Component in the Source Mechanism

• Main Authors: Hrvoje Tkalčić, Douglas S. Dreger, Gillian R. Foulger, Bruce R. Julian
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Iceland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.628.7102; http://rses.anu.edu.au/~hrvoje/Tkalcic_etal_BSSA2009.pdf

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 ofM 5 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) con-traction. 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.