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ample, that earthquakes occur both in mainshock-aftershock se-
quences and earthquake swarms, i.e. sequences without an out-
standing mainshock. The sequence in August 1969 (Table 1) on
the Grímsey fault was a typical earthquake swarm, whereas the
large earthquakes 1934 and 1963 were mainshocks with extensive
aftershock activity.
Some of the important tectonic and seismological questions will
be answered when data become available from the seismograph
network presently being installed in North Iceland.
CONCLUSIONS
1. Absolute locations of earthquake within a given region using
teleseismic data often yield diffuse patterns which are difficult to
put into a tectonic context.
Assuming similar depth and similar focal mechanism the earth-
quakes within an earthquake sequence can be located relative to
each other with sufficient accuracy to allow detailed tectonic in-
terpretation.
2. Relative locations of earthquakes that occurred in 1968 and
1969 within the Tjömes Fracture Zone reveal a long and narrow
seismic zone, probably a fault with a WNW trend. The fault is
parallel to and about 40 km north of the Húsavík faults and is
mostly submarine. The sense of motion along the fault is right-
lateral strike-slip.
3. An additional fault is proposed about 30 km south of and
also parallel to the Húsavík faults. The destructive Dalvík earth-
quake of 1934 occurred on this suggested fault and possibly also
the magmtude 7 earthquake of 1963.
ACKNOWLEDGEMENTS
This research was supported by the National Science Foundation
(Grant GA43382) and the Icelandic Science Foundation. I thamk
Dr. Guðmimdur Pálmason for encouragement and helpful discus-
sions. Drs. C. Scholz and P. Richards read the manuscript and of-
fered valuable suggestions for its improvement.