Achievements

• Structure of the active faults and fault systems. The active faults of the seismic zone are expressed at the surface by arrays of en-echelon fractures. The fractures themselves have a NNE to NE orientation. Most of the arrays, on the other hand, have a N to NNE orientation. The en-echelon arrangement is therefore left-stepping reflecting a large component of right-lateral horizontal shear movement along the arrays. Evidence for faulting along the conjugate direction, i.e. left-lateral slip on ENE striking faults, exists but is very rare. Individual fractures are either purely extensional or have a component of right-lateral movement. Push-up structures are frequently observed between the tips of adjacent fractures. These are hillocks of different size, ranging between a few centimeters in height to a few meters. They reflect horizontal compression in the direction of maximum compressive stress. The en-echelon arrangement of fractures can be seen on many scales within the same fracture system. Individual fractures ranging in length between 1 and 50 m may thus form an array 100-200 m long. The arrays may then be arranged en-echelon within a system on the scale of a kilometer. The kilometer sized systems may then again be arranged en-echelon to form a super-system of 10 km length. Description and maps of these structural relationships have been published in a series of papers, e.g. [20,21,8,24,23] and Einarsson et al. (1999 in preparation).

• Surface ruptures of the earthquakes of 1630 and 1784 identified. Contemporary accounts of a large earthquake in the year 1630 mention ".. that large fissures were formed where there had been none before, in particular near the farm Minnivellir". These fractures have been mapped and found to belong to an extensive fracture system with the typical structures of the strike-slip faults of South Iceland. The system is more than 7 km long and has a N-S orientation.

  The earthquake of August 14, 1784, is considered to be the largest historical earthquake in Iceland (M=7.1). Its causative fault has been unknown except for the general area. Contemporary accounts mention that large fissures were formed in the northern part of the Holt district. The largest fractures found in this part of the seismic zone can be traced as a faint N-S system for a distance of 8 m. We assume that this is the expression of the 1784 causative fault.

  Maps of the 1630 and 1784 faults are presented at the spring meeting of the Geoscience Society of Iceland [35] and will be published in a paper shortly.

• Digitized fault map. The compilation of fault data is in progress. About 3/4 of the area of the zone has been covered. Only the easternmost part is still to be done. This work has led to some new projects, including detailed mapping of faults around the towns of Selfoss and Hveragerði. The fault map of the South Iceland seismic zone will be published shortly with an accompanying paper.