SHORT-TERM WARNING FOR THE JUNE 21 EARTHQUAKE

It was considered possible that another earthquake of similar size would follow the June 17 mainshock. This was realized 80 hours later. But before that, i.e. 26 hours before the magnitude 6.6 earthquake of June 21, as special warning was issued by IMO to the state and the local civil defence services. In the warning it was stated that the most likely hazard area (Figure 6) of a probable impending earthquake would be as indicated in the figure, i.e. within 1 km of the NS fault line on which the earthquake occurred.

 

Figure 6: The map that was sent to the Iceland Civil Defence 26 hours before the earthquake on June 21. Red dots indicate the location of small earthquakes, which were the basis for the immediate warning. Green boxes show probable hazard areas (in Icelandic: Líkleg umbrotasvæði). An accompanying telephone report explained that an earthquake of similar size or slightly smaller than the first earthquake was to be expected to occur within either of the green boxes, more likely within the larger one. A strictly defined time window was not given, but the advice was given to prepare for that an earthquake with the expected size and location would occur "anytime soon". The red NS line, which was put afterwards onto the warning map, indicates the fault plane of the June 21 earthquake.

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The warning was based on evaluation by IMO seismologists of time and space distribution of microearthquake activity. It was also based on the understanding of the tectonics of the area and the knowledge of the historical seismicity described briefly earlier in this paper. Indications based on seismic history as well as on recent observations that large seismic events tend to migrate approximately with a velocity of 5 km/day were also taken into account. The actual velocity between the 2 epicenters was 4.9 km/day.