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The puzzle of the 1996 Bárdarbunga, Iceland, earthquake: no volumetric component in the source mechanism

Hrvoje Tkalcic·Douglas S. Dreger·Gillian R. Foulger·Bruce R. Julian
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Abstract

A volcanic earthquake with M w 5.6 occurred beneath the Bárdarbunga caldera in Iceland on 29 September 1996. This earthquake is one of a decade-long sequence of events at Bárdarbunga with non-double-couple mechanisms in the Global Centroid Moment Tensor catalog. 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 using regional long-period seismic waveforms and a composite structural model. 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.

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Bulletin of the Seismological Society of AmericaArticle
90° N90° S · 180° W ← longitude → 180° E
Source-reported bounding extent: 64.31611045403284° to 64.92354174306496° latitude; -18.6328125° to -16.820068359375° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Hrvoje Tkalcic, Douglas S. Dreger, Gillian R. Foulger, Bruce R. Julian. 2009-09-23. The puzzle of the 1996 Bárdarbunga, Iceland, earthquake: no volumetric component in the source mechanism. https://doi.org/10.1785/0120080361

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