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Jökull - 01.12.1982, Qupperneq 122

Jökull - 01.12.1982, Qupperneq 122
Fig. 6. Typical arrangement of faults and mounds in the South Iceland Seismic Zone, interpreted as resulting from strike-slip movement underneath. The axis of inferred maximum (a,) compressive stress and least (a3) compressive stress are shown. Mynd 6. Dœmigerb afstaða gjáa og sprunguhóla á skjálftasvæði Suðurlands, sem má túlka sem afleiðingu af sniðgengishreyfingu undir niðri. Sýndir eru ásar mestu (°i) °g minnstu (o3) spennu í 'berggrunninum. This is, in our view, not justified. The structure and appearance of the South Iœland fracture systems is quite different from that ofthe fissure swarms of the volcanic rift zones. En echelon arrangements can be found within the fissure swarms, but the sense is not always the same and often depends on the strike of the fissure with respect to that of the swarm. Each swarm has the structure of a shallow graben, and vertical displacements across the fissures are comm- on. The swarms are active mostly during rifting episodes similar to the current events in the Krafla fissure swarm in NE-Iceland. There the displace- ments have been shown to be in the form of widen- ing ofthe fissures and subsidence of thegraben floor (Gerke et al. 1978, Bjomsson et al. 1979, Möller and Ritter 1980, Sigurdsson 1980). The age of many of the faults is uncertain. The 1630, 1896, and 1912 faults are the only ones with a known age. Yet it would be of great importance to date the faults. One may hope to obtain a seismic history for a good part of the postglacial time, and thus improve previous estimates of the recurrence time of large earthquakes in this zone. So far, no evidence has been found for repeated activity along any of the faults. Each fault seems to have been active during one earthquake only. One should bear in mind, however, that all the known faults cut very young rocks and deposits that may conceal older movements. But it is noteworthy, for example, that the 1896 fault was formed 2.5 km W of and parallel to a fault active 266 years earlier (Fig. 5). In this case it appears to have been easier to form a new fault than to reactivate an old one. The main conclusion of this study seems to con- firm the picture suggested by the destruction zones of the earthquakes, namely that individual earth- quakes are related to slip along faults perpendicular to the main seismic zone. The earthquakes seem to be associated with brittle deformation ofa 10-20 km wide zone overlying an E-W trending zone of aseis- mic deformation in the lower crust or upper mantle. The net displacement along the zone is left-lateral transform motion to accommodate the spreading in the Eastern Volcanic Zone, but the brittle crust responds by right-lateral slip along the conjugate fault planes. The reason for this behaviour is not clear, but it might be related to the apparent youth of this zone. It is possible that the finite displace- ment along the zone is not sufficiently large to break the whole crust and produce a major fault. Perhaps one has to view the South Iceland Seismic Zone as a propagating fracture front instead of a steady state feature. Such a front may be expected to propagate southwards in response to increased activity and southwards propagation of the Eastem Volcanic Zone, and would leave behind it a block of thor- oughly fractured crust. This model might explain the extensive faulting of the Early to Late Quater- nary Hreppar rock sequence exposed north of the Seismic zone. Continued studies of the South Ice- land earthquake fracturesand their relation to the Hrepparsequence may further clarify these ideas. REFERENCES. Bjömsson, A., G.Johnsen, S. Sigurdsson, G. Thorbergsson and E. Tyggvason 1979: Rifting of the plate 118 JÖKULL 32. ÁR
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