trending zone. Each segment consists of
shorter en echelon segments, some of which also
consist of en echelon fissures down to a scale of
metres, as is beautifully demonstrated in the fault
exposures west of the farm Selsund (Fig. 4). Inother
cases the en echelon pattern is relatively uniform,
e.g. near the farm Minnivellir (Fig. 5). These frac-
tures were formed in an earthquake in 1630.
Another characteristic feature of the faults is the
row of small mounds of soil and shattered rocks.
The mounds are up to 3-4 m high and are usually
located between the tips of two en echelon tension
gashes. They have apparently been pushed up dur-
ing the fault movement. The top of some of the
mounds has been cut into slices subparallel to the
direction of the adjacent fissures. Some of the best
examples of fault mounds are found on the 1912
fault west of Selsund shown in Fig. 4, and on an
undated fault cuttingasmooth pahoehoe lava NEof
Réttarnes (Fig. 3). In afewareas, where the tension
íissures are obscured due to local conditions, the
row of mounds is the primary indicator of the fault
trace in the field.
Vertical crustal movements do not seem to be a
prominent feature of the South Iceland Seismic
Zone. Normal faults comparable to those of the
volcanic rift zones do not exist. Only small scale
116 JÖKULL 32. ÁR