Microseismicity reveals the fault geometry and internal structure of the re-inflating Bárðarbunga caldera

Abstract

<p>Between August 2014 and February 2015 the subglacial Bárðarbunga caldera collapsed, subsiding more than 65 metres as magma flowed out from beneath it to feed a dike intrusion and fissure eruption at Holuhraun. Subsequently, the caldera has been re-inflating, likely indicating recharge of the crustal magma storage reservoir. Sustained seismicity along the caldera ring faults – but with reversed polarity compared to the eruption period – further indicates its ongoing resurgence<sup>1</sup>. Between June-August 2021 we installed an array of 6 seismometers on the ice cap above Bárðarbunga, to provide improved constraints on earthquake locations and focal mechanisms, and to improve ray coverage in the region beneath the caldera.</p><p>Tilt-tolerant Güralp Certimus sensors provided high-quality three-component recordings throughout the deployment, despite significant ice movement. We used QuakeMigrate<sup>2</sup> – a powerful migration-based automatic earthquake detection and location algorithm – to produce a catalogue of more than 8,500 earthquakes during the two month deployment, with a magnitude of completeness of M<sub>L</sub> -0.8. These are dominantly composed of high-frequency volcano-tectonic (VT) earthquakes around the caldera margins. Waveform cross-correlation and relative-relocation reveals a sharply defined ring fault, which is consistent in geometry with geodetic constraints obtained during the deflation period in 2014-15. Tightly constrained focal mechanisms provide further insight into the geometry of the caldera-bounding fault system.</p><p>Low frequency earthquakes observed between 15 - 25 km depth b.s.l. in the normally ductile part of the crust below Bárðarbunga signify activity at the roots of the volcano, which may indicate fluid ascent pathways. Further long-period earthquakes in the centre of the caldera, at around 5 km b.s.l., possibly mark the location of the shallow magma storage reservoir. Precise manually picked phase arrival times will be inverted to produce a local body-wave tomography model of the internal structure of the volcano. Together with the seismicity, this will provide the first image of the magma plumbing system that feeds Bárðarbunga. It will furthermore provide constraints on the relative geometry of the caldera ring faults and magma reservoir that drained during the 2014-15 eruption and caldera collapse, and which is now re-inflating to drive the ongoing resurgence. These may be compared to laboratory and numerical models of caldera formation and faulting mechanisms to provide an improved general understanding of this important volcanic phenomenon.</p><p> </p><p>1: Southern, E.O., Winder, T., White, R.S. and Brandsdóttir, B., 2021. Ring Fault Slip Reversal at Bárðarbunga Volcano, Iceland: Seismicity during Caldera Collapse and Re-Inflation 2014-2018. https://doi.org/ 10.1002/essoar.10510097.1</p><p>2: Winder, T., Bacon, C., Smith, J., Hudson, T., Greenfield, T. and White, R., 2020. QuakeMigrate: a Modular, Open-Source Python Package for Automatic Earthquake Detection and Location. https://doi.org/10.1002/essoar.10505850.1</p>