abstract

The seismicity of the Iceland region is concentrated along the crest of the Mid-Atlantic Ridge and in two shear zones connecting the two ends of the eastern volcanic zone in Iceland to the ridge crest on both sides of Iceland. Earthquakes of magnitude exceeding 6 occur in these shear zones, but not on the ridge crests. The volcanic zones of Iceland, with the exception of the Reykjanes peninsula, are characterized by low seismicity with no observed earthquakes of magnitude larger than 5. Earthquake swarms are mostly confined to the crest of the Mid-Atlantic Ridge, the Reykjanes peninsula and the north Iceland shear zone, but very few swarms occur in the south Iceland shear zone or in the eastern volcanic zone of Iceland. The magnitude-frequency relation of earthquakes within each earthquake swarm deviates from the linear relation, Log N = a−bM, in that the “b” value decreases with decreasing earthquake magnitude. A better expression for the magnitude-frequency relation within each swarm is Log N = 3.5 − A · 1.5M, where A is a constant related to the size of the swarm, and N is the number of earthquakes of magnitude equal to or exceeding M. The spreading axes follow the crest of the Reykjanes Ridge and the volcanic zones in Iceland. North of Iceland the axis of spreading is not well defined, but it probably has a nearly north-south strike from the Eyjafjördur deep across the Kokbeinsey Island to the Spar Fracture Zone. The spreading of the eastern volcanic zone of Iceland is confined to two or more parallel bands of volcanic activity and fissure swarms, with more stable strips in between. The transfer of spreading from the Reykjanes Ridge to the eastern volcanic zone and from the eastern volcanic zone to the spreading axis north of Iceland takes place in zones of shear deformation, but not along single transform faults. These shear zones are the only areas in the Iceland region where earthquakes of magnitude 7 or larger are known to occur. The postulated central Iceland shear zone (or transform fault) is seismically inactive and, therefore, not similar to the shear zones in south Iceland and off the coast of north Iceland.

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