Relative locations of earthquakes in the Tjörnes Fracture Zone

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 pattern 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 the reference earthquake. This method of analysis reduces the errors in the locations caused by source or 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 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 of 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 motion 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 distinctly off these two faults, notably the magnitude 6 1/4 earthquake that caused extensive damage in the village of Dalvík in 1934. It is suggested 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 volcanic zone in northern Iceland is thus demonstrated to occur along two, and possibly three or more parallel strike-slip faults.