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Tectonic earthquake swarms in the Northern Volcanic Zone, Iceland

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Winder, Thomas 


Microseismicity offers an opportunity to image subsurface deformation at exceptionally high spatial and temporal resolution. This may be related to a diverse range of processes, including fore- and aftershocks to destructive earthquakes on large faults, magma movement at volcanoes, or the gradual advance of glaciers. Microearthquakes at faults might also signal more exotic behaviour, including due to transient events such as fluid injections or pulses of aseismic fault creep.

The study of small earthquakes has advanced significantly over the past decades, as seismology has entered the digital age. Denser networks, with larger numbers of more sensitive seismometers allow ever smaller seismic events to be detected and analysed. The study of larger numbers of earthquakes brings a raft of benefits, including more robust statistical analyses, higher temporal resolution, and the opportunity to achieve significantly greater spatial resolution by harnessing the power of relative relocation algorithms. However, the fundamental task has remained the challenge of extracting an earthquake catalogue from continuous waveform recordings. I present new software that offers a powerful, efficient, and highly automatable method to produce exceptionally complete and robust earthquake catalogues from continuous seismic data. Instead of reducing the information contained in waveforms to “picks” – discrete timestamps marking candidate phase arrival times – within QuakeMigrate the seismic data is transformed to continuous functions which are then combined across the network by migration. Retaining as much information as possible until the point of comparing the recordings from across the entire array allows phase arrivals at or below the signal-to-noise ratio to be successfully associated with events, improving both detection capability and location resolution.

I use QuakeMigrate to detect and locate microseismicity in the Northern Volcanic Zone, Iceland, from 13 years of continuous seismic data recorded on a dense local network. Through comparison with a comprehensive catalogue of previously analysed events, I demonstrate that this new, automated algorithm is capable of location accuracy equal to or surpassing the widely held gold standard of manually refined earthquake locations. Within my new catalogue of more than 155,000 events, including related to magma ascent and intrusion in the lower crust, shallow geothermal activity, and seismicity triggered by fluid injection, I focus on persistent earthquake swarms that delineate a network of faults surrounding Askja volcano.

Tightly constrained earthquake hypocentres, refined by cross correlation and relative relocation, reveal an intricate mesh of strike-slip faults, with fine-scaled structures resolved at length-scales of 10’s of metres. This comprehensive image of the subsurface fault structure fills in the gaps between manually analysed events, and combined with tightly-constrained fault plane solutions reveals kinematics which point to a complex deformation field, and the role of elevated pore pressures or mechanical anisotropy in generating anomalous fault geometries. Comparison of the temporal evolution of the major sources of deformation with the seismicity provides evidence that deflation of a shallow magma reservoir in Askja caldera plays a principal role in driving these earthquakes.

Within individual earthquake swarms, seismicity systematically migrates along fault planes, indicating the role of aseismic processes in controlling their evolution. Analysis of the migration velocities and b-value statistics points to transient creep events as the primary driving mechanism, indicating the role of unusual frictional properties. Aseismic slip has recently been recognised as an important process in releasing strain at subduction zone interfaces, as well as on faults at a range of depths both in the oceans and within the continents. Such well-resolved examples of this process are crucial in gaining a deeper understanding of the wide spectrum of fault behaviour, and the links between slow and largely silent, and fast and potentially devastating fault slip.

The Bárðarbunga-Holuhraun dike intrusion provides a unique opportunity to analyse the triggering of seismic swarms due an extremely well-resolved static stress change. As the dike intruded towards Askja, a surge in swarm seismicity was observed across the network of faults studied in this dissertation. In the absence of complicating factors present in the study of stress changes caused by large earthquakes, I determine that swarms initiated at Coulomb stress increases of as small as 0.025 MPa. Furthermore, swarms do not show any seasonal tendency in occurrence, despite strong seasonal loading of the crust, which dominates measurements of relative crustal velocity changes. This further indicates that these faults experience very low effective normal stresses.

Together, these observations indicate that this network of faults possess an exotic rheology. The absence of seismicity triggered elsewhere by the dike intrusion provides confidence in my observation of their spatial extent, which is consistent with earthquake catalogues derived from analogue seismic recordings extending back to the 1970s. I suggest the potential roles of fluids released at sites of vigorous hydrothermal activity in Askja volcano, and from numerous melt feeders in the lower crust, in producing the conditions that allowed these faults to develop, setting this area apart from the remainder of the Northern Volcanic Zone, which is otherwise largely aseismic during the current inter-rift period.





White, Robert


Volcano seismology, Seismology, Geophysics, Iceland, Volcanology, Geology, Microseismicity


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Natural Environment Research Council (NE/F011407/1)
Natural Environment Research Council (NE/H025006/1)
Natural Environment Research Council (NE/M017427/1)
European Commission (308377)
NERC (NE/L002507/1)