Rapid resurgence of the subglacial B&#225;rdarbunga caldera following collapse in 2014-2015, quantified with repeated Bouguer gravity surveys
Authors: Gudmundsson Magnús Tumi , Högnadóttir Thórdís , Sigmundsson Freysteinn , Geirsson Halldór , Li Siqi , Reynolds Hannah I. , Magnússon Eyjólfur , Pálsson Finnur 
Published: March 2020

&lt;p&gt;The 65 km&lt;sup&gt;2&lt;/sup&gt;&amp;#160;B&amp;#225;rdarbunga caldera is located in the NW part of the Vatnaj&amp;#246;kull glacier in central Iceland.&amp;#160;&amp;#160;The caldera floor lies under 500-800 m thick ice and the rims are fully subglacial as well.&amp;#160;&amp;#160;The caldera subsided by 65 m during the B&amp;#225;rdarbunga-Holuhraun eruption in 2014-2015, when about 2 km&lt;sup&gt;3&lt;/sup&gt;&amp;#160;of magma drained out from a magma reservoir at ~10 km depth leading to the largest eruption in Iceland since Laki in 1783.&amp;#160;&amp;#160;Deformation surveys outside the caldera have indicated inflation since soon after the end of the eruption in February 2015 and seismicity has been elevated.&amp;#160;&amp;#160;The extensive ice cover precludes conventional microgravity surveys or detailed surveys of caldera floor elevation.&amp;#160;&amp;#160;However, we have studied gravity changes by comparing results of repeated Bouguer anomaly surveys.&amp;#160;&amp;#160;We perform a full Bouguer correction using detailed DEMs of both the ice surface and the ice-radar-derived bedrock.&amp;#160;&amp;#160;Ice surface changes are also mapped, allowing the removal of effects on gravity by ice mass changes.&amp;#160;&amp;#160;Possible sources of significant anomalies are either changes in bedrock elevation between surveys, other more deep-seated mass changes beneath the volcano, or changes in the water table and pore pressure.&amp;#160;&amp;#160;Surveys were carried out using a Scintrex CG-5 in 2015, 2016, 2018 and 2019, with measurements done at 25-50 locations each time.&amp;#160;&amp;#160;As no benchmarks exist on the ice the spatial difference in station location of 10-20 m exists between survey years.&amp;#160;&amp;#160;&amp;#160;However, post-processing provides kinematic GPS position and elevation accuracy better than 0.1 m. Analysis of the data and error sources indicate an accuracy in estimates of changes of 50-100 &amp;#181;Gal. The results obtained indicate change with an amplitude of a few hundred &amp;#181;Gals; over the four years between 2015-2019 a clear Bouguer anomaly increase is recorded over the caldera relative to the surrounding area.&amp;#160;Sharp gradients in the gravity difference near the caldera boundary point to a shallow source, consistent with the gravity signal arising from or near the ice-bedrock boundary.&amp;#160;&amp;#160;This indicates fast resurgence at B&amp;#225;rdarbunga since 2015. The elevation of bed reflections delineated from radio echo sounding profiles (~2 MHz), measured within the caldera in June 2015 and accurately repeated in June 2019, further supports this.&amp;#160; The suggested deformation mechanisms can be compared to geodetic observations outside the caldera for further evaluation. If all the signal is interpreted in terms of magma movements, a rise of the caldera floor by several meters and the inflow of 0.2-0.3 km&lt;sup&gt;3&lt;/sup&gt;&amp;#160;of new magma is inferred.&lt;/p&gt;