Variation in b-value of caldera earthquakes during recent activity of the Bárðarbunga Volcano in Iceland

— The magnitude distribution of caldera earthquakes in the subglacial Bárðarbunga volcano in Central Iceland, characterized by the b-value, shows a systematic variation that is consistent with stress changes anticipated in the roof of an inflating magma chamber beneath the caldera. The b-value was 0.83 prior to the rupture of the chamber in August 2014 when a dike propagated laterally from the volcano to feed the eruption in Holuhraun. The b-value was relatively high following the collapse of the caldera, reflecting low stress in the magma chamber roof. Half a year later a decrease was observed in the b-value, concurrent with an increase in the seismicity, consistent with indications of recharging of the volcano magma chamber. The magnitude distribution was anomalous during the slow collapse of the caldera in association with the eruption. During this period the earthquake sequence appeared to consist of two populations, only one of which followed the conventional Gutenberg-Richter distribution. For a subglacial volcano, where geodetic methods are difficult to implement, the b-value of caldera earthquakes provides an important additional parameter for the monitoring of magma pressure variations.