Jökull


Jökull - 01.12.1979, Page 41

Jökull - 01.12.1979, Page 41
earthquake, which is the most recent major earth- quake in this zone, can still be observed in the districts Land and Rangárvellir. Most of the faults are arranged en echelon within a narrow, about 9 km long fault zone trending almost due N-S. The faulted zone was originally considerably longer, but has now been partly obliterated. The en echelon structure appears on many different scales, ranging from meters to a kilometer, and suggests right- lateral movement on a N-S striking fault. Surface faulting during earlier earthquakes seems to be of the same type. The sense of motion implies a least compressive stress in a horizontal NW-SE direction and a maximum compressive stress in a NE-SW direction. The E-W epicentral belt may be interpreted as the expression of a deep seated deformation zone. The sense of motion conforms with a transform fault interpretation of the zone. The brittle crust responds to this motion with right-lateral strike-slip on northerly striking surface faults. The strain in the South Iceland seismic zone is released in sequences of large earthquakes with a recurrence time of 50—fOO years. The sequence often starts with a magnitude 7 — IV2 shock in the eastern part of the zone which is followed by slightly smaller shocks in the western part. This propagation of activity is documented in the sequences of 1732 — 34, 1784 and 1896 (Fig. 2). Tjörnes Fracture Zone The Tjörnes Fracture Zone is a broad zone of faulting and seismicity that connects the southern end of the submarine Kolbeinsey Ridge and the volcanic zone in North Iceland. The seismicity is too diffuse to be associated with a single fault or a simple plate boundary. The epicentral locations shown in Fig. 1 are based on teleseismic data and Fig. 2. Map of the seismic zones of SW-Iceland, showing some of the tectonic features of the Reykjanes Peninsula and the South Iceland seismic zone. The seismic lineaments are taken from Klein et al. (1973 and 1977) and Foulger and Einarsson (1979). The destruction zones of the historic earthquakes of 1732—34, 1784, 1896 and 1912 are shown. Within these zones more than 50% of houses at each farm were ruined. Corresponding intensity is MM VIII — IX. JÖKULL 29. ÁR 39
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