Jökull - 01.12.1979, Síða 41

earthquake, which is the most recent major earth-
quake in this zone, can still be observed in the
districts Land and Rangárvellir. Most of the faults
are arranged en echelon within a narrow, about 9 km
long fault zone trending almost due N-S. The
faulted zone was originally considerably longer,
but has now been partly obliterated. The en echelon
structure appears on many different scales, ranging
from meters to a kilometer, and suggests right-
lateral movement on a N-S striking fault. Surface
faulting during earlier earthquakes seems to be of
the same type. The sense of motion implies a least
compressive stress in a horizontal NW-SE direction
and a maximum compressive stress in a NE-SW
direction.
The E-W epicentral belt may be interpreted as
the expression of a deep seated deformation zone.
The sense of motion conforms with a transform
fault interpretation of the zone. The brittle crust
responds to this motion with right-lateral strike-slip
on northerly striking surface faults.
The strain in the South Iceland seismic zone is
released in sequences of large earthquakes with a
recurrence time of 50—fOO years. The sequence
often starts with a magnitude 7 — IV2 shock in the
eastern part of the zone which is followed by
slightly smaller shocks in the western part. This
propagation of activity is documented in the
sequences of 1732 — 34, 1784 and 1896 (Fig. 2).
Tjörnes Fracture Zone
The Tjörnes Fracture Zone is a broad zone of
faulting and seismicity that connects the southern
end of the submarine Kolbeinsey Ridge and the
volcanic zone in North Iceland. The seismicity is
too diffuse to be associated with a single fault or a
simple plate boundary. The epicentral locations
shown in Fig. 1 are based on teleseismic data and
Fig. 2. Map of the seismic zones of SW-Iceland, showing some of the tectonic features of the
Reykjanes Peninsula and the South Iceland seismic zone. The seismic lineaments are taken from
Klein et al. (1973 and 1977) and Foulger and Einarsson (1979). The destruction zones of the historic
earthquakes of 1732—34, 1784, 1896 and 1912 are shown. Within these zones more than 50% of
houses at each farm were ruined. Corresponding intensity is MM VIII — IX.
JÖKULL 29. ÁR 39