The seismic and petrological signature of magma migration beneath the Reykjanes Peninsula, Iceland

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The Reykjanes Peninsula is located in southwest Iceland, and if one includes the nearby capital city of Reykjavik, hosts more than 60% of the population of the entire country. From a geological perspective, the peninsula features a segment of the Mid-Atlantic Ridge system that forms the boundary between the North American and Eurasian tectonic plates. It is delineated by a series of segmented volcanic zones featuring crust that is heavily fractured and contains fissures associated with historic eruptive episodes. Intriguingly, evidence points to these eruptions occurring periodically with an interval of approximately 800 years. Dating of lava from the most recent eruption places it in the 13th century.In late 2019, elevated earthquake activity along the Reykjanes Peninsula was detected, possibly associated with magma intrusion deep in the crust. Over the subsequent 12 months, this activity repeatedly waxed and waned, and included several sizable earthquakes, including a magnitude 5.7 that shook buildings in Reykjavik in February this year. In the last several month, this activity has gradually escalated, with thousands of earthquakes being detected every day, and was accompanied by the intrusion of an ~10 km long dike at a depth of 1.0-1.5 km. On 19th March 2021, this activity culminated in an eruption near Fagradalsfjall, located at the southern end of the dike, the first on the entire peninsula in ~800 years. This relatively small but spectacular eruption is ongoing (for instance, two new volcanic fissures opened up approximately 700m from the original eruption site on April 5th), and is accompanied by earthquake activity and volcanic tremor (continuous shaking associated with the passage of melt in the crust).The goal of this project is to deploy an array of seismic stations in the region of elevated activity on the Reykjanes Peninsula. This will allow us to record ground motion associated with earthquakes that rupture on tectonic faults, which form part of the plate boundary, and magma migration in the crust. We will also use powerful microscopes to map out the compositional variation in samples of newly-erupted lava. The composition of the magma and the crystals tell us about the depths that the magma was stored before it moved towards the surface and erupted. Maps of individual crystals can be used as "geological stop-watches" to time how long magma took to rise before eruption and our new methods to do this will be applied to the ongoing eruption. By combining evidence for magma movement from the earthquakes and lava, we can build models of how the magmatic system works and inform more effective future volcano monitoring strategies in Iceland and elsewhere. Such advances may help us to understand the periodicity of the volcanic activity, the potential for future eruptions and large earthquakes that may pose a hazard to surrounding population centres, and the nature of the interaction between the plate boundary system and the volcanic system, the latter fueled by elevated levels of melt (magma) in the lower crust and mantle beneath.