Seismicity associated with magmatic intrusions: insights from the Reykjanes Peninsula, southwest Iceland

Since the volcanic re-awakening of southwest Iceland’s Reykjanes Peninsula in 2021, there has been significant interest amongst the scientific community in trying to improve our understanding of the processes governing the ongoing eruptions and the magmatic intrusions that precede them. Microseismicity is a powerful lens through which to view these processes, providing the means to track melt movement as it triggers earthquakes in the surrounding crust. This dissertation makes use of an exceptional seismic dataset recorded by the Cambridge seismic network on the Reykjanes Peninsula, supplemented by instruments from several other institutions, which has made the recent series of magmatic intrusions and eruptions among the best instrumented events of their kind. In the first part of this dissertation, I analyse the intense seismicity produced during the February - March 2021 Fagradalsfjall dyke intrusion, which preceded the peninsula’s first eruption for almost 800 years. A catalogue of over 80,000 earthquake hypocentres was produced covering the 9.5 km-long two-segment dyke throughout three-week period of dyking. Through a combination of linear regression on subsets of relatively relocated hypocentres; earthquake focal mechanism determination to analyse the geometry and slip of the fault planes in individual earthquakes; and analysis of the orientation of mapped fractures, I infer that pre-existing structures exert primary control on the orientation of dyke-related faulting, almost all of which occurs on approximately north-south striking faults that exhibit right-lateral strike-slip motion. In the second part of this dissertation, I analyse the February - March 2021 Fagradalsfjall dyke seismicity further by relatively relocating an earthquake catalogue for the full intrusion period. This reveals even greater detail in the propagation and dynamics of the dyke-related seismicity thanks to exceptionally fine spatial and temporal resolution. It also builds on the previous analysis by illuminating, through alignment of hypocentres, an even greater number of north-south striking faults than previously visible. The increased resolution afforded by the fully relocated catalogue also allows the identification of a small swarm of earthquakes with dip-slip faulting mechanisms occurring at∼ 6 km depth during the early stages of the intrusion, which is likely related to the initial opening of the dyke and its earliest propagation within the brittle crust. In the final part of this dissertation, I apply similar analysis techniques to those previously applied to the study of an inflationary event that occurred in the neighbouring Eldvörp-Svartsengi volcanic system in April - May 2022, believed to be in response to the intrusion of a sill-like layer within a pre-existing mid-crustal magma domain. I produce a catalogue of∼ 15,000 relatively relocated earthquake hypocentres covering the intrusion period and the month prior, from which I manually analyse events selected from structures delineated by alignment of relatively relocated earthquake hypocentres. Fault plane solutions reveal nodal planes which align well with these structures, and also with mapped fractures in their vicinity. I use this to infer that the intrusion is mostly triggering slip on pre-existing structures. I also identify a sharp base to the seismicity, quantified by the decrease in occurrence of earthquakes with depth, with the shallowest point of this base situated above the modelled centre of inflation,∼ 1.5 km below sea level. These two contrasting case studies provide insight into the strengths and limitations of using microseismicity to inform the properties of different magmatic intrusions on the Reykjanes Peninsula.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Southern, Esme

Advisors dc:contributor.advisor

• Rawlinson, Nicholas
• White, Robert
• Winder, Thomas

• Earth Sciences
• Geophysics
• Seismology
• Microseismicity
• Magmatic Intrusions
• Volcanoes
• Iceland
• Reykjanes Peninsula

Licence

• https://www.repository.cam.ac.uk/bitstreams/d58219ee-a9ac-4fd2-9471-84b56fb23ac5/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

Author Identifier

0000-0002-7259-3613

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/398524