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

Bulletin of the Seismological Society of America
By: , and 

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Abstract

A volcanic earthquake with Mw 5.6 occurred beneath the Bárdarbunga caldera in Iceland on 29 September 1996. This earthquake is one of a decade-long sequence of Graphic 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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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title The puzzle of the 1996 Bárdarbunga, Iceland, earthquake: no volumetric component in the source mechanism
Series title Bulletin of the Seismological Society of America
DOI 10.1785/0120080361
Volume 99
Issue 5
Year Published 2009
Language English
Publisher Seismological Society of America
Contributing office(s) Earthquake Science Center
Description 9 p.
First page 3077
Last page 3085
Country Iceland
Other Geospatial Bardarbunga
Online Only (Y/N) N
Additional Online Files (Y/N) N