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A portrait of the Bárdarbunga volcano, Iceland, earthquake and insights into the kinematics of the caldera drop

Hrvoje Tkalčić1, Andreas Fichtner2, Douglas S. Dreger3, Gillian R. Foulger4 and Bruce R. Julian5

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Earth Sciences, Utrecht University, 3508 TA Utrecht, Netherlands
3 Berkeley Seismological Laboratory, University of California at Berkeley, Berkeley, CA 94720, US
4 Department of Earth Sciences, Durham University, Durham DH13LE, UK
5 United States Geological Survey, Menlo Park, CA 94025, USA

 

The Bárdarbunga volcano lies beneath the 500-m-thick Vatnajokull icecap, the largest glacier in Europe. An earthquake with Mw=5.6 occurred beneath the caldera on 29 September, 1996 and produced an unusual radiation that cannot be explained by a shear slip on a planar fault. A peculiarity of this earthquake was that it was the first in a sequence of seismic and magmatic events and that it was followed, not preceded or accompanied, by a major eruption, which ultimately led to a breakout flood from the subglacial caldera lake. It was hypothesized that the observed source mechanisms results from slip on an outward-dipping cone-shaped ring fault beneath the caldera, as a result of a change in pressure in the volcano’s shallow magma chamber. The earthquake was recorded well by the Iceland Hotspot Project seismic experiment. 

The absence of a volumetric component in the source mechanism is surprising, however a possible mechanism that can produce an earthquake without a volumetric component involves two offset sources with similar but opposite volume changes. We show that although such a model cannot be ruled out, it is unlikely. We simulated different caldera geometries and rupture scenarios on the walls of a conical surface. These experiments support a super-shear rupture extending unilaterally accross one-half perimeter of the caldera or a bilateral rupture extending accross full perimeter of the caldera as likely scenarios for the Bárdarbunga earthquake.

If studied in different frequency bands, synthetic seismograms based on a point source approximation fail to simultaneously explain the observed data, and this indicates the presence of finite-source effects. Using a 3D model of the Icelandic crust and upper mantle, we perform a probabilistic finite source inversion. One of the most robust outcomes of this is a well-constrained source duration with approximately equal amount of energy radiated by individual segments. This indicates that the caldera dropped coherently as a single block. We also hypothesise that a smaller subglacial eruption that triggered the caldera collapse occurred and went unnoticed. The caldera drop could have increased the pressure in the magma chamber thus inducing the principal eruption. The major eruption after the earthquake is consistent with the classical model where the ring fault is located above the magma chamber.