Jökull - 01.12.1979, Page 40

Reykjanes Ridge. Epicenters are taken from the PDE lists of USCGS, later NOAA and USGS, for the
period 1962—1977. Open circles denote epicenters determined with fewer than ten P-wave readings
or epicenters of earthquakes smaller than mb = 4.5. Dots are epicenters of events of mb = 4.5 and larger
that are determined with ten or more readings. Large dots are epicenters of events of mb = 5.0 and
larger. The focal mechanisms are shown schematically as lower hemisphere equal area projections.
The compressional quadrants (containing the least compressive stress axis) are shown black. The
bathymetry is taken from a map by the Icelandic Hydrographic Service, Reykjavík, 1975. Depths are
in meters. The volcanic rift zones of Iceland are shown.
nes, earthquakes occur in swarms, i.e. in sequences
where no one event is much larger than the others.
Normal faulting is the most common faulting
mechanism. Towards east the earthquakes tend to
occur more in mainshock-aftershock sequences and
strike-slip faulting becomes more prominent. The
plate boundary in the eastern part of the peninsula
has not been mapped in detail because of the low
seismic activity there in recent years.
South Iceland seismic zone
The South Iceland seismic zone bridges the gap
bctvyeen the two volcanic zones in South Iceland.
Few instrumentally determined epicenters are
available for this zone, but the destruction areas of
some of the large, historic earthquakes are shown in
Fig. 2. In spite of the clear E-W orientation of the
seismic zone, there is no indication on the surface of
a major E-W fault in this area. Many of the earth-
quakes, however, have been associated with surface
faulting and the faults generally seem to be
oriented N-S or NE-SW. The destruction zones are
elongated in the N-S direction.
The surface faults formed during the 1912
38 JÖKULL 29. ÁR