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Abstract:
A day and a half after the earthquake (mb =5.3, MS =5.6, MW =5.6) in the Bárðarbunga central volcano on Sept. 29th 1996, a volcanic eruption broke out under the Vatnajökull glacier. The eruption was located approximately 20 km SSE of the earthquake epicenter, midway between the Bárðarbunga and Grímsvötn central volcanoes. Course of events suggests a connection between earthquake and eruption and therefore a connection with a sequence of earthquakes of the same characteristics in Bárðarbunga during the years 1973–1996. The earthquakes in question are of an unusually low frequency character (corner frequency), explained by exceptionally low dynamic stress drop (< 10 bars) at shallow depth (≤5.0 km). The sequence which lasted for 22 years is characterised by ∼annual main events of magnitudes in the range of 4.5–5.7 (mb). It intensified in the 1990s, with some of the largest earthquakes of the whole episode occurring at that time. Moment tensor solutions of teleseismic signals and locally recorded waveforms reveal that the main events are thrust faulting earthquakes with a significant non-double couple component. Arguments are presented that the faulting occurred on a steeply inward dipping caldera fault, with reactivated motion on a weak fault. As a consequence of this hypothesis magma inflation in Bárðarbunga is the most probable cause of the 1973–1996 events. However, the loading force (the magma) may or may not have resided at a similar shallow depth as the earthquakes. Cast in the frame of the inflation model, the Bárðarbunga 1973–1996 sequence implies a resurgent caldera of at least 0.2–0.7 km 3 for approximately a quarter of a century, exceeding its magma storage capacity in 1996. However, these calculations are model dependent. Bárðarbunga and neighbouring area were relatively calm during the period mid-1997 to 2004. There was a renewed activity of small earthquakes during the years 2005–2009. From the beginning of continuous seismic recording in Iceland in 1925, all eruptions in Vatnajökull on record have been accompanied with earthquake(s) of magnitude ≥4.0, within two months of the initial eruption.
