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Until mid-January 1973, Heimaey was considered volcanically inactive both by volcanologists and its residents. Thus, the eruption came as a complete surprise. Premonitory seismicity was registered on seismographs in South Iceland, but at that time no seismic network existed, there was no way of precisely locating the epicenters of the tremors, and an eruption from Katla volcano would have been considered more likely at that time. A swarm of earthquakes began on the evening of 21 January 1973, with 200 events registered during a 14 hour period. Focal depths were below 20 km depth. A second seismic swarm began at 2200 on 22 January.
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