Geophysical constraints on the formation of the volcanic island of Surtsey in 1963-1967 and its internal structure

The volcanic island of Surtsey, off the south coast of Iceland, was formed by shallow ocean-emergent basaltic volcanism in 1963-1967. There were several episodes of activity, migrating between five vents (Surtur, Surtla, Surtungur, Syrtlingur and Jólnir) to form a discontinuous ~5.8 km-long mostly submarine ridge. Analysis of data from the rudimentary analog seismic network existing in Iceland in the 1960s reveals that the magnitude of completeness for the Surtsey area was ML=2.5. Precursory seismicity (ML<3) is detected for nine days before onset of continuous tremor on 12 November 1963, signaling the onset of a 40-hour long submarine eruption, followed by subaerial explosive activity. Earthquakes during the eruption were mostly related to intrusive activity preceding opening of new vents. A complicated connection emerges between seismic tremor and magma flow rate or style of activity. The highest magma flow rates in the first 10–20 days were associated with relatively low tremor; higher tremor was observed in the weeks that followed, after discharge rate had dropped substantially. No correlation is found between minutes-to-hours variations in explosive activity and tremor. A detailed aeromagnetic survey was conducted in 2021. Spectral analysis, Euler deconvolution and 2.5D forward modeling of selected profiles reveal shallow origin of magnetic anomalies in the area. The subaerial lavas on Surtsey cause the strongest anomalies, while other sources are mostly 0-300 m below seafloor. No indications of significant intrusions are found within and below the structures formed in 1963-1967, confirming that magma fragmentation was dominant in the submarine phases of the Surtsey eruption. ; Surtsey myndaðist í eldgosi undan suðurströnd Íslands á árunum 1963-1967. Gosvirkni færðist til nokkrum sinnum og alls gaus á fimm stöðum, í Surti, Surtlu, Surtungi, Syrtlingi og Jólni. Myndanirnar eru samtals um 5.8 km á lengd, mynda breiðan en slitróttan hrygg, að stórum hluta neðansjávar. Greining gagna úr því frumstæða ...