Ring Fault Slip Reversal at Bárðarbunga Volcano, Iceland: Seismicity During Caldera Collapse and Re‐Inflation 2014–2018
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American Geophysical Union (AGU)
https://doi.org/10.1029/2021gl097613
https://doi.org/10.1029/2021gl097613
Abstract
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Abstract Microearthquakes reveal the kinematics of the Bárðarbunga caldera ring fault; during the 2014–2015 rifting event and gradual caldera collapse, and its subsequent, ongoing re‐inflation. Tightly constrained focal mechanisms during re‐inflation have reversed phase arrival polarities from events during the caldera collapse. Thus, the inner side of the steeply dipping northern caldera faults (averaging 81 ± 8°) has been moving upwards during the post‐eruptive period. Both precise relative relocations of the seismicity and fault plane solutions confirm that this is due to slip reversal on the same ring fault structure.
Plain Language Summary Bárðarbunga is a highly active volcano that lies beneath Iceland's Vatnajökull glacier. It last erupted for 6 months between August 2014 and February 2015—the largest eruption in Europe for 250 years—with tens of thousands of earthquakes. These earthquakes occurred in response to molten rock moving within the volcano. We analyzed the earthquakes to map active faults along the large bowl‐shaped depression that represents the volcano's caldera, and investigate the fault movements where they occur. We observed faulting on almost vertical faults around the edge of the volcano throughout 2014–2018, but in opposite directions during and after the eruption. During the eruption, the caldera floor moved downwards relative to the surrounding flanks of the volcano. After the eruption ended, the motion reversed and the caldera floor began to move back upwards. The downwards movement during 2014–2015 occurred as the molten rock—stored at a depth of about 6 km beneath the surface—moved laterally to feed the eruption over 40 km away from the volcano. We suggest that the ongoing upwards movement since the eruption ended is due to new molten rock accumulating beneath the volcano.
Key Points Fault motion reverses on the same fault structure between the eruptive and post‐eruptive periods Tightly constrained focal mechanisms show motion on steeply dipping faults