Seismic activity associated with the 1963–1967 Surtsey eruption off the coast of South Iceland

The emergence and growth of Surtsey during volcanic activity in 1963–1967, from an oceanic depth of 130 m off the south coast of Iceland, remains one of the best-documented island-forming eruptions to date. By extracting information from seismic bulletins and analyzing archived analog seismic data, principally from SID, the most sensitive station in the seismic network existing at the time (131 km from Surtsey), we address key questions on precursors, timing of eruption onset, and duration of the submarine phase. We estimate the magnitude of completeness for earthquakes at Surtsey to have been M_L = 2.5. Several seismic events of magnitude M_L < 3 in November 3–10, 1963, with epicentral distance fitting with Surtsey are considered to be precursory earthquakes. Continuous tremor of dominant frequency 1–2 Hz, starting in the early afternoon of November 12, is considered to mark the onset of the submarine eruption, about 40 h before the start of subaerial eruption at 06:30 on November 14. The number of earthquakes larger than the threshold magnitude of 2.5 detected in 1963–1966 was 123, with the largest being of magnitude 4.5. Six earthquakes ranging in magnitude between 3.7 and 4.5 were felt in Heimaey, 22 km to the NE of Surtsey. Changes in vent location occurred five times in 1964–1966. Significant earthquake activity after the eruption start occurred in swarms considerably stronger than observed at the start. These swarms preceded vent migration and lasted days to weeks, apparently caused by intrusive activity prior to eruption at the new vents.