Earthquake activity related to the 1991 eruption of the Hekla volcano, Iceland

The 1991 eruption of the Hekla volcano started unexpectedly on 17 January. No long-term precursory seismicity was observed. The first related activity was a swarm of small earthquakes that began approximately half an hour before the eruption. Intensive seismicity, both earthquakes and volcanic tremor, accompanied the violent onset of the eruption. Almost 400 events up to M-L magnitude 2.5 were recorded during the first few hours. During the later phases of the eruption, the earthquake activity was modest and the main volcano-related seismic signal was the persistent volcanic tremor. The tremor died away, together with the eruption on I I March, and Hekla was seismically quiet until the beginning of June 1991, when a sudden swarm of numerous small shallow earthquakes occurred. This activity is atypical for Hekla and is interpreted to be a failed attempt to resume the eruption.