abstract

The earthquakes of 1974 in Borgarfördur occurred west of the mid-Atlantic plate boundary in Iceland. The earthquake sequence lasted more than 2 months and was a combination of swarm activity and a foreshock-mainshock-aftershock sequence. The largest earthquake of the sequence occurred on June 12, 1974 and was of mb = 5.5. No change in the b-value with time could be detected. An array of nine short-period seismometers was operated in the epicentral area for about 3 weeks. The main epicentral zone as defined by 563 accurately determined hypocenters is about 4 km wide, 25 km long and trends nearly east-west. The depth of most hypocenters is between 0 and 8 km. No earthquake was located below the crust-mantle boundary.

The mainshock on June 12 probably occurred near the center of the epicentral zone where it is intersected by a secondary zone which has a northeast trend. A focal mechanism solution of the largest shock indicates normal faulting. One nodal plane of this solution strikes NE which matches the secondary epicentral trend. Focal mechanisms in the western end of the epicentral zone also indicate normal faulting. Nodal planes of these focal solutions, the trend of epicenters, and observed surface fault breaks all have an E-W trend in the western part of the seismic zone. The Borgarfjördur area thus seems to be undergoing horizontal extension. The minimum compressive stress rotates from a horizontal northwesterly orientation in the center of the epicentral zone to a horizontal north-south direction in the western branch. This extension may be caused by a mantle plume under Iceland or by thermal contraction of the lithospheric plates as they move away from the accreting plate boundary.

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