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The Gridavík 2023 dyke and graben formation and growth imaged by seismic and geodetic monitoring

Tomáš J. Fischer1, Pavla Hrubcová2, Josef Vlček1, Gregory de Pascale3, Thorvaldur Thordarson3

  • Affiliations: 1Charles University, Faculty of Science, Prague, Czechia 2Institute of Geophysics, Czech Academy of Sciences, Prague, Czechia 3Faculty of Earth Sciences, University of Iceland,Reykjavik, Iceland

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 126

  • Programme No: 2.1.36

  • Theme 2 > Session 1

Abstract

The current volcanic unrest on the Reykjanes peninsula began in March 2021, when a fissure eruption occurred in the area of Fagradalfjall, and was followed by two further eruptions from the same volcanic dyke in the following two years. All these events were preceded by intense seismic swarms and were accompanied by surface deformation indicating extension and vertical movement. In autumn 2023 the seismic activity moved westwards - to the Grindavík/Svartsengi area, where an intensive seismic swarm activated the Reykjanes oblique rift zone. On 10 November the seismicity highlighted a narrow NE trending zone of about 15 km in length, which was accompanied by rapid subsidence resulting in the formation of a graben 9 km long and up to 4 km wide. The subsequent seismic swarm gradually diminished until 18 December 2023, when the eruption of Sundhnúkur began, starting a subsequent series of eruptions of the same dyke.  We carried out precise hypocentre locations of the seismic swarms in the period ěř October - 18 November 2023 and analyse its space-time distribution. We find a bilaterally propagating tensile rupture of dyke-like character. Comparison with surface displacements measured by the GPS network shows that dyke growth derived from seismic migration is closely correlated with graben-related surface displacements. The character of earthquake focal mechanisms varies along the dyke and correlates with graben occurrence. We also propose a model linking the volcanic and tectonic drivers of the Reykjanes Peninsula extension.