Seismic phenomena associated with the 1996 Vatnajokull eruption, central Iceland

During late September 1996, a major eruption took place at the NW part of t he Vatnajokull glacier in central Iceland. The eruption was preceded by int ense seismic activity, which began with a M-w = 5.6 earthquake two days pre viously. Two very active volcanic systems, Bardarbunga and Grimsvotn, are s ituated in that area underneath the permanent ice cap. The volcanoseismic p henomena associated with the eruption were recorded on both temporary (HOTS POT) and permanent (SIL) seismic networks, covering most parts of the count ry. The recorded events were categorised, according to their waveform shape and frequency content, into three groups: (1) low-frequency events; (2) mi xed-frequency events; and (3) volcanic tremor. The large earthquake at Bard arbunga volcano, which initiated the seismic activity before the eruption, was located inside the caldera and had the characteristics of a non-double couple event. The epicentres of the earthquake swann that followed it initi ally delineated the caldera rim and then migrated towards Grimsvotn, possib ly indicating lateral movements of magma from a shallow chamber beneath Bar darbunga. The eruption affected an area much larger than that between these two volcanoes, since seismic activity was also observed at distances 20 km away, at the Tindafjallajokull volcanic system. The spectral analysis of t remor, recorded at the nearest station to the eruption site, revealed its e xistence before the onset of the eruption in five narrow frequency bands (0 .5-0.7, 1.6, 2.2, 2.8, 3.2 Hz) representing fundamental frequencies with th eir half- and quarter-subharmonics. This pattern continued until the last d ay of the eruption. It is believed that the eruption was caused by a dyke i njection that had been going on beneath the Vatnajokull area for a period o f 10 years. (C) 2000 Elsevier Science B.V. All rights reserved.