The Puzzle of the Bardarbunga, Iceland Earthquake: No Volumetric Component in the Source Mechanism

The Puzzle of the Bardarbunga, Iceland Earthquake: No Volumetric Component in the Source Mechanism Map showing the main tectonic and volcanic features in Iceland. Glaciers are shown in white and spreading segments in dark gray. Volcanoes are shown with thin lines. The Bárdarbunga earthquake is shown with the white star. Triangles are locations of the eleven Iceland Hotspot Project broadband stations used in the moment tensor inversion. Lines indicate the locations of the six different one-dimensional models used between the source and stations.
A volcanic earthquake with Mw=5.6 occurred beneath the Bárdarbunga caldera in Iceland on September 29, 1996. This earth- quake is one of a decade-long sequence of M5+ events at Bárdarbunga with non-double-couple mechanisms in the global CMT catalogue. Fortunately, it was recorded well by the regional-scale Iceland Hotspot Project seismic experiment. We investigated the event with a complete moment tensor inversion method [Tkalčić et al., 2009] using regional long-period seismic waveforms and a composite structural one-dimensional model from receiver function and surface wave constraints. The moment tensor inversion using data from stations of the Iceland Hotspot Project yields a non-double-couple solution with a 67% vertically oriented compensated linear vector dipole component, a 32% double-couple component and a statistically insignificant (2%) volumetric (isotropic) contraction. This indicates the absence of a net volumetric component, which is puzzling in the case of a large volcanic earthquake that apparently is not explained by shear slip on a planar fault. A possible volcanic 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, the circumstances under which it could happen are rare.
</p><p>Tkalčić H., D. S. Dreger, G. R. Foulger, and B. R. Julian (2009). The puzzle of the Bardarbunga, Iceland earthquake: No volumetric component in the source mechanism, Bull. Seismol. Soc. Amer., 99: 3077-3085.
</p><p>Acknowledgements: We are grateful to IRIS DMC center for efficiently archiving and distributing continuous waveform data.</p>


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