The fault nature of the Ms 5.4 volcanic earthquake preceding the 1996 subglacial eruption of Grimsvِtn volcano, Iceland

The seismic activity preceding and accompanying the September 30, 1996 subglacial eruption of the Grimsvötn volcano, Iceland, developed in three stages. (1) The first stage (first 19 h) included the main shock of magnitude Ms 5.4 and its aftershocks along the northern slope of the Bardarbunga volcano, situated about 20 km NW from Grimsvötn. (2) Seismic foci during the next 17 h marked a line connecting two volcanoes, Bardarbunga and Grimsvötn. This stage culminated in the opening of a fissure and the beginning of eruption. (3) The third stage was observed during the eruption (midnight of September 30–October 7), and included continuous seismic activity along the northern slope of the Bardarbunga volcano as well as more western distributed epicenters, which together had clustered the ring-fault epicentral zone. A finite-fault, broadband teleseismic P waveform inversion was applied to the main shock of September 29. The inversion showed that the rupture had developed downdip from the hypocenter in about 3 s, and the main asperity was ruptured at depths between 4 and 6 km. A comparison with the deep distribution of aftershocks showed that their hypocenters were situated above the destroyed asperities at the depths between 0 and 4 km. These space-time seismic patterns and the results of the finite-fault inversion may be interpreted in the following way. There was a magma chamber under Bardarbunga volcano covered by the strong rocks, or asperities. The main shock of September 29 broke the asperity and formed a path for magma transport. Then a fissure was formed connecting the opened magma chamber with the feeding structures of Grimsvötn volcano, and magma was injected laterally to the site of eruption. The continuous seismic activity during the third period was related to the collapse of rocks around the released parts of the magma chamber. The role of the Ms 5.4 earthquake in this sequence was decisive.