Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 47

RELATIVE LOCATION OF EARTHQUAKES IN THE
TJÖRNES FRACTURE ZONE
PÁLL EINARSSON
Lamont-Doherty Geological Observatory and Department
of Geological Sciences, Columbia University,
Palisades, New York
ABSTRACT
The seismicity of the Tjörnes Fracture Zone is distributed over a zone 150
km long and 80 km wide. Teleseismic locations of earthquakes within this zone
show a diffuse pattem of epicenters that does not easily lend itself to tectonic
interpretation.
An attempt was made to locate the earthquakes of 1968 and 1969 relative to
one reference earthquake by using relative P-wave arrival times at a fixed set
of stations. To ensure consistent picking of the arrival times at a given station,
the P-wave signals were correlated visually with the P-wave of fhe reference
earthquake. This method of analysis reduces the errors in the locations caused
by source and station irregularities and mis-picking of arrival times of small
earthquakes. The method further reduces the scatter of the epicenters introduced
by using different sets of stations to locate the different earthquakes.
The relocated epicenters appear to define a narrow seismic zone, possibly a
fault, with a WNW trend. The absolute location of this proposed fault cannot
be accurately determined, but most probably it passes within a few kilometers
of the island Grímsey. A focal mechanism solution of one of the earthquakes
shows strike-slip motion along the fault in a right-lateral sense. The sense of mo-
tion and the strike of the fault is therefore similar to that of the Húsavík fault
about 40 km to the south.
Some significant seismic activity is known to have occurred close to but dis-
tinctly off these two faults, notably fhe magnitude 6% earthquake that caused
extensive damage in the village of Dalvik in 1934. It is suggested here that the
Dalvík earthquake occurred on a fault parallel to the Húsavík fault, but 30 km
to the south.
The transform motion between the submarine Kolbeinsey Ridge and the vol-
canic zone in northem Iceland is thus demonstrated to occur along two, and
possibly fhree or more, parallel strike-slip faults.