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| Titel |
Earthquake focal mechanisms associated with dyke propagation and caldera collapse at the Bárðarbunga volcano, Iceland |
| VerfasserIn |
Martin Hensch, Simone Cesca, Sebastian Heimann, Eleonora Rivalta, Vala Hjorleifsdóttir, Kristín Jonsdottir, Kristin Vogfjörð, Torsten Dahm |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106199
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| Publikation (Nr.) |
 EGU/EGU2015-5854.pdf |
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| Zusammenfassung |
| The dyke intrusion at the Bárðarbunga volcanic system which started on 16 August 2014, as
well as the subsidence of its caldera were accompanied by an intense seismic swarm along
the propagation path of the dyke and around the caldera ring fault. In this study we analyse
focal mechanisms of both clusters, along the dyke and around the caldera rim, to reveal
driving forces of the seismic and volcanic activity. Full moment tensors are determined for
events larger than MwÂ4.5 to obtain more complex mechanisms, especially for earthquakes
associated with the caldera collapse.
Within the first two weeks between the onset of the seismic swarm and the opening of the first
fissure eruption, an approximately 45 km long dyke migrated northeastwards from the
caldera to the eruption site a few kilometers outside the glacier margin of the Vatnajökull ice
cap. The dyke propagated with irregular velocities, alternating phases of arrest and spurts
were accompanied by thousands of earthquakes of magnitudes up to MwÂ4.5. The focal
mechanisms range from strike-slip to normal faulting around the dyke, with a stable tension
axis perpendicular to the dyke. Seismicity along the intrusion was driven by tensional stresses
accumulated in the shallow crust and pressure induced by the magma intrusion itself. This
assumption is supported by the significant drop of activity following the onset of the fissure
eruption.
In contrast, the seismic activity around the caldera rim remains at high levels since the onset
of the crisis while the underlying magma reservoir continues deflating. The caldera floor of
Bárðarbunga has subsided by around 60Âm since August. The strongest earthquakes on the
ring fault reached up to MwÂ5.7 and several dozens of events were larger than MwÂ5. A
moment tensor inversion of these events revealed consistent steep normal faulting with a
significant compensated linear vector dipole component (CLVD). This mechanism can be
explained by two different models: (1) A rupture propagating along at least 120° around the
ring fault. While the movements on each element on the fault is purely double-couple, all
movements sum up to a CLVD mechanism due to the curvature of the ring fault. (2) A joint
mechanism of double-couple movements on the fault and a vertical single force
linked to volume decrease in the magma reservoir and the resulting collapse of the
caldera.
The results of this study help to understand the dynamic processes during the dyke
propagation and the caldera collapse. Especially the observation of such a large
number of Mw > 5 earthquakes related to a caldera collapse is globally rare and
the complexity of the derived solutions calls for a deeper analysis of the involved
mechanisms. |
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