Jökull - 01.12.1982, Blaðsíða 121

Fig. 5. Major faults and fracture systems of the
earthquakes of 1630 and August 26-27th 1896.
Mynd 5. Helstu sprungur og sprungukerfi frá jarðskjálft-
unum 1630 og 26. til 27. ágúst 1896.
vertical movements are occasionally found locally
within the fracture systems, mostly related to the
fault mounds. In one case the senseof motion along
a fault can be determined directly from a manmade
structure. A wall built of turf and stones is displaced
75-80 cm right-laterally where it is crossed by the
fault active in the earthquakes of August 26-27th
1896, near the farm Lunansholt in the district Land
(Fig. 5).
INTERPRETATION
All the en echelon fissure arrays identified so far
are oriented in a northerly direction, a few slightly
to the east ofnorth (Fig. 2).Theenechelonarrange-
ment is nearly always in the same sense, and indi-
cates right-lateral displacement along an underly-
ing fault. VVe therelbre interpret the fracture arrays
as the surface expression of right-lateral strike-slip
faults with a N-S strike. This interpretation is in
agreement with the only fault plane solution avail-
able for the South Iceland Seismic Zone, made by
Ward (1971) for an earthquake in the Flói district in
July 1967. The solution is not well constrained be-
cause of the small magnitude of the event, but indi-
cates strike-slip faulting, right-lateral on a north
striking, or left-lateral on an east striking fault. The
displacement of the turf wall near Lunansholt also
supports the interpretation. From this we can go a
step further and infer the approximate orientation
of the principal stress axes in the South Iceland
Seismic Zone (Fig.6). The maximum compressive
stress axis (oj is horizontal and trends in the same
direction as the surface íissures, i.e. N30-40°E. The
least compressive stress (03) is also horizontal and
perpendicular to the fissures. The intermediate
stress axis is vertical.
It is noteworthy that thisstress field is identical to
that expected around a transform fault. The dilfer-
ence is, however, that instead of rupturing along a
major E-W fault, the crust in South Iceland chooses
to fracture along the conjugate fault planes.
Faults of the South Iceland Seismic Zone were
previously described by Tr. Einarsson (1967, 1968)
and Tryggvason (1973) who both interpret them as
right-lateral strike-slip faults, as we do here. Tr.
Einarsson, however, only looked at individual seg-
ments, which have a more north-easterly trend than
the overall fracture systems and generalized his
interpretation to the whole of the Icelandic rift
zones, where individual fissures have similar trends.
JÖKULL 32. ÁR 117